Motorola Mitrek conversion to repeater or link

by Karl Shoemaker, AK2O



This document is written in attempt to include everyone interested in serious construction of a quality product. Its very technical, however, written openly and honestly. It's designed for Amateur Radio (not commercial) at no cost to obtain and is open for discussing, changes and improvements without notice. Should you feel qualified you are welcome to deviate from the Author's design. You need to have a basic electronics background with some repeater building experience. Understanding schematic drawings is required. If you are new at the repeater operation you might want to check out additional technical books relevant to this documentation. Anticipate about 40 hours to construct each radio, especially the first one. No free technical support is available, however, printed documents are available on an occasional bases for a modest cost for P & H.

About this page

Several images (pictures) on this document, while on-line are enlargeable simply by clicking on them. This gives you an opportunity to "zoom in" for better details should you need it. Other images are not, being unnecessary large files being stored on the server. Some images on this page show only general practice and may not be the completed product or bear accuracy. For example, one image shows a jumper for 9.6v switching with the soldering not completed. Production was paused to take some pictures at the time. This is brought up to clarify the final product may be finished without documentation and may vary from what you see here.

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Acronyms, Definitions, and Theory basics:

Amateur Radio is to develop the art of radio and improving operating practices. This can set a good example for others, including the commercial industry, to what Amateur Radio system(s) are capable of doing to provide public service communications in time of need. This includes the technical side, to produce good operating repeater. To be very clear on this philosophy, we will start with very basic theory. "Two-way" Radio systems send intelligence (voice, data, etc.) by modulating the originating transmitter and decoding (detecting) this modulation at the far end receiver back to something usable to be understood. How well this is understood depends greatly on how well the system is set up. Just about anyone can "throw" a system together to make it work, somewhat. SRG design specifications call for a better way as you will see in this documentation.

A typical (commercial) system uses the audio portion of 300Hz~3KHz for modulation. This document covering modulation frequency and levels will be somewhat different. Also it calls for good technical management. For one, technician organization and discipline is necessary. Plan on what you want to do for a system design and stick to it. Force yourself to keep good practices. One good practice is to establish level references. Some call these "benchmarks", or "baselines". While old Amateur methods used linear (volts, watts, etc) units of measure, most SRG designs and operations use logarithmic units in "dbm". Once accustomed, it's easier to see the entire picture this way, when designing a system, checking frequency response, and keeps the guesswork out of troubleshooting a subtle level problem. More information on this subject can be found by clicking here. References can be expressed in a few acronyms. This is very dry reading, however, you need to spend time on this to better understand advanced circuits, later on.

Normally, a tone of 1 KHz (sometimes 1004 Hz) is used for a testing a "2-way" VHF-UHF transmitter or receiver. TTL ( Test Tone Level ) is referenced to 100% system modulation; in this case F.M. (Frequency Modulation). FM is also referred to "deviation" (of the carrier, at an audio rate). For Amateur Radio 100% system modulation is normally + - 5 KHz. Other areas/States and/or commercial services have different bandwidth standards, presently. In that case it would be + - 2.5 KHz. For this documentation we will only cover the former (5 KHz deviation).

TLP ( Test Level Point ) refers to a measurement point, on equipment, in the system, in reference to TTL. TLP provides easy reference to any parts of the system for measurement and alignment. 0 dbm is referenced to 1 milliwatt at 600 ohms. Therefore, a transmitter AF input with a TLP of 0 dbm, with a TTL of 0 dbm tone input, would fully modulate the system. If the far end receiver was set up the same its output a 0 dbm tone as well. A 6 db drop in (voltage) level would reduce the modulation in half, and so on. In general, levels are stated in transmit-receive (Tx-Rx) order. Therefore, an audio (VF) "drop" TLP of 0/0 would mean a Tx TLP of 0dbm, Rx TLP of 0dbm. Absolute levels are specific-measured (operating) levels, not to be confused with TLPs.

Sometimes operating levels are not at TLP. In this case a level would be so many db "down" from TLP, or just called "xx down". For example, CTCSS tones normally are 18 db down. (1/8 deviation from voice, or 18 db down from max-voice). To avoid technician confusion two sets of numbers are sometime used in diagrams and on the physical equipment's ports or I/O connections. Figures in parenthesis are the TLPs. Non-parenthesis figures are (absolute/actual) operating levels, and as mentioned before, may be at different levels from the TLPs.

Levels below 0 dbm are negative, while above are positive. Take this into consideration when working with system gains or losses. Normally the negative levels have a minus in front of the number, while positive have a plus sign. This is also true for absolute levels (as opposed to TLPs). For example, most transmitters run a +42 dbm while most receiver's sensitivity run a -117 dbm for 20 db quieting, as in the case of the radio being modified for this project. These levels are at the transmit and receiver ports, respectively. Also known as "TOR" (Top Of Radio), AKA, Top of Rack is before the transmission line and antenna outside on the tower. The latter parts can be figured in for the entire system's losses or gains.

Single digit numbers of "1" and "0" in parenthesis are not to be confused with TLPs. In this case these 1s and 0s identify the logic state of a gate, or other TTL/CMOS I/O driver circuit, and so forth. Another aid to avoid confusion between logic states and a TLP is that the latter normally would have a " + " or " - " before the number. For example a TLP of -14.8 is the audio input controlled by a logic gate of [1], being a normal logic "high". One last word on the logic state. The parenthesis indicates a state in normal standby/no activity condition.

Other definitions, acronyms and other "shortcuts" are for practical reading. For example, in the parts list several manufacturers are listed. To save space the name is truncated. For example "Mouser" is for Mouser Electronics, a major parts supplier.

The project

The Motorola Mitrek UHF radio makes a nice repeater. The radio modifications make a rack mounted full duplex (4 wire) link radio. If using the Canadian version (from C.W. Wolf Comm.), which comes with the higher clearance top cover, so you can use this area for addition control boards, such as the cor/AF or (future designed) 4wire link board, designed by AK2O. Either board has its own documentation, as a separate project, however, the former will be mentioned several times in this documentation, known as "cor board". Final design called for the normal "flat" cover, however. The receiver is like the Micor, in frequency response, making it rather flat audio at the discriminator area for "Amateur" use. SRG specifications call for something better. The top end response can be extended to meet this requirement. The Mitrek "plus" version adds more IF filtering, thus, more selectivity. The Mitrek doesn't have the Micor silent squelch. If you wish to get that quick squelch, and cor drop out time, similar to the Micor, you will need to change some of the squelch time constant capacitors. Also, there are three cor points, and depending on what cor point you use, will determine how many caps, you will need to change out. Some of the OEM wiring points, are changed per Author's specifications. When studying the OEM drawings keep this in mind.

The radio will duplex without any desense to itself and if necessary, will work with only a band-reject duplexer. With the optional preamp this will still be true in most cases. The PA will have to run reduced power. Most applications use the T34 or T44 JJA and running the PA down 3 db from spec, say, around +42 dbm. (That's about 18w @ 50 ohms, for math challenged people). You could push it more with a fan, say, with a thermostat mounted on the sink. Use a 12 volt fan for safety sake. You should have the top, bottom, and PA covers normally installed, except for testing and aligning. SRG's Westlink repeater uses the T34, while stand-alone repeaters use the T44 or T54 power option. The transmitter uses channel elements which have a direct F.M. input (DPL input) already, so you don't have to modify the radio for F.M. When changing frequencies it's recommended to send in the entire channel element instead of just the crystal change. That way the vendor can check compensation and warranty the device. As of 2009 only one vendors is known which is International Crystal Manufacturing Company. The search continues for a vender with good customer service. All these subjects, plus more, are discussed later in detail to provide you with the information to make the radio into a repeater or link. If you want a flat audio repeater or link this is a good one to use.

  • Flat audio

    Long haul RF links are made of several transmitted and received signals. Each time this occurs some reduction in signal quality happens. Stock two-way radios are designed for single path operation, with it's own pre-emphasis, deviation limiting (clipping) and receiver de-emphasis, and "forgiving" squelch operation. For multiple links, these stock radios can add gross problems, such as excessive distortion, audio frequency response being very poor and very long squelch bursts. All these conditions will cause a system to operate badly and be rather annoying and fatiguing to listen to. These conditions can be corrected, and are done so in most of SRG projects. Modification details are discussed further into this document. Standards are important and more information on this subject can be found by clicking here.

    For the transmitter section the mic input is not used for the repeat audio. Instead, the (flat) DPL (channel element) input is used. Each time you limit deviation for each hop will add more distortion. To correct this condition, only limit the modulation at one point, such as the system's controller. This is why the links should not be limited, rather passively 1:1. If you do have to limit, one option would be to set the system limit at 6 KHz and let the system user's transmitters limit at 5 KHz deviation. Passive mode requires system management and user responsibility. This may require some enforcement on the owner's part. There are ways to "punish" or filter over modulated users, however, is beyond the scope of this documentation.

    For the receiver section the speaker output is not used for the repeat audio. Instead, the discriminator output is used. All receiver's discriminators should have great low end response, however, (due to IF filtering restraints) the top end always rolls off too soon. There is also the impedance-loading and level issues to deal with in some receivers. The cor board, mentioned before, has circuitry to address these issues. Documentation can be located on this web site about that board.

  • Squelch Operation

    For squelch modifications, some theory is needed to be discussed. FM receivers have large IF gain. At the discriminator there is plenty of noise available during signal absence. This noise can be filtered at the top end (i.e.,8-10 KHz), amplified, rectified and DC amplified to usable DC levels. This is known as a noise operated squelch, used on every 2-way radio, and "scanner" today. A signal into the receiver that is stronger than the noise will "quite" the discriminator audio output, which changes the DC levels in the squelch circuit and turns on the audio amplifier to drive the local speaker for listening. A squelch circuit can also be used to key an associated transmitter, thus, making a repeater.

    A slight confusion issue:

    We just discussed a noise operated squelch. This is also known as a "carrier operated squelch", however, with a twist. The term "COR" is commonly used for repeater projects. In the old days, it came from "COR" ( Carrier Operated Relay ) in the case of tube repeaters, used a high voltage relay in series of the tube's squelch circuit. When the cor would turn on (conduct) current through the relay's coil activated it. The (dry) contacts of the relay were used to key other devices, such as a transmitter. Later on in the solid state world the term sometimes got changed to COS, whereas, a receiver's Carrier Operated Squelch was used for the same purpose. Another reason this is recognized and discussed here, that some FM systems use a sub audible squelch system, better known as CTCSS ( Contiunous Tone Coded Squelch System ). A carrier operated squelch can work together with a CTCSS to make either an "AND" or "OR" squelch. More can be read on this subject by clicking here . Companies produce repeater controllers. Depending on what company used either acronym. Other than possible polarity differences COR and COS mean the same thing. For this document we'll stick with the acronym COR in either case.

    Stock radio receivers have squelch constants (time for squelch to close and mute the audio path) designed for both fixed (base station) and mobile (moving station) signals, therefore, are a fairly long (200 msec.) time for squelch closure. This is noticed by a burst of loud noise at the end of a received transmission. For a single site this is tolerable, however, for multiple links (hops) this can quickly add up to something annoying to listen to. It also slows down switching paths, causing user frustration. For links this problem can be corrected by lowering the R/C constants in the squelch circuits. To shorten the squelch burst the capacitors in several areas of the squelch rectifier area can be reduced. If they are too low the circuits will be unstable, therefore, require some careful selection which is discussed later.

    Links are not intended to receive mobile (moving) signals. Therefore, this squelch modification will be transparent to fixed (links) station use, which should be full quieting, strong signals. Only multiple "clicks" would be heard with this modification. The remote user receivers will still have "stock" squelch constants, therefore, will provide for moving (mobile) signal changes, plus "cover up" the multiple link "clicks". The result will sound like a simple, small, single site system.

    That covers the basic theory of system design of this type. Now, on to the radio construction, starting with.....

  • Mechanical modifications

    The radio is to be mounted horizontally, on a #2 (3 1/2") 19" rack panel with several #10 screws into the radio's right side. It's offset for panel space for local controls. This position was chosen to provide easy access to the top and bottom of the radio while on the rack or (temporarily) pulled for maintenance. The front panel will need to be drilled out with several holes. More information on this subject can be found by clicking here.

    The old mobile mounting plate and accessory group are discarded. The inner bottom (dust) cover and top cover are still used. The radios's old front now becomes the unit's "left" side and the radio's old left side becomes the unit's "rear". The antenna connectors are now on the unit's "left" side to allow close (rear) clearance in small cabinets. There's an interface board inside the radio (for audio and PTT functions) which is removed. More on that later. Additionally, (external) I/O functions run through the stock control cable connector (P1) at the front of the radio, then to TB1, a terminal strip, on the panel which provides spade lug type connections. The screws will accept a #2 phillips or straight blade screw driver. It's designed to hold a # 8 spade lugs, although, a #6 will work if that's all you have on hand. To mount TB1 you will need to drill and tap 2 holes. Suggestion tap size is 8-32. You will should also put some glue on the backside of TB1. Most of the maintenance components such as local speaker, "S" meter and local mic are on the panel. The local volume and squelch controls are either on the panel or inside the radio. The latter arrangement discourages "sticky fingers" (unauthorized persons) at the site playing around with the equipment that's not locked. This makes up a nice compact, self-contained unit. All you add is DC power and some R.F. connections.

    The SRG version ("A") has a handy feature of a panel mounted AGC meter (Non technical Amateurs would call it an "S meter"). After plotting an AGC curve on the finished product, the RSL ( Received Signal Level ) can be determined at the station. It's also useful for tuning the front end, checking path or antenna alignment, RFI searching or even tuning the Rx side of a band pass cavity. This meter takes the place of a test set, using the "M-1" function, plus can be calibrated in a more meaningful scale, logarithmically speaking, and provide a 0-to-full scale reading. Since the radio's to be mounted on a 2U rack (3 1/2") the meter needs to be small, and more importantly, have a small hole required for mounting to keep the structural integrity of the panel itself. More information on this subject can be found by clicking here.

    Several radios were modified (at one time) for a more efficient "production" type operation, since there were several plans for the radios, to serve different proposes. Therefore, some of the pictures will show many of the same parts being worked on. Remember that some of the pictures may not pertain to certain options. Several versions have been built, for example a 2-channel scanning repeater for the Westlink repeater, a stand-alone repeater for the "Wenatchee HUB" and transceiver operation for the VHF club's packet stations/nodes. This next section is for duplex mode, or repeater operation. (for simplex-transceiver mode, skip ahead to that section).

    The RF I/O connections

    This section discusses the coaxial RF connections for the radio for duplex operation. If you are configuring this radio for simplex operation some of this section won't apply. If you need an overall view go forward to the section of "Configurations" for clarification. Then return to this section for relevant information. For the radio to properly duplex you need separate Tx and Rx RF connectors for the coax runs to the duplexer (or two antennas). Both connections go out the "side" of the newly arranged unit. The first major modification is the mechanical/chassis. For DUPLEX radio option, you need to remove the T-R relay, 2135 core/tumbler and handle parts. These and the mobile mounting plate are discarded.

    The next challenge is to provide for a proper mounting area for both RF connections (Tx & Rx). Since the chassis is aluminum, it's practical to use a reciprocating saw to cut away certain portions, to allow proper surfaces to be fabricated for proper mounting of connectors. You can perform the cutting with or without the radio electronics mounted to the chassis. It's recommended the latter to prevent metal contamination. First, unscrew all the main board screws, unsolder the wires at the feed through caps in the rear, and lift out the main board and RF front end chassis as shown in the left picture. There may be some miscellaneous straps to unsolder as well. By clamping the radio (using the rear PA heat sink area) in a vice you can perform this criticle task.

    If you choose to leave the board in, and take very special care, you can run the blade between the chassis and board. You need to cover the main board with something such as 1" foam to protect it from the aluminum "dust". Cut the one side, over to the far edge, then stop. The pictures show which way the cut was made, by observing the surfaces where the metal was. Any slight debris can be blown away with an air nozzle. In early (prototype) versions the cutting was done this way. The next picture points out the areas of this task, cutting it and afterwards, with the board in place.

    Another difficult area is cutting the front of the (aluminum) chassis straight, to eliminate the sloping front, which is a bad angle for the (Rx) BNC port to mount, with the nut on the outside of the chassis. More information on this subject can be found by clicking here.

    After you get the proper and flat mounting "front" for RF connectors, select your type of connectors for the transmitter and receiver ports. By using different connector types it's improbable to connect the coax cables backwards, thus preventing radio damage.

    Some pictures of the power amp (PA) section. On the right is the exciter tuning coils, L9 through L12.

    Some detail on the output area and where you need to unsolder the wiring going the filter, mounted on the bottom of the chassis.

    This is with the completed antenna connectors (ports) installed.

  • Transmit Port

    For the development of this project the Tx (transmit) port a type N connector was selected. This was not an easy task. After considerable research and trial and error either the bulk head Fem-Fem (UG-30/U) or the Fem-coax termination was considered. The final selection was an N chassis type. More information on this subject can be found by clicking here which covers sources, part number and lot of details on installation.

  • Receive Port

    For the development of this project the Rx (receiver) port a BNC type was selected (chassis mount). The selected type will need its mount modified to seat into the inside surface of the chassis. File down the edges and round the bottom half of the connector, then mount and tighten with the supplied washer and nut. More information on this subject can be found by clicking here which covers sources, part number and lot of details on installation.

    The interconnect board can be intermittent at times, mainly from the pins not making contact. To increase reliability it was discarded, but the P1 (control cable connector) was re-used, because of the nice feed-thru caps for RFI filtering. Connections from the main board to P1 were made with new wires, color coded per the spread sheet list. Also, because of this discarded board, there will be some other components to replace, which are discussed, later, under "Radio Mods". First, P1 needs to be removed. It's real tough to get out, so by removing the big diode across the PA leads, then sucking out most of the solder for all 19 pins. A torch could be used, by "hitting" all the pins at once and working the connector out, unharmed.

    One can't say the same for the board, but it's to be discarded.

    The last parts to be saved are the speaker output caps. If in doubt of their age you may consider installing new capacitors. The first radio built for version A used radial caps, however, axil were ordered for future radios. Both have advantages. With all the stock lined up the Author's ready to assemble the first parts of the newly modified radio..........

    Here's what the empty eyelets on the main board (J10) look like.

    Here's with most of the panel controls installed. Version B is shown in this picture.

  • Wiring

    For either mode you will need to install some "lost" parts from the interconnect board being removed. C1, C2 and R4, are for the speaker output circuit. (R4 goes across the caps). The best place the author found to mount them is glued on the inside chassis, just behind the escutcheon. Also, C3 is for DC blocking of the detected audio for the volume and squelch pots. The negative lead goes towards the pots. The final version has C3 soldered to the squelch pot tab, feeding both pots. (for note: the later cor board version has its own blocking cap.)

    The P1 wiring harness is made up separately, then installed in the radio for further hook-up. The pictures show the P1 and some of the wiring installed. A good way to do this is hold the P1 in a "jig" such as a little vice and solder all the colored wires on at one time. You will have plenty of them going to the right, towards the middle of the radio. Installing some clear heat shrink around the bundle keeps it manageable, while still being able to trace wires, should the need arise.

  • P1 Power leads

    For clarification, the red and black wires, power and grounds, pins 19 and 17,respectively, are discussed here; For the red leads, will be a total of four; three going to the main board's J10 and one for the cor board. (not to be confused with the large red lead for the PA, on pin 18, discussed later). For the black leads, will be total of seven; three going to the main board's J10 and one for the cor board. One short jumper to the P1 ground ring and one jumper to the chassis (with a soldered ring) plus one more for the PA's "A-". (not to be confused with an additional black wire for the mike "low" which goes to P1, pin 2). Even though these runs are fairly short with little potential differences the Author decided to follow OEM wiring as much as practical. Take all of this into account when applying the heat shrink to the bundle.

    The left shows where the new speaker coupling caps can be installed, in this case, radial leads were used. Right shows the overall view of the new wiring.

    The prototype cor board was built with the tan color type with no silk screening. A silver felt pen marked the holes for easy location for the wiring. The right picture identifies the red LED location.

    A local speaker is real handy and having it part of the one-piece unit is even more convenient. Some surplus (new) front mount Radius type speaker housings were found at Hosfelt electronics . With drilling a couple of 1/8" holes and mounting it with some 4-40 screws and standoffs, makes a pretty nice local speaker.

    Production of the local speaker assemblies.........

    After the speaker assemblies were installed it was decided a good way for part connections would be to mount the tie points and other parts, such as the load resistor inside the speaker housing. The load resistor is a 4.7 ohm, 2 or 3 watt value. The left showing wiring and the right the complete and mounted speaker housing good better wire management.

    Overall view of the nearly complete wiring inside the prototype radio. The panel wiring is yet to be done. The right image is from a radio near completion. It's enlargeable as well.

    Another problematic area was the heat sink thermo switch which tells the cooling fan to activate when needed. A quick way to disconnect the wires going to the fan unit was needed for radio servicing. The original way had a couple of "spade" type female slip-ons crimped/soldered to the fan wires as shown in the left image. While moving, packing or transporting this left the issue of this device catching on a seat, clothing, etc and pulling off. The right shows an improved version by shortening the original tabs and soldering the quick disconnect pins to them. However, this still presented some problem of catching and pulling on outside objects.

    Another improvement is to use a "pigtail" for the disconnect. That gives it some flexibility should it catch on outside objects. Later, it was decided to install the molex "shell" so the pins stay together. Doing this also improves the reliability, so that one wire does not come loose by itself and unplug. The fan wires were now the mating end. It was undecided which connector sex to use. This is reasonably secure and still can be quickly unplugged. The lower image is a small production for the switches being "gooped" up with heat sink compound. Finally it was decided to use what's been around for about 100 years, which is a terminal barrier block, such as the front one. Therefore, a 2-position 140 series block was mounted on the (new) back side of the unit. Since the thermostat is mounted about 7mm back from the edge, Mount the block a little further back. Drill two pilot holes, starting with 28mm OC from the edge for the left mount and about 57mm OC for the right mount of the block. Arrange these holes so the block is centered, vertically. Once you've verified the holes are centered, Use a 7/64" drill to enlarge them, then tap for a 8-32 x 1/2" machine screw. Epoxy glue was also added behind the block. This now is called "TB2". Simple spade lugs can be used to make the wire connection to the FCU. (Fan Cooling Unit).

    Radio Modifications

    Modifications made inside the radio are documented on a copy of the transmitter and receiver schematic diagram, usually penciled in. This is a good time to discuss some of the functions of transceiver switching. In order for the receiver to be protected during transmit, the receiver is disabled, or, "turned-off" during transmit. This can be accomplished a couple ways. For the Mitrek, both the receiver crystal (entire channel element) and the speaker amplifier are turned off during transmit. Most of the other receiver circuits are left on during transmit. The transmitter is "turned-on" by turning on Q701 in the early stages, (along with pin 2 of the transmit channel element) plus a few other power control circuits. The transmitter P.A. is "hot" all the time. Since the P.A. is a class-C device there's no power out during receive. It's important for the receiver to "recover" (turn back on) as quickly as possible. This is usually controlled with values of capacitors on these "control lines". No modifications are recommended at this point; these functions are mentioned in the event your version B has a receiver "recover" problem, such as sometimes noticed with high speed (9600 bps) packet operation.

    Transmit section

    Flat audio

    The Tx AF TLP was based on the channel element's "IDC" set at maximum (which no longer functions as a deviation limiter). As previously discussed, the Tx audio input TLP can be either set up for that or 0 dbm as well. Otherwise, if you choose to leave the Tx TLP a little higher (ie. +5 dbm) thus, producing some headroom for minor level adjustment (using the IDC pot) this will allow for little differences in crystal characteristics.

    The stock PL buss connects all the CE's pin-4 to modulate the transmitter. Therefore, the channel element positions will be isolated, since they will be used for various functions. The procedure for isolation is covered further into this document, under the "Configurations" and "Transceive, Duplex Repeater, Duplex Link, and scanning Link" sections. This section covers the transmit audio input which uses the F4 line.

    The transmit audio needs to be flat in frequency response, by using the PL input. Only the F4 position is used for this, via J10-21. To do this remove CR604 and install a jumper in its place. Solder a 100uf/25v coupling capacitor across pins 4 and 5 of the F4 Tx position, with the positive lead on pin 4. This is to block the DC on the line from the outside, while maintaining the good low end response. Since the associated diode CR604 was permanently removed, this new cap will be called "C604".

    If you are building out version "B" for packet this section will apply.

    For version "B" C604 should be in the 4.7 uf area or less, because of data waveform's eye pattern gets distorted with high coupling. More information on this subject can be found by clicking here.

    Otherwise, the following applies to either version:

    You can lower the Tx AF input TLP to about 0 dbm for the UHF radio. Some older TNCs, such as the MFJ-1270C, do not have enough drive level, plus, they get loaded down too easily, (higher impedance). To accomplish this, change R513 from 200 ohm to 10K ohm in the UHF radio. It's located between Tx CE2 and CE3 positions. To avoid confusion the VHF version stock is a 560 ohm, stock. In the UHF stock version it's a 200 ohm. Next, change R515 from 360 ohm to 6.8K ohm. It's located near Q503 and Q504. Another twist, in the VHF radio, it's a "L515" choke. Remove it and install a resistor in it's place. We will now call it "R515". In the UHF radio it's already called R515 , so just change the value. The next paragraph talks about the TLP for the UHF radio.

    This does not increase the sensitivity of the modulator, in fact, does the opposite, however, this is not the point. The point is, by changing some resistors on the output section raises the impedance, thus, reducing loading to the external device (TNC, link source, etc.) therefore, effectively lowering the Tx AF TLP. The channel element's deviation pot adjustment could be left at maximum. With a 0 dbm input tone should give you about 6.5 KHz deviation. A much better way is to set the Tx AF input for a standard, such a TLP of 0 dbm. This will give you enough headroom for crystal variances to run it at 5 KHz deviation.

    NOTE: For the VHF radio has one-third less multiplication, resulting in one-third the deviation at the operating frequency. The modulator needs three times the amount of deviation to make the operating frequency of 5 KHz. Therefore, the TLP will be 3 times higher; or about 9.5 dbm higher. Take this into consideration when using the cor board or other controller-line driver for your repeater or link.

    This will also lower the sensitivity for the local mic audio, however, has low impact, since the local mic is used only for testing. The only exception to this would be in the case of using the mic input for TNC input. If this is the case, make the "R515" (the old L515) a 1K ohm, plus, change out C503, C504 from .047uf to .22uf. Also, change R501 from 560 ohms to 4.7K. These three parts changes will allow (weak) TNCs to modulate the transmitter, sufficiently for packet operation, say, around 3 KHz deviation, bringing the Mic TLP in the -30 range. This will also raise the TLP back up for the flat Tx AF in, but this is only normally used for 9600 bps operation. Since most VHF packet is 1200 and pre-emphasized, using the mic input has priority over the flat AF input path. The latter is normally only used for higher speed operation, such as 9600 bps on UHF. Obviously, if you need to use 1200 bps/pre-emphasized on UHF, then set it up the transmitter modulation changes like the VHF radio, as just described.

    The point is, prioritize which audio input you need to use and modify it, if you need more level sensitivity. Yes, you could add an IC amplifier for better control of the TLPs, however, the Author chooses to keep it "simple" by working with the OEM circuits and (slightly) modifying them.

    Another note: From a packet radio site , (TAPR.ORG) recommends: for some RFI protection on the 9.6v line; install a .1 uf disc cap. on Tx #4 channel element pins 1 and 3.

    If you are building out version "A" these changes are not required, however, are recommended. Typical frequency response was plotted to find this result.

    Leaky Transmit circuit

    As you probably found out, many manufacturers of two-way radios sometime do strange things to make a circuit work. Motorola is no exception. The Mitrek power control and receiver netting circuits are strange and poor in design. We'll first cover the latter circuit which was built in the early 1978 radios, but left out in the latter, 1981 radios. Two service manuals numbers reflect this:

  • # 68P81037E75-A of the 1978 period used the transmitter netting circuit. This consisted of P905, E5, and CR903 .

  • # 68P81045E75-A of the 1981 period, omitted this circuit.

    For the early radios: The idea was to short P905 which put "9.5" to the "Tx SW 9.5 line via CR903. The transmitter oscillator would turn on so you could net it to the receiver. The receiver had its own rudimentary netting function, where you would short "P4" to turn on a mulivibrator circuit to give "M4" of the receiver. With the two netting circuits you could net the transmit frequency to the receiver's. This, of course, only worked if the receiver was already properly netting and was on the same frequency as the transmitter's. For this (duplex) project this method is useless.

    More importantly, the early version's circuit had a potential problem. This is especially important since most of the SRG's link radios are the early version. Let's review the (stock) OEM arrangement with the radio and interconnect board for the early version:

    As its understood, part of the power control circuit, U901, was apparently designed to "see" 12v power (voltage) at the PA, during receive and transmit modes. The normal path for the "big red lead" ("A+" 12v+) for the P.A. is from pin 19 (OEM) of J1, then to a red lead in the radio (next to the chassis) that goes through C884 in the PA section. This circuit, however, is a secondary path. At the J1 connector, pin 19 (OEM) also runs through the interconnect board. "A+" is applied to pin 17 (OEM), of J10 on the main board. This runs to the power control circuitry, through L901, JU905 and the surrounding components of Q902. With the "A+" applied, Q902 barely has enough bias to keep it turned off.

    In the event the big read lead fuse is blown, the transmitter may be active (very low power level). This happens from Q902 leaking some voltage, going through (believed to be) CR902, and CR903 , which turns on the netting circuit, as previously described. While this condition may not damage the front end of the receiver, the condition will be that the receiver will be "hearing" a local signal all the time. This is obvious in simplex operation and was observed on the bench with one of the radios. This condition could exist until discovered at the remote site. Of course, if the radio was set up for repeater (duplex) or cross frequency mode this problem would not be so easy to find. Even though the interconnect board is to be removed (for this project) and certain runs and connections are bypassed or otherwise modified (pin 19 is now not PA power, etc.) this condition still can exist, therefore, you need to be aware of this. For further clarification of this circuitry on the UHF radio, a copy of the OEM drawings are available:

  • Early version Schematic; netting circuit and CR 903
  • Early version Board layout; netting circuit and CR 903
  • Late version Schematic; no netting or CR
  • Late version Board layout; no netting or CR

    P.A. power switch on the front panel

    As you know, one of the modifications in this document is to have a separate PA power switch on the front panel. This is very handy for testing, transmitting without any power going out to the antenna for netting or other testing, however, complicates the (previously mentioned) condition, thus, when this switch is off is the same condition as the (previously mentioned) red lead fuse blown condition. Another note is an led on the front panel indicates PA power. The PA red lead powers this led. When the PA power switch is off there is some leakage from the power control circuit presumedly in the Q901, Q902 area. This was verified by temporarily lifting JU906. This condition does not affect the radio performance. You only need to know this to avoid a false indication because the led will glow dimly when the switch is off. In fact, the dim led might indicate a "normal" condition for the power control circuit when the switch is off.

    Note this project now uses pin 18 for the PA's "A+" as apposed to the "stock" discussion (above) concerning pin 19.

    After considerable research, another modification can be performed to correct this threat. Since the receiver channel element frequency (netting) can be accomplished by sweeping the front end with a signal generator. The "M4" function can be disabled, which is a preferred method over the (almost useless) "M4" netting function. Therefore, CR903 can be removed, so no voltage gets to the Tx sw 9.5 line during receive mode, with the PA power turned off (or fuse blown). There's a partial picture to view the area near the end of this document. One more point on this; the VHF version does not seem to use this CR903; at least the version of radios and manuals available to the Author at time of this printing. If this is the case, then the VHF radios don't have the problem; only the UHF early versions.

    Another area of research is the power control circuit. Currently research continues, however a few things have been found by the Author. Remember, by modifying this section is at your own risk of damaging the radio. It's for understanding only, at this point.

    The power control consist of several parts on the main board, plus some on the PA deck. The former uses U901 and some transistors. As discussed earlier should the PA power be off (blown fuse or the panel switch) off some leakage came from CR903, which was one of the modifications to remove it. However, another source of leakage comes from Q902. If you wanted to stop that source you can either lift the green sense wire, or take out JU906. Another thought to eliminate R909 you can remove JU905 and just control the power out with R911 (blue pot). Another note is the 91K resister from the Tx 9.5v to the junction of R910 and top of R911. Without this resistor the power would be wide open and run too hot. The U901's input on pin 2 goes lower to increase the power out. Input going lower cause the output on pin 5 to increase and further conduct Q903, which increases conduction of Q904 to increase the control voltage to tripler, Q704 and the PA control voltage input as well. As time permits further explanation can be posted on this page.

    A+ for the PA (big red lead)

    While we are discussing the transmitter and its PA section, some words about heat loss is appropriate. You may have read in other documents on this site about lowering the PA (A+) supply voltage from the usual (stock) level of 15 volts to 10. This works fine with the Micor, however with this model higher PA (A+) is required. For rule of thumb the minimum should be 12.0 volts to make rated power out. Anything lower will cause unstable operation with the power control circuity. Higher voltage promotes stability, however at the expense of heat loss. Suggestive A+ for the PA (only) to set the supply at 13.0 volts, for this model of radio.

    Sometimes Q703 runs a little to hot. To correct this install a 47 ohm, 2 watt resistor in series of the junction of L724 and L723. This now will be called R723. Also, R706 needs to be installed, if not already (normally for the low range radios) in. Now, retune the transmitter, especially the affected circuits, which includes L705, L706 and exciter filter L9~L12. If you can; check for a clean signal on a spectrum analyzer. Then set the power out for repeater operation. Recommended is a maximum of +44 dbm at the Tx ("N") port using a fan for cooling. For a better location of R723 click on the image to enlarge it. Note it's on the "cold" (DC) side of the RF filter, L723. When you do this you can also see just below where CR903 has been removed. (that was the leaky PA circuit described earlier)

    In noting the transmitter efficiency does not seem to be affected by this modification. Being rather low the typical figures are:
    at 13.1 v DC supply it should draw 6 amps. That's 78.6 watts of power on the DC side. For the AC side, AKA RF, +44 dbm converts to 25.119 watts, at 50 ohms. That works out a 31.958% or just 32%, efficiency.

    Frequency stability

    As previously discussed, it's recommended to send in the entire channel element for the transmitter (and receiver) for compensation on your frequency, instead of just replacing the (H18) crystal. The Author currently uses International Crystal Company since 1975. They accept credit cards and possibly even personal checks. MO's of course.

    In the event you don't have any elements (or crystal) to check or tune the radio there's an alternative using a signal generator as a local oscillator. Click here for details.

    Being a mobile frequency stability should be good enough for most repeater projects. You may note that new crystal will drift around during the first part of the aging process. Receiver elements run all the time; the transmitter does not during standby. Therefore, it will take much longer for the Tx frequency to settle down. An option is to run the Tx element all the time as well. This will require modifying the 9.5v lines for the early stages of the transmitter section. The images above show some of the modifications, on both sides of the main board. For more information click here .

    Receiver section

    The OEM Rx audio output TLP is spec at about a +2.5 dbm. This is at the "detected audio output" from pin 9 of J10. For amateur standards this point is fairly flat. Further improvement for frequency response can be provide by using the (separate) cor board designed by the Author. If you wanted to standardize Tx and Rx levels, such as 0/0, you could install a simple pad on this output, before it gets to the external equipment. A good place to perform this might be on the inside of the I/O connector, J1 pins.

    "S" meter

    The F3 CE position is used for the M1/AGC meter function. The M1 function is picked up from the junction of R222 and C233, then processed externally with the cor board's built-in limiter DC amplifier, then goes back out through J1 to dive an external meter (panel mounted) to indicate the receiver's limiter. To accomplish this jumper J1001, pin 1 to a run going to J10, pin 20. There's a handy eyelet for this modification. Details on this circuit can be found on the cor board documentation found on this web site. Even though some other board versions will work with this radio, 6.3 is the intended one to use.

    COR points

    The wires are tucked away for later, for when the COR audio board is to be installed.

    As you probably know, when a signal enters a FM receiver, it quiets the receiver, which activates the noise operated squelch. This squelch has several circuits to handle this condition, which also provide several voltage points that changes DC level. There is a choice of using one of the three cor points, "L", "E" OR "H", all which are controlled by the panel squelch pot, of the point at which the local speaker and repeater squelch gate opens. (In the Micor repeater the squelch gate has it's own noise amp and switch for independent opening point). One of these points can drive a high impedance DC buffer/amplifier. The cor board has a DC comparator to perform this function. In order for this buffer to sense carrier activity, a reference voltage (bias) need to be adjusted on a one-time basis, depending on which squelch point is used. cor points of L, E and H, each have their own characteristics. Earlier mentioned was the cor board and the versions, depending on what configuration you are doing. Refer to the cor board documentation about polarity of the cor input buffer. Its found on this site.

    "L" is a negative going active point (less positive). Being a DC "analog" point, it sits about 1.8 volts positive with the squelch closed, but near the threshold. As a signal quiets the receiver, this point goes less positive, to .04 volts with a full quieting signal. (Never goes negative). This point is DC analog, therefore, you have a "quieting" choice where the cor will change logic state on the control board. This might be handy to set the cor and local speaker activity points differently. This arrangement is similar to the "repeater squelch gate" used in the Micor station repeater. If using this point, set cor board bias at un-squelch, at desired level of quieting, but less than the cor standby voltage, but more than the active low voltage. It's at the junction of R410 and R411 and the base of Q405.

    "E" is a negative going active point (less positive). Being an almost completely logical point, it sits about 2.8 volts squelched and 0.16 un squelched. It's at the squelch switch and used for the stock consolette interface board's carrier indicator. The advantage is time and "stock" proven for reliability. If using this point, set the bias for a + .924 volts. Point "E" is at the junction of R430 and C418 and at collector of Q406

    "H" is a "low" in standby (squelch closed) and goes positive on squelch open. Being a logical point, it sits about zero squelched and 6 volts unsquelched. Point "E" drives the input of U401 which is acting as a DC comparator to switch on the audio. Point "H" is pin 4 of U401 which is one side of the balanced audio output to drive the local speaker. The advantage is this active high point will drive any cor/circuits you might already have in mind, and is simple to set up. The disadvantage being audio is riding with the cor voltage, so if you crank up the local volume too high, the cor/PTT function will drop out erratically. If using this point set the bias around 4 volts (lower than the cor active voltage).

    For (single) conventional repeater skip this section and leave the squelch constant caps "stock". For links, as previously mentioned, the squelch time constant can be shorten (5-10 times as less) to get away from (linked) additive long bursts. Depending on which cor point is used, will determine how many caps are needed to change to a lower value. You have the option of performing all the cap changes to allow cor pick off changes in the future. All the SRG link radios are intended for long haul links, therefore, most or all the changes are performed. The violet circles around the caps will help you locate them. Most of the new capacitors are yellow or blue, being a tantalum type. The pictures are made larger by clicking on them. For the short squelch constant change the following capacitors:

  • For cor "L" method change C416 from 15uf to 1uf and change C417 from 4.7uf to 2.2uf only if there is no potential of interference or other unstable conditions.

  • For cor "E" method perform the first change, plus, change C418 from 10uf to .68 uf. (typical is .47~.1uf)

  • For cor "H" method perform the first two changes, plus, change C427 from 15uf to 4.7 uf.

    The "E" point is used in all SRG links and repeaters.

    As previously mentioned, the cor board version 6.3 is used for this radio, for interface to the outside world. It mounts, upside down (components down) in place of the stock PL deck, with a slight twist. It does not mount on the interconnect board because it was removed as part of the mods. Also, the stock screws for the Pl may be hard to find, especially if your radio is a carrier squelch model. Most local HW stores will not carry such screws. After considerable research they were identified. At first thought they would be a # 6-40 (not a typo) machine screw with Pozi drive head. Even though the 6-40 may work in the chassis, the manual showed them to be a M3.5 x 0.6 x 12 mm screw. This type was not found. However, an equivalent was found in phillips pad head; about $5 for a box of 100 on Mc Master Carr's site on the Web.

    Other issues

    As you are aware this radio was originally designed as a mobile. That's means a transceiver , running half duplex, AKA, transmit or receive, but not both at the same time. Most of the modifications discussed in this document converts the radio nicely into a repeater and/or link radio able to run in full duplex mode. One minor issue concerning the B+ line should be noted. The receiver's audio output amplifiers, U401 and 402 are capable of driving an external speaker very loudly with several watts of audio power. When the volume control is cranked up this audio will "ride" on the B+ line. If there are other radios or devices sharing the input power will be affected by this audio on the DC line. This is mostly observed as a "noise" on the other devices, while the radio's squelch is left open. Another symptom is "crosstalk" where is the receive audio from the radio is heard on other radio (channels) that share the same DC power source. This can cause additional time troubleshooting if it's a problem on your system. One easy cure is leaving the receiver's volume down low, or off. This is a good (curtesy) practice, anyway, for a station at a site with other tenants there. One small modification is to change the value of R416 from the 10K to 47K. This will reduce the TLP at point "K" 11 1/2 db. The image above (cor points) show the caps, but also this resistor change location. The range of the volume control is too high to begin with so this won't be an operational factor.

    There was a minor inconsistency in the Motorola Manual. It's about the dropping resistors for the volume and squelch. The documentation, here, by the author, is correct for this application and will work fine. More information on this subject can be found by clicking here.

    Using the manual's schematic, remember to align the receiver properly once you obtain the proper (compensated) re-crystaled channel element for the operating frequency you plan to use. A tip for adjusting the receiver; assuming the IF is properly tuned you can "sweep" the receiver to get the channel element on frequency. Inject an on-frequency RF modulated signal with a 1 KHz tone, of 7 KHz of deviation. This will be at the clipping point of the IF. Increase the RF level to the point of no clipping. Typically this will be around a -103 dbm. The "sound" of the tone will mostly clear up from the "raspy" sound at this point. Then "rock" the warping device in the channel element back and forth for the clearest tone.

    Radio Versions


    Made for SRG; The mic connector is a 4-pin, two coupling capacitors to route the local speaker lines to the outside local speaker, and an AGC meter is mounted on the front panel. It's anticipated future versions for SRG will only be built and will be version "A". As side-note earlier versions before 2000 such as the Westlink repeater have different arrangements, such as a separate control head. This version uses the cor board version 5.x or 7.2. This version is a duplex radio.


    Made for I.E.VHF R.A.; A.K.A. the VHF club; The mic connector is an 8-pin, 2 pin jacks and 2 banana jacks are mounted on the front panel for the local speaker monitor for testing. The meter is left out; the pins mount where the meter would be on version A. Also, for packet the T-R relay is left in and operational, because this version is built for packet simplex operation. There were five radios build (serial 1~5) for the VHF Club in this version around 2004 for a very specific use of 2 packet radios for two different sites, plus a fifth one for central control them and a BBS. This type of quality design and attention was a bit over done for the club for their needs. Therefore, it's anticipated these will be the last of this version, although the readers are welcome to produce more. This version is a simplex radio.


    In addition, the old frequency select lines for F3 and F4 will be modified to for new functions of M1 and transmit audio, respectively. However, they will need to be isolated from the matrix. To do this several jumpers will need to be removed, as discussed below. There are about four configurations for this unit depending on the intended purpose. Even though SRG's main configuration is "Link" the other ones may be useful for the reader, therefore, are discussed.

    The versions can be configured different ways. Being that a morse code IDer may be the main component of a repeater that is not addressed with this project, you may want to take that into consideration when choosing the configuration. Otherwise, in some cases, most basic functions can be used, such as timers and controls. Referring to the interconnect diagram has many I/O functions on TB-1. A few of them are dual purpose, depending on which configuration you wish for the unit. Consideration was made not to interfere with the cor board's functions either, nor a TB-1 function conflicting with another configuration. These dual purpose TB-1 connection-functions will depend on which version board you use.

    Transceive, Duplex Repeater, Duplex Link, and Scanning Link

  • For Transceive (SIMPLEX) configuration you will need all the (stock) receiver mute/channel element functions enabled by leaving in CR1, CR2 and CR403 on the main board in. Since the interconnect board will be removed (loosing CR2, etc), you will need to run a jumper from J10-7 and J10-14, so the receiver audio amplifier will mute during transmit. (It was mentioned here so you understand what will be affected by leaving out the board). The OEM circuit used a diode, however, because of the simplicity of the modifications a jumper will be fine. You will be leaving the T-R relay alone as stock. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

    For the cor board, PLI or CON 2 won't be a function in this case. Terminal 17 could be used for a rudimentary CON 1; if so leave JU611 in for single frequency operation. Or terminal 17 could be used for control of F1; if so leave JU611 out for the same reason. Transceiver is used for packet operation. To note: The web site , (TAPR.ORG) recommends: for some RFI protection on the 9.6v line; install a .1 uf disc cap. on Rx #4 channel element pins 1 and 3 for both the Tx & Rx side.

    For Repeater configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

    Use cor board for this configuration for internal control and timing. Also, if you are also using the stock PL deck, you may be using the mode function on terminal 13. More on this later. You also may be using CON 1 and CON 2 on terminals 17 and 18, respectively. If so, leave JU611 for single frequency operation. More on this later.

    For Link configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

    For Scanning Link configuration you will need all receiver circuits operating all the time for duplex operation, therefore, the receiver mute functions need to be disabled. Since the interconnect board will be left out that covers the removal of CR2 on that board, as well. (not to be confused with the second "CR2" described, below). For this configuration do not jumper J10-7 to 14. There's also a receiver channel element off/mute function plus a secondary function of "M4" test. The "M4" circuit is explained on the receiver schematic. It's a poor solution to Rx frequency netting. Also, the mute function goes the way of Q3 and Q1 of the "M4" circuit. Neither will be used and can be disable by leaving out CR1 and CR2 on the main board. One last mute circuit needs to be disable by leaving out CR403 on the main board. The antenna port modifications were covered, earlier. Also, in the PTT circuit, optionally, change R1012 to 1K and install a red led in the holes where the relay wires were. This is handy as a transmit indicator. In the receiver section remove JU606, JU607, JU608, JU609, JU610, CR607 and CR608. In the transmitter section remove JU601, JU602, JU603, JU604, JU605, CR603 and CR604.

    For the cor board, in this case you won't be using CON 1, CON 2 on terminals 17, and 18 respectively. Most likely a link will be carrier squelch. If not, you could use an external controller and/or decoder. In this case terminal 13 might be a PLI for tone squelch. You need to leave JU611 out. An external "scanner" will control the F1, F2 lines. Ideally this controller could be built to do all three; scanning, control/timing and tone decoder in the case of co-channel RFI). The unit would now be a "control station" for two distance "HUB" repeaters in opposite directions. This configuration will have two control pairs, adjacent channel, especially when using a single duplexer. This is an advantage where a site owner charges per radio, when you need to link to other (zero-tail) repeaters together. This is the case with SRGs eastlink repeater.

    More on Terminal 13 and tone control

    Most of the TB-1's connections go to P1, then various point in the radio and cor board. Most of the positions are fixed, dedicated to the radio's functions and I/O. However, a few positions can change assignements on a perminent basis. Previously mention was "robbing" 17 and 18 for other functions, such as CON1 and CON2, respectively by leaving JU611 in. Also, terminal 13 is a triple assignment, however, only one at a time; which are either PLI, HUB or CON (PL input, hang up box or control, respectively). This discussion involves using the COR/audio board version 6.3 of course. Other version don't work the same as 6.3. You need to remember this.

    If you choose to use an earlier version the following rules apply:

  • By default 13 is the PLI. That means using an external tone decoder, it's (DC) output connects to 13, then goes to the internal cor board's PLI. If you wish both audio and PTT 1 (out) to be AND squelch, connect 13 to COR board's PLI point for external decoding and control of the mode (from tone to carrier squelch).

  • If you wish to control the PTT out, 13 could be a control, type "1" or "2" while connected to the COR board's CON1 or CON2, respectively. Or, if you wish to control both Tx and Rx operation by disabling both channel elements leave JU611 out and use the F1 select for this. This would be rudimentary "CON 1" as well, even in the case of a scanning repeater. If not, leave JU611 in for single frequency (pair) operation.

  • In either case CON2 on terminal 18 is not available for a scanning repeater. Another point to remember is if you want use the F1 line for control, once active, will disable the entire radio because it would disable any audio path for controlling it back to enable. To address all these features you could use an external controller and/or decoder with an independent audio path. For example, a multi-port controller is typically used for a 4 wire, 2~4 way matrix/link station. Such a controller either hand-made or bought can address most of these features for the configurations.

    Using an external control for mode change was previously discussed. There is another way, using cor board version 6.3. On-board is a "JU4" berg type jumper to change modes locally (at the site). For more information on this jumper seek the COR/audio board version 6.3 found on this web site There is a "catch" to this jumper and is discussed in detail in that document. Obviously, if you feel qualified, you can deviate from the Author's design and wire up a solution for your particular needs.

    Additional information

    Tip: When you suck out the solder in the holes they make a handy eyelet location tool. (you can find the open hole on either side of the board for referencing other eyelet locations).

    Many of the wire and circuits may appear to be redundant and not needed. Wiring color and functions were selected for most any configuration you may need. Either install them or leave them out. The former is preferable to avoid "tearing" into the harness, management shrink and glue holding the wire. Another possible option is to use the extra wires for another function. This is another reason redundant (black) wires for additional grounds were installed in the original design specs.

    Diagrams and documents to download:

  • Front panel layout Where controls are located.

  • Diagram For the pin assignments on J1 and other notes. (jpg)

  • Diagram Same as above but in pdf format.

  • Wiring chart for above diagram Color assignments, etc.

  • "P1" I/O wire colors for above diagram

  • Modifications to the radio

  • Transmitter frequency option Improve stability

  • Radio alignment Tx and Rx tuning

  • Production examples For version A

  • Production examples For version B

  • Operator's Manual

  • Poor receiver sensitivity From "wiskers"

  • Thick top cover Multiple channel deck

  • The old HUB repeater Wenatchee 1995~2010

  • Using cor board prototype (Ver 6.2)

    Mitrek modification parts list: Version specific radios (A or B) are indicated by colored fields. Optional parts that are grayed (lowlighted) on the schematic diagram are not listed here.

    QYT Description: Values/notes: Part number: Approx cost:
    1 panel, 19" rack # 2 RU PBPA193000B 12.00
    1 Meter, panel analog ~200uA,1.6 x 1.6 x1" MFJ 400-0026A 7.92 + ship
    7 Screw, machine 10-32 1/2" for radio TBD 0.04
    2 Screw, machine 4-40 1" ? TBD 0.04
    2 Screw, machine 4-40 3/4" TBD 0.04
    2 Screw, machine 4-40 1/2" TBD 0.04
    2 Screw, machine 8-32 1/2" For TB1 TBD 0.04
    9 Nut,hex 10-32 . TBD .10
    1 Nut,machine,hex . TBD .02
    1 Lug, ring #8 For ground TBD .02
    6 Lug, spade For I/O connections TBD .02
    2 Standoff, 4-40 Fem 1" Speaker mount .30
    1 Terminal strip 20 position,.TB-1 TBD 3.00
    1 Jack, pin type . TBD .02
    2 Jack banana type Blue TBD .02
    1 Jack, mic Pin-4 . TBD 2.00
    1 LED, T1 3/4 Green, diffused TBD .10
    2 LED, T1 3/4 Red, diffused TBD .10
    1 LED, T1 3/4 Yellow, diffused TBD .10
    1 LED, T1 3/4 Green, blinking TBD .10
    1 LED, T1 3/4 Red, blinking TBD .10
    4 Resistor, 1K ohm 1/4w TBD .02
    1 Resistor, 4.7 ohm 1w TBD .10
    1 Resistor, 220 ohm 1 w TBD .05
    1 Resistor, 6.8K . TBD .02
    1 Resistor, 1/4w 10K TBD .02
    1 Resistor, 41 ohm 2 w, R723 TBD .25
    2 Capacitor, 100uf/25v Electric, radial leads TBD .30
    1 Capacitor, 4.7uf/25v Electric, radial leads TBD .30
    2 Capacitor, .22uf/25v Electric, radial leads TBD .30
    1 Speaker, with hous . TBD 3.00
    2 Pot, 25K, LT 1/8" shaft Vol & Sq TBD 3.00
    2 Knob, round . TBD .30
    3 Switch, DPDT, 10 amp TBD .30
    1 Switch, SPDT 3 amp TBD .30
    1 Switch, thermo NO, closes 120 TBD 4.00
    1 Connector, RF Type N female 523-82-5378-RFX 6.00
    1 Connector, RF BNC female 523-31-318-RFX 6.00
    6" Cable, coaxial RG-415 teflon 50 ohm TBD 6.00
    1 Channel element With Tx crystal KXN1089B/86 35.00
    1 Channel element With Rx crystal KXN1088A 35.00
    1 Board, COR Version 6.3 TBD .02
    24" Wire, hookup 22 AWG various colors 5.00
    4-ish Wire, bare 22 AWG For jumpers .10

  • Unless otherwise specified, resistor values are in ohms 1/4 w, 10%, chokes in milli-Henries, caps in Micro-Farads. Color of wires: Black, red, white, green, yellow, orange, blue, brown, violet, pink, slate.

    This may be copied or printed in complete form only for non-profit purposes, such as for the knowledge for the Amateur Radio Service, with AK2O credited as designer. Other arrangements please contact the author. Most of the developments and corrections were made in June~August of 2004, with updates in Sept. 2005 and October 2008.

    Copy write: AK2O 2008 and current

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