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  Diagnosing, Troubleshooting, and
Repairing Motorola MaxTrac Radios

By Robert W. Meister WA1MIK
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This article pertains to MaxTrac and Radius radios. GM300s and their variations are very similar but there are subtle differences that prevent you from swapping modules between GM300s and MaxTracs. MaraTracs use the same RF boards as MaxTracs but that's about all you can swap; the logic board is similar but the speaker audio section is on another board. Consult the appropriate service manual if in doubt. They're all here on repeater-builder so you have no excuse.

The Building Blocks:

These radios are composed of several major assemblies or modules:

Except for 900 MHz radios, assemblies from one radio can be swapped with assemblies from another radio. Of course, you can't swap RF boards unless they match the band of the Power Amplifier, or vice versa. 900 MHz radio modules are all different and they can't be swapped with non-900 MHz modules. For the purposes of this article, the VHF, UHF, and 800 MHz radios are one group; the 900 MHz radios are a second group. Don't mix and match modules between groups.

The Front Panel / Control Head contains a volume control and mike jack board that will work on all radios in a group; the 900 MHz radio board has Hear-Clear circuitry on it that will only work on the 900 MHz radio. The channel selector / display board is different for the 2-channel and the multi-channel assembly. All radios can be configured to use either one but you have to blank and initialize the radio to select the particular display board. The speaker plugs into the Logic board with its own cable. On GM300 radios, the speaker is wired to the connectors that plug into the front of the Logic board.

The Logic circuit board contains the microprocessor, microphone audio circuit, and receive audio amplifier. Any Logic board can be installed in any radio in a group. A row of 14 pins sticks out from the bottom of the Logic board; a socket on the RF board mates with them. On the GM300 radios, these pins are in a separate assembly that's mounted to the chassis rather than sticking out from the Logic board, and both the RF board and the Logic board have mating sockets.

The RF circuit board contains the receiver and transmitter RF components and the 14.4 MHz reference oscillator. It is frequency-dependent. As long as it matches the frequency range of the Power Amplifier, any RF board can be installed into any radio in a group.

The Power Amplifier is the unit at the rear with the black heat sink. The frequency range of the Power Amplifier must match that of the RF board. The MaxTrac/Radius radios use a 5-conductor cable; the GM300 uses a 6-conductor cable.

The Chassis accepts all RF boards, all Logic boards, and all Power Amplifier modules. On the GM300 radios, a block of interconnect pins that fits between the RF board and the Logic board is mounted to the chassis.

Special Tools:

The most common hand tools you'll need are Torx drivers. You'll need some other common tools to remove the Power Amplifier circuit board. The MaxTrac Service Manual has disassembly instructions.

You need a Torx T-15 driver to remove the two long screws that hold the Front Panel to the chassis.

You need a Torx T-10 driver to remove the four flat-head screws that hold the top and bottom covers to the chassis. This same driver is also used for all the internal screws on the other modules.

You need a Torx T-8 driver to remove the two flat-head screws that anchor the Logic board's heat sink to the side of the chassis on MaxTracs, and hold some parts to the heat sink on GM300s.


Always connect a dummy load to the antenna jack when testing the transmitter.

Always disconnect DC power from the back of the radio before removing or installing any connector or circuit board.


The quickest and easiest way to troubleshoot these radios is to swap entire modules until you isolate the problem to a particular module. Spare radios are inexpensive on ebay. These radios are very robust and don't break often, but when they do, simple troubleshooting procedures will quickly isolate the problem to one board.

Usually, the radio:

It's rare that multiple problems occur on one radio unless the radio has been subject to excessive voltage or the wrong polarity voltage.

Each section of the service manual has some theory of operation and detailed troubleshooting flowcharts that have you check a lot of items, but the eventual solution is to replace the affected circuit board or assembly. For MaxTracs, section 1 covers the Front Panel; section 2 covers the Logic boards; section 3 covers the RF boards; section 4 covers the Power Amplifiers. The GM300 manual isn't organized as neatly.

Common Problems:

The radio won't turn on.

The radio could be configured for Ignition Sense. This means that in addition to the main DC power via the two-pin power connector on the back of the Power Amplifier, you also need to feed +13VDC into the Ignition Sense pin of the Accessory Jack on the back of the radio (pin 10 on the 16-pin connector, pin 5 on the 5-pin connector). An internal fuse (F801) or simple wire jumper on the Logic board normally provides this voltage so you don't need to feed anything into the Ignition Sense pin, but if a previous user has removed the jumper or the fuse has blown, which will happen if you only provide DC power into the radio via the Ignition Sense line and then press the PTT button, the radio will need +13VDC fed on this pin as well. I've replaced the fuse with a 10 ohm 1/4 watt resistor or just a short piece of #22 solid wire.

The on/off switch on the back of the Volume Control could be bad. DC power comes in from the two-pin power connector on the back of the Power Amplifier, and feeds the Logic board via J7 pin 5. It then goes out to the Front Panel at J8 pin 6, through the power switch, and back to the Logic board at J8 pin 5. With the power switch turned on, check each of these points. If you don't have voltage at J7 pin 5, then there's a problem between that point and the DC coming into the radio (very rare). If you don't have voltage at J8 pin 6, an internal trace on the Logic board is open; repair it with some insulated wire. If you've got voltage at J8 pin 6 but not at J8 pin 5, the power switch or its wiring is bad. Bypass the switch with a clip lead from J8 pin 6 to J8 pin 5 and see if the radio powers up. Once you've got power on J8 pin 5, any failures are related to the power supply regulators or the microprocessor.

The volume control can be damaged if the radio is dropped on its front or if something hits the volume knob. This will push the switch back into the radio, usually destroying the volume control action and possibly the switch itself. Replacement is the only way to fix either condition.

If the radio turns on, there should be an audible "beep" heard on the speaker. If you don't hear anything, the jumper on the 16-pin accessory connector might be missing, or there may not be an accessory plug inserted into the connector. You need to connect pin 15 to pin 16 to activate the internal speaker, and of course there must be an internal speaker and cable, which must be plugged into the Logic board. I've seen radios with the cable unplugged or missing entirely. No jumper is required on radios with 5-pin accessory connectors.

Pressing any Front Panel button should also produce an audible "beep". If you press and hold the MON button, the squelch should open and the volume control should vary the level. If someone replaced the Front Panel with the wrong one (i.e. put a 2-channel panel on a multi-channel radio, or vice versa), the radio will partially work but you won't see any LEDs light up. The only fix is to put the right panel on or blank and initialize the radio to work with the existing panel.

If the wrong polarity DC power was applied to the radio, the reverse-polarity protection diode on the Power Amplifier circuit board probably shorted out as a result. The rest of the radio may still work fine. You need to pull the PA circuit board out to fix it. See the hints in the "No Transmit" section below.

The radio won't receive properly.

First make sure there's a speaker connected (and the correct jumpers are installed on the 16-pin accessory plug), then press and hold the MON button; you should hear squelch noise. That verifies that at least the audio section is working. If not, start checking at the Logic board and follow the audio up to the volume control and back. Also check for audio at the back end of the large 1000uF tubular capacitor on the Logic board. If you've got it at the capacitor, that leaves the speaker, its wiring, and a 1 ohm resistor on the Logic board that's in series with the loudspeaker.

Feed a weak signal into the antenna jack. All radios should easily receive a signal of 0.5 microvolt. If it takes 10-50 microvolts, the front-end protection diodes are most likely shorted, or the front-end preamp transistor is bad. Both of these parts are mounted under shields under the RF board.

If considerably more signal is required, the antenna switching circuit in the Power Amplifier could be bad. Feed a signal directly into the RF board at the small coaxial connector on the left side of the board (when viewed with the Front Panel towards you). Good sensitivity here means there's a problem between that coax cable and the antenna jack, usually the antenna switching diodes. Replacement requires removal of the Power Amplifier circuit board. See the hints in the "No Transmit" section below.

If the receiver hears nothing, even a very strong signal, it could be due to a defective second oscillator crystal on the RF board. This crystal runs at one of several frequencies depending on the operating band: 10.245 MHz for VHF-Low, 44.645 MHz for VHF-High, 44.645 MHz for UHF, 44.645 MHz for 800 MHz, and 38.695 for 900 MHz. To quickly verify the oscillator is working, tune to its frequency on a nearby receiver; you should hear a solid signal. If not, the second oscillator crystal is most likely bad. This is a common problem on GM300s.

If you can hear a signal fine with the squelch opened, but not with it closed, this indicates the RX VCO is not locking properly. This could be due to incorrect programming or because someone previously adjusted the VCO tuning for out-of-band operation. Check the "SL" test point on the RF board with a DC voltmeter. It should be between 2 and 7.5 VDC if the VCO is locking. If the radio works on some modes but not others, you should program two modes with the correct band limits for the radio in question and adjust the appropriate VCO coil for 2VDC at the lowest frequency and 7.5VDC at the highest frequency, or as close to those values as possible, then verify that all frequencies are within the legal range for the radio.

If the volume can't be controlled or is always very soft or very loud, it's usually due to a broken volume control, which can be damaged if the radio is dropped on its front or if something hits the knob. This will push the shaft back into the radio, usually destroying the pot and possibly the power switch behind it. Replacement is the only way to fix it.

If the radio seems to take 10-30 seconds for the receive audio to be heard when first turned on, as if a tube is warming up, replace C551 on the logic board. They get electronically leaky and upset the voltages in the receiver's audio stages. Anything from 4.7uF to 33uF at 16 volts will work. Just make sure the new capactor is small enough to fit in the space occupied by the original one. This can affect MaxTracs, Radiuses, GM300s, and MaraTracs.

The radio won't transmit properly.

The most common issue is no or low output power. This is most likely caused by a defective mini-UHF antenna connector: the center contact has expanded and is not making contact with the plug's center pin. The connector must be replaced and you need to remove the Power Amplifier circuit board to do this. The most difficult part is unsoldering the two-pin power connector from the circuit board. The pins are large and the holes have very little clearance. Heat both pins and wiggle the connector while pulling it gently out the back of the heat sink. Clean all the solder out of the holes and off the pins before you put it back. Clean the old heat sink compound off the transistors and heat sink and add new heat sink compound to the transistors when you reassemble it.

Occasionally the antenna switching diodes on the Power Amplifier will go bad, often as a result of a bad antenna connection.

No transmit audio or no transmit activity at all (not even the TX LED lights up) could be caused by a bad microphone, a bad RJ45 mike jack, or bad wiring between the Front Panel and the Logic board. Substitution of any of these assemblies will quickly tell you where the problem is.

If you have no transmit PL/DPL on a GM300 with the enhanced (EPROM) logic board, there's a jumper (JU-701) that selects whether transmit PL/DPL comes from the internal microprocessor or the accessory jack. Make sure it's in the "A" position so the radio uses the internal PL/DPL signal. Note that only this particular radio has this jumper.

If you get a "boop" sound when you try to transmit, this indicates the TX VCO is not locked. This could be due to incorrect programming or because someone previously adjusted the VCO tuning for out-of-band operation. Check the "SL" test point on the RF board with a DC voltmeter. It should be between 2 and 7.5 VDC if the VCO is locking. If the radio works on some modes but not others, you should program two modes with the correct band limits for the radio in question and adjust the appropriate VCO coil for 2VDC at the lowest frequency and 7.5VDC at the highest frequency, or as close to those values as possible, then verify that all frequencies are within the legal range for the radio.

If the radio immediately transmits when turned on, first unplug the microphone. Check the wire contacts of the RJ45 mike jack to make sure they are all straight and parallel. Also check the wiring of the 16-pin accessory jack. There are zener diodes that protect many input signals on the Logic board, such as the PTT line, and if the radio was connected or interfaced to external equipment, higher-than-normal voltage may have been fed into one of these inputs and caused the zener diode to short out. Measure the PTT line at the Front Panel and the Logic board connector; unplug the Front Panel connector to isolate the problem.

I had a GM300 with an odd problem. The internal speaker was bad but I wanted to see if it would transmit. Pressing the desk microphone's PTT button did nothing, but pressing it a second time would cause the radio to transmit properly - sometimes. I could find nothing wrong with the Logic board so I obtained another board and put it in. I eventually replaced the speaker as well. I reprogrammed the radio and it worked as expected. I later discovered that the GM300 has a programming "feature" called "Forced Monitor" that requires the user to put the radio in Monitor mode and listen to the channel before it will allow transmission. If not, a "boop" sound is emitted when PTT is pressed, which I couldn't hear due to the bad speaker. Pressing PTT a second time over-rides this restriction and lets the radio transmit normally, but if you wait too long, you have to start all over. So it was a programming issue, not a bad Logic board.

The radio won't respond to programming or loses code plug.

Programming is accomplished through the RJ45 mike jack. Look carefully at the contact wires inside the jack and make sure they're all straight and parallel and riding in their proper slots. The programming line runs from pin 7 (2nd from bottom when viewing the radio from the front) to the Logic board at J8 pin 7. This line should rest at +5VDC with no programming cable plugged in.

Two transistors (and one section of an op-amp on some Logic boards) buffer and split the single programming line going to the microprocessor's TX Data and RX Data lines. These parts are unprotected from high voltages and they can disable the radio's ability to see or respond to programming data.

The older Logic boards used a Dallas DS1220 non-volatile random-access memory module (NVRAM) to hold some of the code plug data. A lithium battery inside this module only lasts 10-15 years; after that the radio will accept programing but forget some of the information when power is turned off. This memory is only used if the radio has more than 14 modes programmed or uses enhanced features. If the code plug size is more than 64 blocks when read or written, this memory module is being used and data could be lost. You can fix it one of two ways: replace the Dallas module with another Dallas module, which will fail the same way in another 10-15 more years, or replace the Dallas module with a regular EEPROM such as a 24-pin 28C16 with a speed of 150 or 200 nanoseconds, which will never lose memory, then reprogram the radio. An article on this web site details the procedure.

There are several checksums in the code plug. RSS checks these and will display errors while reading a radio's code plug if any are incorrect. These can be caused by the EEPROM going bad or if there's a problem with the RIB or programming cables or if a previous programming session terminated prematurely. Of course, in a perfect world, these would never occur. One quick thing to try is to connect the radio and go into the RSS Service menu (F2), choose the Board Alignment section (F2), then go to the Frequency Warp screen. Change the setting up or down by one value (remember what it was) and write it out using F8. This process will usually correct the checksums, at least the ones in the microprocessor memory where the tuning values are stored. Exit the Service menu and see if you can read the code plug now. If so, immediately write it back to the radio. Remember to go back and restore the tuning value you changed earlier.

The radio is off-frequency.

The 14.4 MHz reference oscillator on the RF board determines the radio's operating frequency. Check that the transmit frequency is correct, but if it is way off, chances are the interconnect pins between the Logic board and the RF board are corroded or dirty. This is a VERY common problem on GM300 radios but much rarer on the others.

The normal value for the Warp Frequency (in the Service Menu) is 101. If you have to adjust it by more than +/- 10, something is not right. If the radio is off frequency by more than a couple of kiloHertz, something is not right. Don't adjust the Warp Frequency to compensate. Remove the RF board and Logic board and clean the interconnect pins. Reinstall the Logic board then the RF Board. On GM300 radios, remove and clean the pin assembly that's mounted to the chassis between the two boards, then reassemble. Set the Warp Frequency to 101, and if the transmit frequency is still incorrect adjust the coil near the 14.4 MHz oscillator crystal to bring it within 200 Hz.

Motorola, MaxTrac, Radius, MaraTrac, GM300, RSS, RIB, PL, DPL, and a whole bunch of other terms are trademarks of Motorola Inc.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page created on Sunday 23-Nov-2012.

Article text, artistic layout, and hand-coded HTML © Copyright 2012 by Robert W. Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.