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  MaxTrac Adjustments
By Robert W. Meister WA1MIK
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This article deals with the various coils and potentiometers on the MaxTrac (and some Radius and GM300) RF Boards. It describes their function and suggests adjustment procedures. I'm sure Motorola aligns these boards on a test jig, but as we only have entire radios at our disposal, this will have to do.

Adjustable Components:

The following table summarizes the adjustable coils and potentiometers that are involved with frequency, deviation, and squelch. All of these are on the RF board. The location of these components varies somewhat. The photos below are representative of the various adjustments.

All of the pots except R106 are round and blue and can be adjusted with a small flat-blade or Phillips-head screwdriver. R106 is a small trim-pot immediately next to the reference oscillator module; it has a small slotted screw head sticking up.

AdjustmentTypeVHF-LVHF-HUHF800 MHz 900 MHz
VCO ModulationPotR302 - - -R302- - - - - -
Reference Mod.PotR164 R164R164R164 R105
RX SquelchPotR70 R60R60R60 R20
Ref. Osc. FreqCoil/Pot.L151 L151L151L151 R106
RX VCOCoilL202 L202L202- - - - - -
TX VCOCoilL213 L213L213- - - - - -

Most RF boards are logically laid out. The right-front section contains the reference oscillator. The right-rear section contains the VCOs and buffers. The left section contains the receiver front end (towards the rear) and the receiver IF and squelch (towards the front). On VHF-L boards, the noise-blanker (Extender) circuit is in the middle of the board. The adjustable coils have a ferrite or metal slug in them that is best turned with a plastic or wooden alignment tool. A metal tool will affect the tuning and may break the ferrite slug. Some of the slugs have a square hole; others have a rectangular hole, while still others have just a slot. A wooden toothpick sometimes works well in the square holes; just shave or sand it down until it fits.

PartBand Approximate LocationWhat It Does
R302VHF-L Front left corner of right-rear section Adjusts modulation level
R302UHF Left section, rear-most pot Adjusts modulation level
R164V/U/8 Left section, front-most pot Balances low-freq modulation
R164900 Round pot next to Ref. Osc. Balances low-freq modulation
R70VHF-L Left of 14-pin connector Adjusts receiver squelch
R60V/U/8 Left of 14-pin connector Adjusts receiver squelch
R20900 Left of 14-pin connector Adjusts receiver squelch
L151V/U/8 Right front, next to crystal Adjusts reference osc. freq.
R106900 Tiny trim pot next to Ref. Osc. Adjusts reference osc. freq.
L202V/U Rear-most coil of right-rear section Adjusts RX synthesizer VCO
L213V/U Front-most coil of right-rear section Adjusts TX synthesizer VCO

As a quick rule of thumb, the Squelch pot is always located to the left of the 14-pin connector, the REF MOD pot is always located very close to the 14-pin connector, right at the front edge of the RF board, and the VCO MOD pot, if present, is the only other round pot on the board. It could be near the 14-pin connector, in the middle of the board, or next to the Reference Oscillator. The photos below show the variation.

Here's a marked-up photo of a VHF-L RF board. Click on any of the photos for a larger view.

max-adj/vhfl.jpg

Here's a marked-up photo of a UHF RF board.

max-adj/uhf.jpg

And here's a marked-up photo of a 900 MHz RF board.

max-adj/900.jpg

Test Equipment and Setup:

You'll need a lot of equipment, most of which can be found in a decent service monitor. I have separate pieces of test equipment so I'll list the capabilities here. A computer with the appropriate RSS for your radio, a RIB, and a radio cable are always necessary.

Remove the radio's covers and take the shields off the RF board. Read the existing code plug and save that to disk. Program three channels, all simplex, all with carrier squelch and no PL or DPL. Determine the lowest and highest frequencies for the radio band you're working on, and program one channel to the lowest frequency, one channel to the highest frequency, and one channel somewhere in the middle that has all zeroes after the decimal point (i.e. it's an exact MHz frequency). For 800 or 900 MHz radios, choose a transmit frequency in either the normal (repeat) or the talk-around band.

Adjustments:

These procedures assume that the radio is otherwise working properly and has been initialized by the factory or been blanked and initialized by following the steps in the RSS manual. This implies the correct crystal and tuning data has been entered.

These adjustments should be performed in the order they're listed below.

  1. REF OSC. Connect the radio to the test equipment so you can measure the transmit frequency. Go into the Service Menu, Alignments, and adjust the WARP Frequency for a value of 101. This should produce about 5.2VDC on J6, pin 9 (that's the connector that goes between the logic and RF boards). Make sure you save this out to the radio (press F8) and exit the Service Menu. Select the mid-band frequency programmed earlier. Key the transmitter and adjust the Ref. Freq. control L151 so the radio is on frequency to within 100 Hz. It should take a very tiny adjustment to do this. If the radio is more than a few kHz off frequency, chances are the interconnect pins between the logic and RF boards are dirty. You should remove both boards, clean the pins, reassemble the radio, and repeat this procedure. For 900 MHz radios, adjust the WARP Frequency for a value of 38. This should produce about 1.70VDC on J6 pin 12. Make sure you save this out to the radio. Then adjust R106 so the radio is on frequency to within 100 Hz.

  2. RX VCO. Skip this step for 800 and 900 MHz radios, as the VCO is not adjustable. Select the low end frequency channel and measure the DC voltage (to ground) on the Steering Line (SL) test point, located somewhere on the RF board. It may be marked. This point should be between 2.0 and 4.6 volts. Repeat this measurement on the high end frequency channel. It should be between 5.0 and 7.6 volts. If the voltage is too low on the low channel, or too high on the high channel, adjust the RX VCO coil L202 to bring the SL voltage within that range. Do the best you can. It's OK to be a bit below 2.0 and a bit above 7.6. Try to center the SL voltage so it's equidistant between those two voltages.

  3. TX VCO. Skip this step for 800 and 900 MHz radios, as the VCO is not adjustable. Repeat the above measurements when the radio is transmitting on those two frequencies. If the SL voltage is lower than 2.0 or higher than 7.6, adjust the TX VCO coil L213 to bring the SL voltage within that range.

  4. RX SQUELCH. The squelch setting is something that is a matter of preference. Motorola usually adjusts the squelch so it just opens at the 20dB quieting point, which is around 0.3 microvolts. Hams usually adjust it to the threshold, so it opens with the slightest signal. Connect the radio to the test equipment so you can feed a weak signal into it. Generate a signal on the mid-band frequency at 0.3 microvolts (or less if you desire), select that channel on the radio, and adjust the squelch pot so the noise just goes away, then back it up a bit so the radio unsquelches. If you want it the radio to be the most sensitive, turn the signal generator off and adjust the squelch control so the radio just squelches up.

  5. VCO MODULATION. This step only pertains to VHF-Lo and UHF radios, as the VCO MOD is fixed on the other bands. This adjustment interacts with the deviation settings in RSS. I'm sure there's a level it should be set to, but as long as the radio can make and maintain full deviation at all 16 calibration points, the setting is not critical. I've found that low-band radios tend to have the most variation of those 16 calibration values, and if the deviation won't go low enough at the highest frequency, then the pot on the board should be turned down to bring the RSS values back into a controllable range. A procedure I'd use would be to step through the board replacement procedure until you get to the deviation screen, then feed a 400 Hz tone at 1Vrms into the MIC jack and measure the deviation at tuning points 01 (will probably require the highest deviation setting) and 16 (will probably require the lowest deviation setting), and if the radio can't be adjusted to provide 5 kHz deviation at those two points, adjust the pot on the RF board until it does, then readjust all 16 points. Another method might be to set all of the deviation settings to mid-range and adjust the VCO MOD pot for 5 kHz deviation, then go back and readjust all 16 points.

  6. REF MODULATION. This is probably the most difficult to adjust because you need to see the demodulated waveform. I used my spectrum analyzer with its built-in FM demodulator. A service monitor with an oscilloscope will also do, as will a separate oscilloscope and some sort of receiver or modulation meter, if it has a demodulated audio output. I fed a 200 Hz audio signal through a 10uF capacitor into the MIC jack of the radio at 0.7Vrms, although you could use more. (You can use a lower audio frequency but it will have to be at a higher level because the MIC circuit is trying to filter out the lower frequencies.) You want a lot of signal here such that the audio circuits clip and limit the audio. I keyed the radio and adjusted the REF MOD pot for the flattest and most level top and bottom plateau of the waveform. If you go off in either direction, the flat areas will tilt one way or the other. You can also do this by programming the radio to transmit DPL on one channel, but the deviation will be a lot lower. You can see the effect of this pot in the oscilloscope traces below. The 900 MHz Trunking manual says one of its test modes generates a 125 Hz square wave and tells you to adjust R105 for the optimal undistorted square wave signal on the scope.

At the conclusion of making these adjustments, you should go back into the Alignment menu and adjust the overall deviation and frequency. It might be worthwhile to go through the Board Replacement procedure and adjust all the deviation settings

Demodulated Audio Oscilloscope Traces:

Here is what a properly adjusted radio looks like with a 200 Hz sine-wave signal fed into the MIC audio at 0.7Vrms. Note that the top and bottom straight parts of the waveform are flat and level. The vertical deflection is 2 kHz per division, so the radio is deviating just over 4.5 kHz.

max-adj/200hz-good.gif

When the REF MOD pot was turned too far one way, the top and bottom flat areas are now tilted and no longer level.

max-adj/200hz-ccw.gif

Similarly, when the REF MOD pot was turned too far the other way, the top and bottom flat areas of the waveform are now tilted the other way.

max-adj/200hz-cw.gif

This is a DPL 445 signal after proper adjustment of the REF MOD pot. Note how the top and bottom straight parts of the waveform are flat and level. The vertical scale is now 1 kHz per division, so the deviation is just under 1 kHz.

max-adj/445-dpl.gif

And for completeness, here's a 100.0 Hz PL signal coming out of the same radio. The vertical scale is now 1 kHz per division, so the deviation is just under 1 kHz.

max-adj/100hz-pl.gif

Acknowledgements and Credits:

The component identification came from the MaxTrac service manuals. Additional procedural information came from the GM300 service manual and experimentation.

The REF MOD procedure was actually copied from the Motorola R100 UHF Repeater manual.

Contact Information:

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

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This page created on Wednesday 14-Dec-2011.



Article text, artistic layout, photographs, images, and hand-coded HTML © Copyright 2011 by Robert W. Meister WA1MIK.

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