VE7XEN's electronics blog

Out of Band Power Control in the Motorola Radius M100 VHF

I recently purchased a Motorola Radius M100 radio used on eBay, intending to use it for an APRS tracker or perhaps a digi. The radio arrived in perfect working order, making its 25W rated power across the rated band split (146-174MHz). It was a couple KHz low in frequency, but easily “warped” up to within a few Hz.

However I ran across some trouble when testing it with the local APRS frequency of 144.390MHz; the 25W rig was making nearly 45W and the software power control made no change at that frequency! After doing some research I found several articles detailing the issue with MaxTrac radios. Essentially the problem is that the power output of these radios is controlled by the microprocessor, which is given a series of tuning values during alignment that establish a power output control curve. However the tuning values only cover the band split the radio was built for, so when operating outside the normal band, the microprocessor just runs the power amp at full bore. Not healthy!

Two solutions were proposed in the articles I found: either cut the power control DAC output trace to the PA and replace it with a potentiometer - manual control, or blank the radio and reprogram it as a 136-162MHz band split. Given that I recently destroyed a board trying to cut a trace, and generally not liking the idea of reworking circuit boards, I thought I would give the blanking solution a try first - worst case I could restore my old codeplug and then do the hardware mod instead.

It turned out not to be as difficult as I though it could be, there were suggestions that manual bit-banging of the EEPROM might be necessary to get the Radius RSS to recognize the board as blank, but this proved to be unnecessary.

Before you start, you will need the Radius RSS, the Maxtrac Lab RSS, a decent DMM, and the normal RIB and programming cables, and the Torx driver to open the radio. A dummy load, power meter and/or service monitor would be ideal. See links below for software.

First you should record as much of the tuning data you can find on the radio. Some of it needs to be measured or found and some can be read from the RSS:

  1. Open the radio, remove the shield on the RF deck and measure the voltage at Pin 1 on the red connector (Pin 1 is toward the rear of the radio, and is marked on the PCB). It should be near 9.6V.
  2. Remove the secondary shield from the synthesizer section and hope that your crystal tuning values haven’t been lost to the ages. I got lucky. You’ll find two rows of 4 digits typed on the crystal itself, and a 7-digit number handwritten on a sticker on the PCB (or perhaps floating around in the can).
  3. Reassemble the radio and connect it back to your RSS computer. In the F2 SERVICE, F6 BOARD REPLACEMENT menu choose F2 LOGIC BOARD OR RF BOARD. The software forces you to go through the steps one by one, but you can press F10 to cancel without making changes. Go through each option and record all the tuning values. The crystal data doesn’t appear, so just skip that section. You should be able to record:
    • TX POWER
    • TX POWER CAL (16 values)
    • TX DEVIATION CAL (16 values)
  4. Record the serial number and exact model number of your radio, either from RSS or the label.
  5. You should definitely save the current, working codeplug before continuing.

Now it is time to completely blank the EEPROM in the radio using MaxTrac Lab, so start that program on your programming machine. It will not program the radio as it recognizes it is not a MaxTrac, but it does allow both bit-banging (manually writing to EEPROM addresses) and blanking. We just want to blank it, so go to F2 SERVICE, F8 MAKE BLANK BOARD and select F8 BLANK BOARD WITH EXTENDED CODEPLUG / CLEAR TUNE DATA. This takes a minute or so and completely blanks the logic board. When it’s done exit the Lab RSS and start the normal Radius RSS.

At this point you have a completely blank logic board, so the software will allow you to tell it the parameters of the radio you’re putting it into. To accomplish this, go to the F2 SERVICE, F6 BOARD REPLACEMENT and F2 LOGIC BOARD section again. The radio should recognize that the logic board is blank and present you with a screen asking for the model information. I’m not sure what the Product Line refers to, but I chose Radius there. Model, Range and Model Number should be pretty self explanatory, just scroll through the options until you get your correct model number that you recorded earlier. Apparently you can convert a 16 channel to 32 channel radio (or add/change features) this way too, but mine is only a 2 channel so I don’t have that option. Panel Number should be 000 for the 2 channel head or 001 for anything else. Enter the serial number you wrote down and hit F8 to store the values.

Finally, press F2 on the board replacement menu again and go through the initialization procedure. First the crystal parameters (enter the values you wrote down from inside the radio), then F8 to store. Continue for the rest of the parameters. If you have the appropriate test equipment you should use it on the power output and deviation calibration pages; if not I would interpolate the trend from the values you recorded before blanking the radio. Value 9 IIRC is the ~146MHz one that would correspond to Value 1 on the radio’s natural split, so follow the trend down and copy the values above it.

The VCO might not lock all the way down to 136MHz, but mine had no trouble at all, and produced a full 25W up to 150MHz or so and tapered off to about 22W at 162MHz. Not bad at all. If necessary, information on retuning the VCOs is easy to find, unlike the blanking details.

Congratulations! You now have a Radius that can control its power properly inside the ham band, and you don’t need to do any tricks to program frequencies either!




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