Full Coverage Maxtrac 900MHz Modifications

The original version of this page can be found at Michael Roche's page, http://www.geocities.com/aa2ls/, it has been reproduced here in case Geocities dies...


After trying the VCO modification using the conductive pen, I was amazed with the fine level of tuning right where you want it (within about 200kHz). What bugged me was that it still did not cover the whole band on both transmit and receive.

I found a way to do just that. The two problems are the VCO range and the TalkAround (T.A.) frequency switch in the firmware at 903 MHz.

With 15 Volts on the VCO steering line (S.L.) circuit, the VCO has a 30 MHz range. Rewiring the VCO Pin Shift to the PTT line allows the radio to cover below 903 using the T.A. side. Now it doesn't matter where the RX side of the VCO range sits.

Both modifications required are on the RF board. Supplying 15VDC can be done with a DC-DC converter and a small regulator (TO-92). The 24 Volts out of the Converter should be filtered (140kHz design). [Bat Wing's Addition] You could also reduce the component count even more by getting a DCP010515B from Texas Instruments. This is a single DIP IC which takes 5VDC in and gives you rectified 15VDC out. Just steak 5VDC from on the board somewhere are you are set.

Anyways, on with the modification... Cut the trace where R119 is fed from the 9.6 Volt Supply and feed the 15 Volts to R119. The S.L. circuit draws about 21 mA, so this design is perfect.

Re-wiring the VCO to use the right side (T.A.) can be done with a 27K resistor tied to J6 Pin 4. Don't forget to cut the trace from J6 Pin 14 (leaving C250 connected to the pin).

That's it! With the two front end filters changed and FL51 in the 455kHz stage (for 5kHz deviation), you'd think this radio was made for the Ham band.

If you want to see this modification along with the schematic of the DC-DC converter, along with tune-up instructions, click here.

Mike Roche, AA2LS


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