The STC  MTR25-151  Hi Band FM mobile tranceiver
2M amatuer service conversion

    ralph klimek vk3zzc  June-2010

The STC  MTR25-151  Hi Band FM mobile tranceiver  became surplus about 1985 here in Australia, and I think it was the WIA that made them available in large quantities to the Ham community.  I originally converted mine sometime back in 1986  and was used continuously until 1995 when family duties required me to forget ham radio. Its 2010 now, and after getting my old call back, I ejected some red-back spiders and a 15 year encrustation of hardened dust I reapplied power. It sort of worked, but the squelch has died, and with it all usefull audio as well.  But where was the circuit ?  A collection of circuits had been supplied with the unit. The Internet has forgotten this radio, but after a short battle with some determined silverfish I have recovered this collection of documents about this radio from my bottomost basement.

This radio would benefit by the substitution of a hotter transistor for the front end, something like a BFR96 or MRF901, but otherwise it provided very effective service, long after its commercial life had expired.  All circuit nodes are exposed and can all be probed whilst the radio is in operation, however repair requires disassembly to free the PCBs from their cages.

Getting the squelch and adequate audio signal level on the reciever part going again was an interesting and enlightening exercise. The fault was that no to little  detected audio was leaving the squelch circuit.  The squelch sort of   still worked but its action was very mushy. When the squelch opened, little audio came through.  It turned out that there was not just the one fault, there were many.  The ratio-detector diodes  both had very high forward resistance. They were germanium diodes and they should have a very low forward resistance. They were replaced. This radio is an old one, and really old fashioned bakelite encapsulated carbon composistion resistors had been used. It turned out that nearly all composition resistors in the 1K to 100K range  had drifted high in value, some had just gone open circuit. I have seen this sort of bad behaviour before , but only in thermionic valve circuits where these resistors do actually dissipate some power, not the case here.  It was a surprise to see this behaviour in low power solid state circuits.   So there is the clue, if you have one of these units that you are trying to rejuvenate, just test ALL resistors and replace if out of tolerance.

transmitter part Atransmitter part B
reciever part A
reciever part B

top view showing RX audio, if strip, master oscillatorsIF , detector and audio ampcheck all resisttors of this vintage, they have drifted high


page created with raw unedited scans Wed Jul  7 14:15:38 EST 2010