The STC MTR25-151 Hi Band FM mobile tranceiver
2M amatuer service conversion
ralph klimek vk3zzc June-2010The
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.
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.
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
|transmitter part A||transmitter part B|
|reciever part A||reciever part B |
|top view showing RX audio, if strip, master oscillators||IF , detector and audio amp||check all resisttors of this vintage, they have drifted high|
page created with raw unedited scans Wed Jul 7 14:15:38 EST 2010