GM300 Dead receiver

[Josh]

I've been sitting on an 8 channel VHF GM300 for a while with a dead receiver. It worked fine for a long time but sat being stored until one day I went to use it and its receiver was dead. TX is dandy, no issues.

I've heard tales of a certain crystal going bad on the RF board, so I finally got a replacement, known good crystal installed and low and behold, the receiver is still dead. I can get it to open up with an extremely strong signal, but this is pretty absurd.

I'm running out of places to look. I've even tried feeding a signal directly into the 'RX' port of the RF board thinking it was something in the PA section but to no avail.

What would cause a radio to just magically quit RX'ing after first being on and in service for about 6 years or more to being stored, then dead?

-Josh

[SD70MAC]

Josh,

I have one that done the same thing ,its an 8ch 45w GM300(1993). I had got it out to give to a friend ,and the RX is dead. 3-4 month ago it worked ???

[jackhackett]

In a GM300 no receive at any signal level usually indicates the 2nd oscillator crystal is bad, receive only with a very strong signal usually indicates a problem in the RF front end, either the RF amp transistor blown or the overload diodes across the input to the amp shorted, or both.

There could be other things wrong, such as a bad IF filter, or the receive IC, but those aren't anywhere near as common.

You should also always check the feedthru connector on a GM300, if the pins are in a rectangular metal plate they're almost certainly bad, there's a replacement with an irregular shaped plate.

[Josh]

In a GM300 no receive at any signal level usually indicates the 2nd oscillator crystal is bad, receive only with a very strong signal usually indicates a problem in the RF front end, either the RF amp transistor blown or the overload diodes across the input to the amp shorted, or both.

There could be other things wrong, such as a bad IF filter, or the receive IC, but those aren't anywhere near as common.

You should also always check the feedthru connector on a GM300, if the pins are in a rectangular metal plate they're almost certainly bad, there's a replacement with an irregular shaped plate.

I've ruled out the PA section as even feeding a signal directly into the RF board, via homemade wire antenna with a portable transmitting right on it with 5 watts only broke the squelch and didn't return any distinguishable audio to the speaker in my case.

I have replaced the second oscillator crystal, as that one is what I always assume will go bad.

Maybe it would be in my best effort to just replace the silly RF board or radio! Tell me about this bad IF filter idea. Only RX is affected on my radio as TX is just as intended.

[Bat2way]

I've had just a few filter crystals, either Y51A or B go out = very poor sens. Check your junk pile and substitute. Very easy to check with a RF millivoltmeter and input signal.

[Josh]

I replaced three crystals on the RX part of the board, using similar known good parts from a MaxTrac.

No dice,

I removed the interconnect unit, yes this one does have the rectangular piece, but I cleaned it of corrosion and tested all the pins with my multimeter and it seems that it should be fine.

I reconnected everything and closed the hood, still no RX... this is pretty annoying. Next I may have to start swapping parts with my other VHF GM300 to see exactly where the fault lies.

-Josh

[PY5FT]

Very good trouble....

Josh, if u press and hold the monitor, the yellow (or amber) led blinks? U ear a noise?
Some defects are in rx audio path, check a smd capacitor in mosfet (MMBF5484) gate -audio mute- 100nF (C563). This capacitor some time have a low resistance.
Chek 14 pins connector (J6), 3rd pin have rx audio or noise. Check 9v6cc, some time this voltage can affect the frequency, it's a Moto error..... act above
temperature sensor. Audio PA IC.... pin enable also, BIPs on turn on, or channel changing?? A BAD TDA1519 can produce a low audio out.
Ah, check audio ICs SMD8. I got a tantalitic defective cap some times, (C551) 10uC from interconnect (J6-3) to pin 3 (U551).
(xxx) from my drawing.
Good luck, Antonio from Brazil, now PY5AB.

[motorola_otaku]

I have two (three?) Radius-series radios that exhibit the same problem, except mine are COMPLETELY deaf.. no on-channel RX whatsoever. TX works just dandy and they'll spit out white noise when unmuted.

Someone in a thread many years back mentioned pin diodes used in the T/R circuit as a possible culprit, but I never could get anyone to tell me where they were on the board and of course I am sans a GM300/M120 detailed service manual.

[PY5FT]

motorola_otaku
PM!

[Edgar]

1.-Clean with contac clnr all J6 pins- removing rf board and logic board from chassis,
2.-Check for CR2 and Y52 for a dead receiver.

good luck

[Josh]

I got rid of the radio months ago.

case closed.