GM300 not receiving anymore

[pe2kmv]

Hi all,

After some years in a box, I've fired up my GM300 VHF again. For about 5 years it was stored dry and dust free in a box. Nevertheless it now has an issue with its RX. When testing it just give a short noise on upkey of another radio. At that moment the TX led shortly flashes. On the other hand TX is fine, voice and 5 tone, with and without PL. But regardless the frequency, there is no RX (even with the squelch permanently open).

Any idea what could have happened to my dear GM300?

Greetz,
Ronald

[AEC]

Check the 2nd oscillator, Should be 44.645 Mhz. See what the frequency error is. If it is off by one or 2 Kc, your receiver will not function, or you get none to litte white noise.
The 16.8 Mhz. refereance oscillator will place your TX frequency off channel as well.

The other thing to look into, are the audio output transistors on the right side of the logic board.
You may even have a loose plug for the internal speaker....

Use a stable frequency counter, or attach a sniffer probe with a service monitor tuned to the 2nd oscillator frequency...Should have a solid +10dBm level or greater, and be as close to 44.645 Mhz as possible.
Same goes for the reference oscillator.

[pe2kmv]

Thanks for the answer! I'll have a look at this a.s.a.p. I've already grabbed a service manual from somewhere to make the job a bit easier.

It really looks as if the receiver is off frequency. Because when sending a tone (either single tone or 5 tone) from another radio I can hear the tone a bit garbled from the GM300.

Greetz,
Ronald.

[AEC]

Sure 'sounds' like a 2nd oscillator issue.
These are known to go south, and when dead, you lose receiver function.
At least the receiver is working, but off channel sounds are a direct lead to a bad 2nd oscillator Xtal.
You can use a service monitor and tune the monitor to clear up the sound, then look at the error.

[pe2kmv]

Already thanks for the help!

Is this 44.645 MHz crystal some special Motorola spare part, or can I order a crystal on this frequency from my local supplier? Is it an overtone crystal? Something special to pay attention to?

Greetings,
Ronald - PE2KMV

[AEC]

I think the crystal is a fundamental, not overtone.
I do not remember the loading required, bu you can obtain that crystal from motorola.

[mike m]

ICM , international crystal also has them as a standard catalog item and they are the same part number as Motorola, PN # 48-80008K02.

ICM crystal Oklahoma City OK, 800-725-1426.

It was 23 buck plus shipping for each of mine, Ouch that hurt my wallet.

[pe2kmv]

And indeed... More than a year later I've found a spare crystal and the radio was fixed within 15 minutes!

Ronald