For some reason, GM300s seem to be prone to losing receive sensitivity. Whether this is due to nearby excessive transmitter power or operator error, the eventual cause seems to be either shorted protection diodes across the receiver's input, or a dead first RF amplifier transistor. All of these components require removal of the RF board to access them, but replacement is quite easy. They're surface-mount, of course.
While not a problem, I encountered a VHF radio with a receiver that was a couple kHz off frequency. In addition, it had very distorted audio when I fed a 3 kHz deviated signal into it. The 2nd oscillator crystal frequency was a couple of kHz high too. Replacing the crystal got the receiver back on frequency but the audio distortion was eventually traced to the fact that this was a 12.5 kHz narrow-band receiver and would only accept a 2.5 kHz deviated signal. I had never encountered one like this before and it was completely useless for 5 kHz reception, which is the standard on VHF.
Another common issue is a dead 2nd oscillator crystal. This operates at 44.645 MHz and is used to convert the 45.1 MHz intermediate frequency signal down to 455 kHz for the detector. When the crystal dies, the receiver will hear absolutely nothing or it may only hear a really strong signal, like a portable transmitting two inches from the antenna jack. If you have a receiver capable of hearing the crystal frequency, you can pick it up if you remove the shield from the RF board and place the antenna within a few inches of the left side of the radio. If you don't hear the 44.645 MHz signal, the crystal needs to be replaced. These can often be found on a popular auction web site. The Motorola p/n is 48-80008K02 and the crystals are marked "44.645/08K02" so you know they're the right ones. The most recent crystal I replaced is marked "44.645 08W05 NDK'9520" but I'm not sure how that differs from the "K02" crystal. In September 2019, Motorola claims the current part number for the 08K02 is 48-80606B02.
These radios often go way off frequency, to the point that the warp adjustment will not get it back where it belongs. The cause is most often dirty interconnection pins inside the radio. These are between the RF board and the logic board. On the MaxTrac, they are attached to the logic board; on the GM300 the strip of pins is mounted on the chassis and both boards plug into it. Remove both boards, clean these pins, and reinstall the boards. While the radio is apart, clean the front panel connectors too. These same connectors get dirty on MaxTracs as well, but for some reason they don't often cause serious problems like they do on the GM300. See this article for details and photographs.
I discovered that the RX Audio output on the accessory jack pin 11, with JU551 in the de-emphasized/muted position (B) has negative peak clipping (distortion) present on a 5 kHz deviated signal with a 300-400 Hz modulating tone. This may also be present on the MIC jack pin 8. It is caused by too much gain in the final audio stage ahead of the volume control. Two 16-channel radios had this problem; I don't know if all GM300s (and their related models) do. I fixed this by soldering a 22k resistor from pin 1 to pin 2 of U553B to reduce the gain by about 50%. The radio now exhibits no clipping with deviations as high as 7 kHz.
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