Batboard

When the mic is keyed, instead of transmit going "on" once, as long as the mic button is held (normal operation)...

... The radio transmit keys repeatedly ~ 1x per second -- as long as the mic button is held.

Example: Imagine if you repeatedly, manually keyed the mic once every 1-second.

I've exchanged mics.
Does not seem to be channel-specific.
Mobile (Ham) installation
16-Channel

Ideas? Thanks, all!

A couple of thoughts.
1-What level is the power set? (15 watts, 25 watts, 40 watts, etc)
2-What is the current capability of the power supply?
3-What gauge and how long of a power cord?
4-Have you checked SWR on the antenna?
Possibilities are its a 25 watt PA and you are driving it to the maximum or over RSS will show the PA's capabilities.
The radio is being supplied from a circuit that cannot handle the current the radio needs. Possibly tripping a circuit breaker that resets automatically.
If the gauge of the wire is too small, the radio may switch off due to inadequate current.
If the antenna SWR is too high, there may be a fold back circuit that shuts the transmitter down.

You might try this. Remove the radio from the vehicle. Put it on a test bench with a heavy duty power supply, with a 12 gauge power supply lead, and hooked to a 50ohm dummy load antenna.
If it is still malfunctioning, look carefully at the RSS when you read the radio. Are the transmit frequency(ies) outside the parameters of the radio. There are different versions of the GM300. VHF, UHF. Different versions of the UHF that transmit in different portions of the UHF band. Ex- You need for the radio to transmit on 448.200mhz but this radio will only transmit from 460- 490mhz.

Since I don't have the radio in front of me, I am taking some wild guesses on what the problem might be.

Andy
W4KDN

Andy, thank you. Looks like my assessment was not accurate:

The issue is specific to 4 channels that have MDC programmed (has worked without issue for 10+years).

If you're familiar, when you key on an MDC channel, the radio emits two short tones from the speaker (I think to signify that the MDC is being transmitted).

So, recently, if I key the mic on one of these channels, those MDC tones play continually, over and over again, as long as the mic button is held.

** I'll bet if I disabled MDC the issue would go away.

Make sense?

Thanks again.

Jim | KB9MIR

Jim, makes sense to me. I have not dealt with MDC on any of my GM300's. If you have no need for MDC in your particular application, make a copy of your current codeplug, then delete the MDC and see what happens. One thing I hated about the GM300, when I am loading a codeplug, the data transfer meter moves across the screen for a ways and then stops. My heart almost stops every time with the fear I just bricked a radio. The data transfer picks back up and I not longer need CPR.