Batboard

I have seen 2 CDM1250 Dash mount develop a short on the input power. One radio was in a fire truck and the other was sitting on a desk with a power supply providing a radio Reference broadcast. This radio never transmitted. The transmitter leads are shorted as measured with a meter. The radio will not light up and the power supply was hot. Any thoughts?

A dead short on the power connector is usually caused by a shorted protection diode. There is what is basically a large zener diode across the power connections, if voltage is hooked up backwards or exceeds the diode's rating it can cause the diode to fail as a dead short. This is to provide some protection to the rest of the radio in the case of problems with the power source.

Some causes for this can be:
Someone accidentally connecting the radio backwards
Someone hooking up jump start cables to a vehicle backwards
A power supply failure causing over-voltage on the radio
A voltage surge

It's also possible that either the transmit output transistor or the audio power amplifier may have shorted, although these *usually* fail open circuit. Incorrect external speaker wiring or a bad antenna system would be possible causes here.

What type of power supply was the desk radio running on? Can you check the output voltage?

Thanks, I will check the diode and/ or cut it out.
The power supply is a standard linear supply putting out 13.5 vdc. Normally the supply runs cool to the touch but it was hot when the radio was shorted. I put a meter on the supply with the radio connected and it dropped to 5 volts. I may have a bigger fuse than the supply rating.

Thank you again, Bill

1. Power supply not capable of supporting transmit current drain;
2. Radio trying to send an emergency alarm because the trigger jumper is missing?

Are you sure you're getting a dead short on the radio? I usually measure with a DMM set on diode test mode, and measure with the leads connected both ways. A radio with a good diode will show something like a half volt drop in one direction, and a high or open reading in the other direction. It may take several seconds for the meter to settle as filter caps charge. A radio with a shorted diode will show close to zero volts on a diode tester in both directions.