CDM1250 Power Pin Short
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CDM1250 Power Pin Short
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?
- jackhackett
- Posts: 1517
- Joined: Tue Jun 10, 2003 8:52 am
Re: CDM1250 Power Pin Short
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?
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?
Re: CDM1250 Power Pin Short
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
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
Re: CDM1250 Power Pin Short
1. Power supply not capable of supporting transmit current drain;
2. Radio trying to send an emergency alarm because the trigger jumper is missing?
2. Radio trying to send an emergency alarm because the trigger jumper is missing?
Karl - WA8NVW AFA5VB
SHARES + NCS
SHARES + NCS
- jackhackett
- Posts: 1517
- Joined: Tue Jun 10, 2003 8:52 am
Re: CDM1250 Power Pin Short
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.