Power Suppy Thermal Protection?

[KA7ZIS]

I have an MG Pro 8 amp, 13.6 volt power supply. Inside the power supply is a small ( 1" x 1/2" )circuit board with a resistor and diode mounted on it. Two of the four wires connected to the board run to the relay circuit breaker. The other two wires (small magnet wire) run parallel to each other into the heat sink. These wires I assume are thermal protection and have been over heated at some point as the insulation is melted. It looks like they were attached directly to the heat sink with adhesive but were damaged from over heating. Can anyone tell me how to repair this type of circuit as no schematics are available. Thanks for any help.

Bob
KA7ZIS

[K5LXP]

This sounds like a crowbar circuit, which is meant to actually short out the output of the supply in case for some reason it's output voltage exceeds a predetermined threshold.  In the case of a 12V supply this might be 15 or 16 volts.  So it appears you have two problems, your supply is no longer regulating and the crowbar circuit did it's job by shorting out the supply (which heated up the wiring).  The place I would start is to disconnect the wiring going to the heatsink and make sure the supply is working correctly.  Then reconnect the crowbar circuit and see if it's working OK.  What's *supposed* to happen is the crowbar draws so much current the line fuse should blow, but that doesn't always happen (or the fuse is too big).  Check it out and let us know what you find.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net

[KA7ZIS]

Thanks Mark for the reply, you've given me a starting point for this repair. When I first checked the power supply, the meter showed something like 16 volts unloaded. When I hooked up a 12 volt bulb for a load, it dropped to zero. I removed the 2N3055 and tested it, and it checked ok. Put it back in and rechecked and it worked. Must have been a cold solder joint. Anyhow, now with the load drawing 1.5 amps the voltage is about 6.8 volts. The two heatsink wires are exposed so I can access them and if I short them together nothing happens. I'll do a little more troubleshooting and read up on crow bar circuits....Thanks again

Bob
KA7ZIS