U.S.AMPS 100HCA Power Supply Turns on 3 secs then looses Power

[Kip McGee]

This overgrown underrated 100 watt amp will fire up and play thru both channels for about 3 seconds and then fades out power supply goes dead
and dosent try to come back on.Can anyone tell me where to begin?

[Perry Babin]

Does it draw excessive current when it's producing audio?

Does either speaker pulse in/out significantly as the audio goes on/off?

[Kip McGee]

I tested for DC voltage on the outputs when the amp was running ,I will test for that again.It seems like one was slight I will answer with more detail in a sec .
As far as excessive current draw I burnt up my Homemade Ampmeter setting the bias in a 45 .Give me a few minutes to rig a new Amp meter I will answer.

[Kip McGee]

No to both Questions ,The Amp drew only a small amount of current around two Amps.
The speakers didnt Pulse noticeably,The DCV on speaker leads measured this: One channel went to .5 volts on turn on.
the other went -.5 then +.5 when it turned off.
Note a little difference in the way the two channels acted is this a clue?
I took the measurement with an analog simpson Meter.

[Kip McGee]

anyone have a schametic to this Amp ? A U.S.AMPS 100HCA ??
I have double checked all measurements they are accurate. On turn on the AMP draws 1.9 amps and slowly draws less till it its power LED fades out.
I am going to start Trouble shooting.
First I checked all output Transistors for unusual resistance measurments having found nothing there.
Next I am going to Fire it up again and take a IRThermometer and see if any of the componants change in Temp the amp has a Temp now of 81.3 degrees F some componants up to 81.9 for a starting temp..

[Perry Babin]

Can you post a photo of the board?

[Kip McGee]

Thanks for the Interest Perry here are some pics of my board..The Power Supply uses no IC KEWL huh..I do not know how to post a pic I will email it to you

[Kip McGee]

I hope I have your email right,I found it while reading over some of your post.
If you could kindly post the pics where the rest of the world can see I would appreciate it.I sent you mail at 9:35 PM or so.

[Perry Babin]

I received the photos. This is a self oscillating power supply. It's much more difficult to troubleshoot than an IC driven power supply. Confirm that all of the passive components are within tolerance. If they are, I'd next replace the smaller transistors in the power supply. The larger capacitors aren't likely to be a problem but the smaller electrolytic caps should be checked.

It appears that 2 diodes are missing from the circuit board. Are they possibly on the bottom of the board?

Does it appear that someone has previously tried to repair this amp?