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Old 24th March 2011, 06:31 PM   #1
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Default Amp only works when turned "off"

Hi

Sorry first off if this is posted in the wrong area or if it's somehow a "bad" question. Anyway, my DIY stereo project was going really well until:

The old GE amp/radio receiver I have suddenly stopped working. I have the whole thing apart and suddenly (after changing the input from radio to aux) the speakers stopped working.

I began to fiddle with things and soon noticed that the main power switch, if switched back and forth (on and off) would cause the music to play through the speakers again for a very short time - roughly two to four seconds. I took the switch apart - it was broken and decided to try to see if I could just get it to work but leaving out the switch and just closing the circuit. Again I got no sound. But then, when I disconnected the wires I got another 3 or 4 seconds of music. The same thing happens when I leave the circuit closed but unplug the whole unit - another couple of seconds of sound. The sound that's being created isn't nearly as full or powerful as the amp was before while it was working but it is clear - I think some capacitor is discharging but I can't figure out why the amp isn't working normally if it's charging the capacitor.

I'm pretty invested in this project and this amp so suggestions to "just throw the whole thing in the garbage" aren't really what I'm looking for. Anything more helpful though would be greatly appreciated.

I'm happy to send any images to anyone interested if that might help.

Thanks so much,

d.
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Old 24th March 2011, 07:26 PM   #2
Mooly is online now Mooly  United Kingdom
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As always a circuit diagram is a must really. Should be easy to fix... it's just basic faultfinding checking rails are correct and tracing signal through. Could be a biasing problem that just crosses a threshold as the rails collapse due to different time constants in the circuitry.

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Old 24th March 2011, 07:37 PM   #3
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IMG_0052.JPG

IMG_0054.JPG

Hey

Thanks for the quick help, I hope it's an "easy fix", mostly because I'm not really an expert on what's going on here.

Hope these pics are what are being asked for. Again - not an expert.

Thanks again,

d
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Old 24th March 2011, 08:08 PM   #4
Mooly is online now Mooly  United Kingdom
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Well tbh the piccys aren't going to help fix this really I suspect...

There's a lot of wiring in there Make sure none have come adrift, it easily happens.

What follows is just based on how it appears from your piccys...

Looking at the main amp piccy it appears to be an AC coupled design with a single main rail.
Make sure the main voltage is present, I would think the large blue cap is the main reservoir and would expect somewhere around 18 to 25 volts at a wild guess. The voltage on the amplifier outputs should be half that supply voltage (the mid point) so see if you can locate and measure that. Those two orange caps near the heatsink I am guessing are the speaker coupling caps and the midpoint voltage can be measured there. Depending on the amp design it could be on either + or - end of caps.

You need to split the fault in two... to determine if it's in the power amp section or the small signal stages before.
I take it both channels are faulty and behave the same ?

That will give you something to go on anyway.
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Old 24th March 2011, 08:10 PM   #5
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Yes, it's quite a bird's nest of wiring.

Thanks for the tips, between this and a few youtube videos I've found. I at least now have some places to start.

Thanks for the help,

d
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Old 25th March 2011, 01:48 AM   #6
Enzo is offline Enzo  United States
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If I had to guess, it seems to me the amp has a DC volatge offset somewhere in the circuit, and that is biasing off the signal flow. WHen you power off, that DC disappears, and you get a couple seconds of signal throughput as the power supply empties.
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Old 25th March 2011, 06:21 PM   #7
jaycee is offline jaycee  United Kingdom
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How old is it? If it's more than 20 years old, replace all of the capacitors..
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Old 25th March 2011, 08:42 PM   #8
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Andrew,

I have higher respect for you than that. Please show some to the forum.

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