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Pioneer SX-650 won't fire up.

cudajim

cudajim

New Member
I'm trying to fix an old Pioneer 650 amp. It's on my test bench and it crapped out just yesterday. The output relay won't engage. I get audio signal all the way to the relay and if I jump it I get good sound, both channels. The relay itself picks with an external supply. I suspect the protection devices are at fault but I cannot figure out where that is. I have a schematic/service manual but it's a bit hard to follow. My supplies look good too. I do have meters and a scope as well. Any help would be appreciated
 
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With no signal input, volume to minimum, measure the dc volts at the outputs for both channels, before the relay contacts (pins 17 and 19). If you have excess dc offset the relay will not pull in, but the amp may still have audio output. Post your voltages, please
 
merlynski said:
Welcome to Audiokarma!
With no signal input, volume to minimum, measure the dc volts at the outputs for both channels, before the relay contacts (pins 17 and 19). If you have excess dc offset the relay will not pull in, but the amp may still have audio output. Post your voltages, please
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I'll measure it soon,(probably tomorrow), will get back to you. Thanks.
 
Today I fired up the amp and the relay did contact. I had audio (fm) in both channels but one channel was not good (lots of static). I measured the offset voltage at the relay. 30 mv and 800 mv on the bad channel. Where to go next?
 
cudajim said:
Today I fired up the amp and the relay did contact. I had audio (fm) in both channels but one channel was not good (lots of static). I measured the offset voltage at the relay. 30 mv and 800 mv on the bad channel. Where to go next?
Click to expand...
30mv is good.
800mv is a problem, the usual culprit is the diff amp transistors in the power amp, which was channel was it?
 
Ok, so I dove in again and the relay was going on & off every minute or so. I first nailed it down to the bad channel then looked at the driver xistor. It checked ok with a VOM so I hit it with freeze spray which caused it to work. I then hit it with the heat gun (slightly) and it crapped out instantly so I decided to change it. Turns out I had the 2SA798 dual xistor in stock. I replaced it and everything behaved as it should. I checked offset voltage several times after it got good and warmed up. Both channels were < 30mv. I now have a working amplifier! Thanks so much.
 
cudajim said:
Ok, so I dove in again and the relay was going on & off every minute or so. I first nailed it down to the bad channel then looked at the driver xistor. It checked ok with a VOM so I hit it with freeze spray which caused it to work. I then hit it with the heat gun (slightly) and it crapped out instantly so I decided to change it. Turns out I had the 2SA798 dual xistor in stock. I replaced it and everything behaved as it should. I checked offset voltage several times after it got good and warmed up. Both channels were < 30mv. I now have a working amplifier! Thanks so much.
Click to expand...
A 2sa798 transistor in stock :eek:, how did that happen?
Anyway, glad you got it going and that I could help!
 
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