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Pioneer SX-550, help needed

L

Lucamonzaz

New Member
Hello everyone,
I'm repairing this amp - I've already fixed a power supply voltage by replacing q14 (which is a 2SC945A) then all voltages looked good, so I hooked up the dummy load and oscilloscope. The right channel waveform is severely clipped (basically almost all the top half) and I can't set the bias on that channel, it's stuck at 0. Left channel looks and adjusts good.

I've checked a few things with the multimeter and didn't find shorted transistors or open resistors (and trimmer) so I'm kinda stuck.
If anyone has suggestions here's the service manual I'm working with pioneer_sx-550_sm (1).pdf

Thanks in advance,
Luca
 
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Dave1027 said:
What are the voltages on Q10? What does the DC offset look like on the right channel?
Click to expand...
Q10 voltages:
Base: not stable, it keeps moving from 0.67v to 0.85 roughly
Collector: 28v
Emitter: jumps around from 60 to 90mv

Right channel DC offset about 70mv but that keeps moving as well like +-15mv

Thanks
 
Well it is the outputs unfortunately. I've moved Q9 and Q11 in place of Q10 and Q12 and now the bias on the right channel adjusts fine.
I'll get some new equivalents and replace all four. Thanks everyone for your suggestions
 
Lucamonzaz said:
Well it is the outputs unfortunately. I've moved Q9 and Q11 in place of Q10 and Q12 and now the bias on the right channel adjusts fine.
I'll get some new equivalents and replace all four. Thanks everyone for your suggestions
Click to expand...
Be sure to check the drivers. Many times one affects the other.
 
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