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New to me AU-666

Checked the voltages across BE on all the transistors - all test between 0.5-0.7V

EXCEPT Tr811 starts at 0.5 and rises proportionally with volume up to 3.1V

Not sure it makes sense, wrong side of the waveform. Maybe the feedback loop does something inverse to it.

Guess I'll try swapping it out...
 
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Well one of the first things I did was disconnect that board completely!

Might be something to do with the bias transistor. I had to change the resistor values in order to get bias in range, will change it back and try a different method. Also need to tack the 47pf caps back on as I now have the oscillation again, I took them off when I thought I had this thing figured out the 4th or 5th time...
 
I believe you can still get the originals but if the problem is on both channels it’s unlikely both of them failed
There was a service bulletin for this model also not sure what it is entails but might be worth looking at it for a clue
 
quadmatrix82 said:
I believe you can still get the originals but if the problem is on both channels it’s unlikely both of them failed
There was a service bulletin for this model also not sure what it is entails but might be worth looking at it for a clue
Click to expand...

Well that's been the discussion the whole time, "if it's both channels then it's probably not the power amp", but everything else checks out - filter caps, rectifier, ripple filters, output transistors. I don't think it's the transformer, I see no sag there even when the voltage from the rectifier starts to sag and the sound distorts. The preamp is completely clean all the way to where it enters the power amp boards.

As are all the resistors on the power amp boards, and all the transistors are new minus the bias transistors. Aside from being modern transistors, the only change I made was switching out R833 for a larger value as otherwise the bias couldn't be set to above 6.5mV across the 0.5w emitter resistors. So I'm going to switch the original resistors back in, try putting one across the VR, see if that does anything.

Then if that fails I'll outsource.. This thing is holding up so many other things that I want/need to do that I'm beginning to hate it. I've spent far too much time on it.
 
Put the correct resistor back in, tacked a 47pf back across Tr807BC, tried an 1815 for the bias transistor. No change. Still distorted and Tr811BE still shows a voltage that increases linearly with volume.
 
Apologies for tossing some advice in here and then skipping out for 2 weeks :whip: I think you got some good advice in the process and I too wish you the best of luck with your tech's repair. I'm really curious what he finds.
 
Picked it up from the tech yesterday - he says - you're gonna hate yourself.

And he was right!

I put in 6.8K instead of 6.8R resistors at R843. Yeah, on both sides.

He also replaced the 2383s with 1220s, and that fixed the limitation in bias. Was able to get rid of the additional 47pf caps put across TR809 as well.


...


Amp sounds fantastic!

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I learned some good stuff here. Thanks!

Glad your tech was cooperative, most could not be bothered when someone else was in it.
 
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