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New to me AU-666

JyriK said:
That's progress!
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1786088644983.png



JyriK said:
What's the output when distortion appears? Is the rail B3 clean as well? How does it behave with high output?
Click to expand...

Sorry I don't understand, 'what's the output?' Basically when the waveform looks like the last picture I posted is when distortion starts up, and then the hump gets very prominent with increase in volume, to the point it eats up most of the negative wave.

Will check the other rails, hadn't thought to do that.
 
Output at the speaker terminals, AC Vrms.
If B3 is clean, then some tr replacement doesn't co-operate, is my guess.
 
Yeah all the rails going to the fuses read solid 39.8vdc.

Curiously the voltage at point 26 (output of the neg ripple filter) is now 36.8 versus 37.8 for the positive filter. Which kinda makes sense because r872 is 10k and r871 is 4.7k (both of which were replaced with metal film 1/2w resistors as they had tested high).


Edit: output distorts at 3.1Vac on the DIMM while using speakers... Not sure about rms?

If I use an 8 ohm dummy load on just one channel then distortion starts at 4.8vac.
 
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JyriK said:
Does it do funny things with higher volume?
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For example? Just distorts.


One point of info is that L and R start distorting at slightly different volumes.

Without a load, nothing distorts in the entire amp board. Maybe that's obvious, I dunno. Once I put a load on then it shows up everywhere on the amp board, from tr801 down.
 
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bhairavi said:
B3 is just off the rectifier so I just see the sawtooth wave.
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Actually there should be no sawtooth wave as C003 should filter it.

E: Damn, nevermind, C003 alone aren't able to filter it altogether.
 
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JyriK said:
Actually there should be no sawtooth wave as C003 should filter it.

E: Damn, nevermind, C003 alone aren't able to filter it altogether.
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Plus it's not connected AFTER the filter cap, it's connected at a common junction.

At work, will report back on possible distortion.

I'm beginning to think you might be right about this just being a result of transistor choice on the power amp.

Which is really *^@#*$# frustrating because it could be some or all of them that are causing a problem. And I've already tried modern outputs to match the modern drivers... although at that time we were still dealing with oscillations. Which, come to think of it, probably had to do with the negative ripple filter not working.

This 666 really is the work of the devil..
 
Meanwhile I made a model of the power amp and power supply of AU-666 with QSpice (using my custom transistor models). Here's what input (green), output (blue), and rail B3 (red) looks like in it, when 0.5V peak-to-peak 1khz sine wave is fed into the power amp:

scope.PNG

The model should be useful for providing voltage readings for the transistors. They should be in the right ballpark, although I already see some discrepancies with the rails. But that may be due to the tr-betas I used (usually an average from a datasheet).
 
This is really nerdy, isn't it? But here are dc volts the model gives for the transistors (with some rounding), with no input signal:

TR801
b=183mv
c=-30v
e=770mv

TR803
b=150mv
c=-29v
e=745mv

TR805
b=-29.3v
c= ground
e= -29.9v

TR807
b=-30v
c=-990mv
e=-30.6v

TR809
b=-450mv
c=990mv
e=-990mv

TR811
b=990mv
c=39.6v
e=532mv

TR813
b=-990mv
c=-39.5v
e=-530mv

TR815
b=531mv
c=-39.6v
e=14mv

TR817
b=-531mv
c=-39.5v
e=-14mv

Running with 20.4mA of bias current, dc offset near optimal. Original transistors modelled.

Phew!
 
JyriK said:
This is really nerdy, isn't it? But here are dc volts the model gives for the transistors (with some rounding), with no input signal:

TR801
b=183mv
c=-30v
e=770mv

TR803
b=150mv
c=-29v
e=745mv

TR805
b=-29.3v
c= ground
e= -29.9v

TR807
b=-30v
c=-990mv
e=-30.6v

TR809
b=-450mv
c=990mv
e=-990mv

TR811
b=990mv
c=39.6v
e=532mv

TR813
b=-990mv
c=-39.5v
e=-530mv

TR815
b=531mv
c=-39.6v
e=14mv

TR817
b=-531mv
c=-39.5v
e=-14mv

Running with 20.4mA of bias current, dc offset near optimal. Original transistors modelled.

Phew!
Click to expand...
Now do mine! :D
 
No thanks! :p

One thing you could do with the real device is to check b-e voltage drops, when in the fault mode, to see if any of trs have gone into saturation, ie. if the drop is 0.3v or smaller instead of the usual 0.5-0.7v.
 
By the way this is what maximum vol looks like on the outputs:

20260807_013154.jpg20260807_013244.jpg


Does this look more like hard clipping than hump from 'current starvation'?
 
I was wondering about putting in a spare mj21193 in place of the pnp output of one of the channels to see if that's the problem.

if it's not the filter cap/rail/rectifier then it seems like it would be in the pnp chain?

but going off what @jmjrcv said earlier in the thread, does that mean I have to take out the nfb circuit in order to accurately scope out where the distortion starts?

I'll also check voltages on the transistors.
 
New output made no difference, changing rectifier and filter cap made no difference.

Very close to calling it quits unless someone has any bright ideas.
 
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