New to me AU-666

[bhairavi]

Roger that. Will also recheck bias and offset.

 

[JyriK]

Forgot to mention that resistors should be used between the base and emitter nodes of outputs, if they are off. 220ohms would suffice. So the feedback loop would still be there. Otherwise it may go wacky.

 

[JyriK]

...and yeah, bad pnp-outputs could absolutely cause it.

 

[bhairavi]

Forgot to mention that resistors should be used between the base and emitter nodes of outputs, if they are off. 220ohms would suffice. So the feedback loop would still be there. Otherwise it may go wacky.

Thanks didn't know that. Will check bias/offset first, then take those 47pf off the L amp drivers just cause I think I don't need those anymore, and then scope with the outputs out. I will still get a (small) signal at the speaker outs if I put those resistors in?

 

[JyriK]

Yeah, from the drivers.

 

[bhairavi]

Nothin doin, waveform without outputs is the same -

 

[bhairavi]

Tr801C:
low volume


medium volume


high volume



Tr803C


Tr803E


Tr805C - totally flat

Tr805B


Tr805E




I rechecked the preamp out, and I have a clean waveform. Also checked at the subsonic filter, same thing.

So if Tr801 already looks like that... what does it mean?

 

[JyriK]

So the signal is clean at B and E of TR801?

 

[JyriK]

Come to think of it, since it is on both channels, you need to check if the rail B7 is supplying clean -32V.

 

[Scott Muni]

Come to think of it, since it is on both channels, you need to check if the rail B7 is supplying clean -32V.

I really enjoy following along diagnosing problems. I try to think what would be the next step. My initial thought here was power supply negative voltage sag.
It's a good way to learn. and I have learned alot.

 

[bhairavi]

Come to think of it, since it is on both channels, you need to check if the rail B7 is supplying clean -32V.

Yeah the rail was what I was thinking about since both channels, and sag, but I've checked that a few times. Will check again. Haven't replaced the 8002/9002 on those boards.

BTW B7 is -38v not -32v, I believe that's an error on the schematic.

 

[JyriK]

Yes, looking at it again, B5 is +38 so B7 is most likely -38. Anyhow, check it with the scope.

 

[bhairavi]

Yes, looking at it again, B5 is +38 so B7 is most likely -38. Anyhow, check it with the scope.

Wait what will I even see with DC? Flat line, right?

 

[JyriK]

It's flat if it's clean.

 

[bhairavi]

Okay so it is NOT flat.



The negative side ripple filter is leaking somewhere... I think it's the transistor? Since C814 and C816 are filtering fine..

Red dots = sawtooth pattern from rectifier.



Stick a BD140 in there, I think, yes?

 

[JyriK]

Let's hope bd140 fixes it. If not, there might be something wrong with the main bridge rectifier (if all the caps are good).

 

[bhairavi]

Rectifier is new, but of course it doesn't preclude malfunction. I suspect transistor is bad and maybe related to the brief time when that filter cap was put in backwards. I already checked the filter cap and capacitance/esr tested good.

 

[JyriK]

A bad cdc9002 most likely then.

 

[bhairavi]

Replaced the transistor on the negative ripple filter. Checked the positive and the negative against each other. Now the positive filter was showing no sawtooth at all on the transistor pins. Took a closer look, one of the leads was broken. Replaced that one with a BD139.

I now have clean DC rails on both sides.

BUT still getting a wonky wave on both side at Tr801.



Took out the 220 ohm resistors and replaced the outputs.

Checked for sag - at very high volume voltage will at worst drop from 39 to 36vdc (transiently) on both filter caps.

Getting clean waves on the speaker outs BOTH CHANNELS up until around 1 o'clock on the volume, at which point I get this:



(and increasing volume after that makes that hump in the trough worse).

 

[JyriK]

That's progress!
What's the output when distortion appears? Is the rail B3 clean as well? How does it behave with high output?