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Sansui AU-919 problem - oscillation

2

2000zx12r

Active Member
Hi there,

New to the area!!

I recently picked up an AU-919 that seemed to be in reasonable shape. Anyway having now cleaned it I started to go through the signal path and have discovered that the left channel appears to oscillate under certain conditions. I have traced it down to the "Flat amp". When the "Jump" switch is engaged the flat amp is bypassed and all seems well. The situation becomes a little worse when the tone controls are engaged - since these are wrapped around the flat amp so this seems logical.
I have taken the board out, cleaned it and resoldered all joints. There was a lot of gunk and crap on the board. It appears a little better on the oscilloscope but not 100%.
I was hoping somebody has run across this and has some pointers. Next steps may be replacing some of the caps and such.

Cheers in advance!!

Steve
 
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A big welcome to AK !!!

I am pretty sure I am right in saying that the culprits are the very low value bypass capacitors - I have not seen a schematic, but 9pf and 3pf are values that come to mind having read others experiences on AK with the AU-919.

These can be replaced with silver-mica equivalents (change them all !) - AK member EchoWars seems to have had the most to say about these. Maybe he will chime in and advise you in depth, and give you supplier details for the ones he uses, his advice is worth waiting for. There are many other truly excellent tech's here on AK who could also advise.

You could use the AK (or Google) search facility to see if you can find out more.

Gonna get me an AU-919 one day...
 
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I have had similar problems with the phono preamp in the AU-X1, where it would oscillate if it was loaded with the next stage of the amp, or a scope probe. Sometimes just installing a 1K resistor in series with the output of the stage in question, is enough to isolate it from the capacitive load and it stops oscillating.
 
Thanks for the input. There are several low value pF caps in the range of 3pF - 5pF etc mainly used for compensation in base to collector spots. I am about to order some mica replacements shortly. I agree interstage coupling can be a problem but only one channel exhibits the issue and there is also a 100 Ohm build out in series right now. I will also check the bias points and compare good to bad channel to see if anything else is going on.

Thanks again!!

S.
 
The 919 will never be a reliable piece until all the Black Flag ceramic caps are discarded and replaced with new silver mica caps. Sansui used them on nearly every board in this amp, and they will eventually contribute to the destruction of those 'impossible to replace' output transistors.
 
Hi there and thanks for the input. I see all of the "black flag" caps. They appear to be on both sides of the board. Looking at the service manual it does not indicate the use of ceramic caps on the flat amp, mainly lytics, styrene types. Am I missing something?
Thanks
Steve
 
So the plot thickens. Last night I took a number of measurements comparing both channels of the flat amp and it appears that on the bad channel the output stage bias if not right. The base to base total voltage of the output transistors appears correct indicating that he double diode and series R are fine but the voltage id offset with 0.4V on the top device and -1.4 on the bottom - so there is no bias in the output stage. As compared to +/- 0.9V on the good channel. More work to be done to find the culprit.

Steve
 
That indicates loss of drive on the positive side of the amp. Possibly an open resistor or a transistor that is no longer passing current from Collector to Emitter.
 
Problem solved. Nothing to do with the Black Flag caps even though I replaced all of them on the Flat Amp board. Turned out to be a broken - open - pnp transistor in the output stage of the Flat Amp. It was interesting since it still had output, albeit no right. The feedback was confusing the trouble shooting. Anyway some new Fairchild equivalent parts and we are good to go. Channels balanced all the way to 100kHz+. Now on to going through the rest of it as time permits!
Cheers for the input.

S.
 
dr*audio said:
Can I call 'em?! Glad you got it fixed!:thmbsp:
Click to expand...

You sure CAN call 'em :thmbsp: - you were spot on with my AU-9900 too :thmbsp: :thmbsp:

Well done 2000zx12r ! - it is particularly satisfying when you can land right on the problem :thmbsp:
 
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