Wanted Schematic (help too) Sansui Au-101

[Raphael Shaw]

I have very little experience with solid state amp cicuits (I usually play with valves), but I am trying to fix a friends amp anyway.
It's a:

SANSUI AU-101

The problem was some distortion in the right channel.
I have narrowed the problem down to the power amp section, which thankfully looks very simple, and is on a seperate PCB.

As far as I can tell, the electrolytics are fine.
I dont know how to test the transistors properly, but they also seem ok.
Could the problem be in the power supply (which is on the same PCB)?

I think I could get along ok if I had a schematic, but I can't find a free one on the net.

Any help or tips will be apreciated, as I said I'm no expert.

[Raphael Shaw]

A have been looking around a lot, but still no luck with a schematic.

However, what was interesting, was that most of the sites that I did find which related to the Sansui AU-101, were all compaints of the same, or similar problems.
Also, none of the advice given to these people on the net seemed to help in the least.

I think I may have picked up on some sort of decieving symptom. It's a problem which seems sympler than it realy is.

I must have visited at least 7 or 8 sites, where poeple complained about distortion in the right chanel. Though aparently no other problems.

Have any of you perhaps done any repair on this amp, or even encountered this problem? any feedback will be apreciated.

[Netlist]

Originally posted by Raphael Shaw

I have narrowed the problem down to the power amp section, which thankfully looks very simple, and is on a seperate PCB.

As far as I can tell, the electrolytics are fine.
I dont know how to test the transistors properly, but they also seem ok.
Could the problem be in the power supply (which is on the same PCB)?

Raphael,

I don’t think it's the power supply. Both channels would be affected. What makes you think it's the power amp? Also, don't assume the electros are fine, the amp is too old. I think mid '70's.
Same goes for the transistors. Measuring them is the only way to know and even then they can be faulty in-circuit.
So, not an easy task but lets try anyway.
Could you make a small list of the transistor numbers on the output board?
I' asking because I might have a similar schematic at hand.

/Hugo

[Elso Kwak]

My routine: replace drivers and output transistors, especially when the amp is distorting while it is hot. I used to burn it hot full power on 4-Ohm loads.
Of course all lytics are suspect at that old age, just as me, LOL

[anatech]

Hi Raphael,
I do not trust the small electrolytics at all in those older amps. They are not expensive, so just replace them in the amp section while you are getting the numbers for Hugo.

If the unit has pre out and main in jacks, substitute another preamp output into the amp. You may be chasing a fault in the wrong section.

Some 2SC458's were bad production runs (small signal transistor). Watch for those.

-Chris

[Jonathan Bright]

Raphael, I emailed you last week but apparently it didn't get through. But if you send me your postal address I'll send you a schematic. (I don't have a scanner) My email address in "Members" should work.

[Raphael Shaw]

The main transistors are C1986's
others on the circuit are C1364, A640, A678.

I suppose the codes might be old, and have equivalents, that are currently available.

I'm in a rush now, but I'll go through your posts again and post again.

[Raphael Shaw]

The amp doesn't have pre out or main in jacks, but there is only one pair of screened wires conecting the pre amp board and the power amp board. So I un-soldered them, and tested the pre out with another amp, It's fine. I also fed another signal into the power amp, and I get the same distortion. So the problem is definately in the power amp.
Thanks for the tip Chris.

I hope my previous post makes sense to you hugo.
The main transistors are C1364's. There are four of them, and the middle leg is lifted (not used). One of them looks a little different which makes me think it might have been replaced before.

There are also quite a few C1364's around the circuit, and two A640's.

The A678's I mentioned, are in an odd shaped green package, but still resemble transistors, but I've never seen them before.

Jonathan, I did get your e-mail, and I have e-mailed you my postal adress.
Thanks a million. I hope it works out.

[anatech]

Hi Hi Raphael,
Okay then. With no speakers connected and no signal into the amp, please measure the voltage across the emitter resistors in the defective channel. I'll assume the DC offset is okay as you didn't mention any speaker thumping.

-Chris

Edit: to correct choice of words .....

[Raphael Shaw]

Please excuse my ignorance, but I'm not sure exactly what to measure.

Should I measure DC from the emitter leg of the transistor to ground?
Should I measure AC between the two emitter legs?

Sorry, my terminology is not up to scratch.

Also, What is the the pin out on these things?