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SANSUI AU-7900 PROTECTION PROBLEMS

Subchannel

Subchannel

Active Member
I have restored and recapped a Sansui AU-7900 which I bought with a problem.
After a few minutes the amp clicks and sound is shut off. the power indicator stays on.
Turning it off, wait a few seconds, and turning it on again helps, and it stays on.
The old C701 (15000uF 63V) measured 0.2uF so strange it worked at all.
I have replaced all electrolytic capacitors with new ones. Every switch and potentiometer is thoroughly cleaned.
Removed solder and resoldered all power transistors pluss many other solder joints on PSU PCB.
Protection relay is opened and contacts are cleaned.
The amp has been playing non stop since recapping (2 whole days), but this morning the same problem appeared, a click and no sound. turned it off and on, and sound is back.
I´ve read that some components in the PSU (other than caps) should be replaced, but I´m a bit lost here. Anyone with experience on same problem?
Thanks!
 
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So you mean that protection relay clicks. First of all you need to measure DC offset when it happens. Also this unit has fusible resistors and in your description above it is not clear if you replaced them(important to do so). Also this unit has infamous 2SA798 dual transistor on the input of power amp section ( or call it Driver board). It is advisable to replace this dual transistor. But first we need DC offset numbers when it happens.
 
Thank you for the info TupolevFan. It will not be easy to measure DC offsett, it goes into protection after 5-15min, and to get that problem to repeat itself I have to for wait hours. As mentioned, it played perfect the first day for 4-5 hours, then yesterday I turned it on in the morning and it played until the evening, no problem whatsoever.
It was this morning it went in protect, but only once. I will try to do what you recommend tomorow.
I have not replaced the fuse resistors, neither the rectifier diodes or transistors, only electrolytic caps.
In lack of knowledge, where to measure DC offset?
Thanks again!
 
DC Offset in your unit can be measured between the ground and either side of R73 for one channel and R74 of the other channel. So locate R73 and R74 on board F-2596. By the way dual transistors i was talking about are also located on the same board (IC01, IC02). Fusible resistors Also on this board and those are R31, R32, R41,R42 replace with this : https://www.mouser.com/ProductDetail/KOA-Speer/MF1-4CCT52R2200F?qs=KVzGhl/cVV1aGF8Vc/%2BvDg==.
I suspect that your issue also related to heat meaning you probably have a bad transistor and that leads me to believe that it is related to IC01 or IC02 .
 
Regarding IC01/ IC02, it´s not esy to find this part. do you have experience with two single transistors? (ref pic).
I´ve not been able to measure DC offset, the error occure very random or not at all. I figure the best way is to replace the components (resistors/ transistors)?
 

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The resistors you recommend from Mouser are not Fuse Resistors and 1/4W, it´s ok to replace with these?
Thanks again.
 
Where are you located? I have replacement for your dual transistors. it is modern Toshiba chip installed on adapter. Can supply you 2 of those.
 
Subchannel said:
I live in 1672 Fredrikstad, Norway.
Click to expand...
OK I can send to you 2 adapters with the chips installed on them if you are willing to pay shipping fees. It will be easy to install those. why i send 2 ? Because you should change both. Registered mail takes about 2 weeks from USA to Norway. ( 10 business days). By the way I am ethnically Russian but was born in Riga, Latvia :)).
 
Dear Audiokarma:
After recapping, cleaning, adjusting and replacing the bipolar transistors in the Protection circuit (thanks for sending them Tupolev) and the belonging resistors, the AU-7900 have been playing fantastic ever since.
But now the problem is back. The amp can play without any issues for days, then suddenly cuts audio out. As before, the amp is still on but no sound. Turning the amp off and on again solves the problem, and the amp can again play for days.
Another issue that´s new is white noise that is getting louder after a while. This is not affected by any of the controls, so my guess is that it´s before the preamp.

Any suggestions are welcome.
Thanks!
 
Probably nothing to do with your issue, but on my AU-7900 I had a similar issue which was solved by re-flowing the solder on the two coils near the relay. There must have been a crack or something.
At the same time I re-flowed all solders in the vicinity of the relay
 
Thanks for the tip Dom123. I have re-flowed almost all solders, but maybe I didn´t do all points around relay. I´l try that as well.
 
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