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Sansui AU-20000 Muting Problem

Rotoflex4

Rotoflex4

Active Member
Hi everyone,

My AU-20000 has a muting problem and I'm hoping someone has encountered this before and might be able to help me focus my search for a solution. The muting switch has 3 settings: 0dB, -10dB and -30dB. The -30dB setting seems to function like it should. But.....there is no difference in volume when switching between the 0dB and -10dB positions. Based on what I'm hearing, it SEEMS like the -10dB position may be working correctly, and the 0dB setting is faulty (it is muting to -10dB regardless of whether I have it set to 0dB or -10dB).

Any ideas?
 
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The switch is selectable resistor pad, on the F-2502 board , switch SO-3 ( sections A-D ) It is the longest switch on that board and I THINK you can get to it by just popping the top off.
Try a spritz of Caig De Oxit.....
 
I have the front end of the amp all apart right now. With a little testing/probing, I can probably figure out whether the switch itself is functioning properly. If a little deoxit could fix things, that would be great.
 
I'm wondering if the muting relay may be the culprit. I'm referring to the HA-224N relay on the back of the F-2504 board. I haven't removed it from the board yet, but I applied 28 volts directly to it and don't get any click.
 
The muting switch is not a MUTE but a DIM and I wouldn't try to activate the relay IN CIRCUIT .. and its a reed style relay you won't hear a click
 
Thanks for the info regarding the relay. I wrongly assumed it would click like any others I've encountered. I might just unsolder the relay and test it on the bench. The muting switch looks a bit intimidating. A heckuva lotta terminals on its backside. I'll try a little de-oxit on it but probably otherwise leave it alone and hope for the best.

It would be nice to discover the source of the problem definitively before putting it all back together. I'm hoping not to have to take it all apart like this again. I think I'm pushing my luck a bit already.



vigman said:
The muting switch is not a MUTE but a DIM and I wouldn't try to activate the relay IN CIRCUIT .. and its a reed style relay you won't hear a click
Click to expand...
 
the mute relay has nothing to do with that... best to not touch it... no easy sub to replace it....
 
vigman said:
the mute relay has nothing to do with that... best to not touch it... no easy sub to replace it....
Click to expand...

Thanks for the tip. Since I have things apart I'm basically trying to test any components that are readily accessible in the hope that I find a faulty one. I'll take your advice and leave the mute relay alone. I'm hoping that the source of the problem is just a dirty switch. If it's not.....any idea what else it could be? Or....what board I should focus my efforts on?
 
This is a well made unit BUT if you do not have the skills & tools the BEST advise is to send it out for a FULL RESTORE..... at this age things start getting FUNKY.. and there is NO just do this one thing and it will be good for another 40 years.....
 
Yes, it's pretty much stock. Changes have been limited to remote mounted resistors on the power supply and Bourns trim pots on the driver boards. I bought it last year as a non-running unit with some evidence of sloppy repair attempts. I completely overhauled (recapped plus many transistors, diodes, relays, trim pots and resistors) all the boards in the back of the amp (F-2511 Protector Circuit Board, Power Supply Board, Driver Boards and the two boards within the heat sinks along with the output transistors). Also recapped a couple of the boards at the front that can be easily popped out (F-2505 and the F-2495). It now runs well, the bias is set and stable. The boards at the front of the amp look like they've never been touched. I find that I'm OK working on things as long as I go nice and slow, take lots of notes and pictures, do lots of reading, and quadruple check everything. My plan going forward is to recap those last few boards, clean the controls and connectors, replace a few transistors that have bad reputations (like the C1313 and A726) and try to spot any other possible problems while I'm in there.
 
Hi Retroflex
I as well have an AU 20000 with exactly the same Muting problem.
Would be great if you can elaborate a little on how this was fixed.
My preamp section is simply too noisy and does not deliver enough output v or the main

Greetings from Germany

Andreas
 
AKohlmeyer said:
Hi Retroflex
I as well have an AU 20000 with exactly the same Muting problem.
Would be great if you can elaborate a little on how this was fixed.
My preamp section is simply too noisy and does not deliver enough output v or the main

Greetings from Germany

Andreas
Click to expand...
Hi Andreas,

I'm sorry, but I never did learn more about this issue. If I ever do, I will post the info here.
 
Thx Rotoflex4
Does that mean your machine still has that muting problem or was it fixed and you don't know how ?
I assume nobody here has muting board fir sale ;)

Cheers Andreas
 
AKohlmeyer said:
Thx Rotoflex4
Does that mean your machine still has that muting problem or was it fixed and you don't know how ?
I assume nobody here has muting board fir sale ;)

Cheers Andreas
Click to expand...

It still has the problem. It had occurred to me that maybe they all function like that. But...I don't know.
 
If you take a look at the schematic for the AU-20000, and look at S03a-d (near F-2502) you will see that positions 1 and 2 of the switch are linked - hence moving the switch between 1 and 2 has no effect - these being the positions for 0dB and -10dB. However, I do notice S03e & f which are not described on the schematic (as far as I can see), and which are part of the same switch, I am not sure what function these switch contacts perform.
 
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Coming back to this after rotoflex reached out to me since I’ve repaired two AU20000’s this month.
After looking at the schematic I’mdefucing that the 10db attenuation resistor is internal to the switch.
Since you can be assured that switch is no longer available - unless it can be opened and repaired - I suspect the solution would be to figure out how to bypass the 10db portion.
 
For the sake of anyone who may be interested: I heard back from my tech guy this afternoon - he's managed to sort it all out. I wish I was technically proficient enough to understand his explanation, but it involved much more than just the switch itself. He believes that it was a factory flaw and that the amplifier was operating with insufficient gain, which in turn affected the muting function (if I'm understanding correctly). He said something about one of the muting settings relying on feedback to function properly. (I believe he said that the different settings accomplish the muting by different means). He says that my amp likely never functioned properly, even when new.....but now it does.
 
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