Sansui AU-20000 Interesting Fault

Hi there, I thought I would share this fault symptom and cure with everyone as it was kind of an oddball fault, it may be a 'one off' but to anyone that is currently repairing one of these monsters it could be of some interest.:yes: Its a long story, so I'll try and make it as condensed as possible so here goes...

Powering the AU-20000 up from dead cold, DC would sometimes intermittently appear on the bases of the output transistors, from the drive stage and if you weren't lucky this would generally kill the output transistors!:gigglemad

But if a variac was used to slowly bring the power on from cold, a reduced DC supply voltage sat on the base and then it would clear and the normal +/- 0.5v would appear on the output bases and the protection would release as it should, and the amp would run fine.
I then was able to turn the amp off and on again as many times as I pleased without the variac and the fault would not reappear. It would only happen 'once' from cold!

A can of freeze was grabbed so I froze everything in that area, driver boards output stage area and no amount of freezing could duplicate the fault. I also checked every supply rail voltages etc,etc in both states cold and after being powered on and off a few times after the fault cleared, everything appeared to be in order.

So I did the usual, caps on the power supply board and driver stage, as they needed replacing anyway and checked/replaced all the transistors on the driver board of the suspect channel etc,etc and those of you who have worked on one of these, know that those driver boards are a pain to get out being modular and hard wired together!

But the fault still remained, and after blowing a set of outputs due to this prick of a fault I wasn't game to power it on directly without the variac until I had narrowed the problem down.:headscrat

I thought I had found the problem as I directly froze the temp sensing/bias transistor thats clamped on the output transistor heatsink and it started to play up so I replaced it and the fault vanished.....for about a month...then it came back and of course another set of outputs went west.....

It wasn't until one day I had the thing running, can't remember exactly what I was doing on it, and accidently knocked that circuit board sandwiched in between the heatsinks and there was an almighty 'crack' from the workshop speakers and it went into protection mode, I lunged at the power, shut it down immediately and checked the outputs - they where ok-phew! I gingerly turned it on again with reduced mains this time and flexed that board quite firmly and got the fault to reappear.....

The problem turned out to be dry solder joints on the through holes on the double sided circuit board in the output heastsink, I also checked the tracks for hairline cracks as well, they were fine. As a precautionary measure I did the other channel as well, just to be on the safe side!

And so the story ends!

After exhaustive testing and cringing everytime I turned the thing on from cold, (fully expecting it to blow up again!) I can the confidently say the AU20000 is fixed, and it has been running now for close to year and it hasn't missed a beat since and it peforms really well sound wise, I actually prefer it to the AU-9900...

Out of all the Sansui's and Pioneer's that I've serviced over the years, this one was definetly the most challenging one I've come across, solder fatique can do some funny things huh?


Thanks for taking the time to read this 'saga!

Cheers!
Glen

Do you have some photo on the compensation transistor and heatsink. I think my AU20000 have same fault before.

Glenz75 said:

The problem turned out to be dry solder joints on the through holes on the double sided circuit board in the output heastsink, I also checked the tracks for hairline cracks as well, they were fine. As a precautionary measure I did the other channel as well, just to be on the safe side!

Click to expand...

Do these circuit boards have copper traces on both sides? Are the solder joints that are faulty, used to connect one side of the board to the other? If so, it's a very similar problem to what we see on the Dolby board of the 9090DB.

On a different note, It's good to hear from a Kiwi. My Daughter is living in Dunedin attending Otago University as an exchange student.

- Pete

Hi Pete, yes they do as there are tracks on both sides of the board and these through holes connect the tracks to each other. I was reading about those 9090db dolby boards, this is obviously a common problem with some Sansui models, at least with the 9090db its all low voltages and signals on that board, with the AU20000 its a different story, like a game of a 'Russian Roulette' with the output stage!

It also makes me wonder how many other 20000's are out there with this similar problem and there some poor bugger pulling his hair out in trying to fix it or given up and the ones that are still going fine, have this fault kind of lurking in the foreground, waiting for the right moment to catch its owner unawares!!

I guess its just a matter to time, would be interesting to see if anyone else has struck this problem....:scratch2:

There don't seem to be many NZ'ers/Kiwis on this site, I know there a couple apart from myself on here, I'm sure there are other NZ 'lurkers'..hehe
Been to Dundein only once, its a nice place, but cold and snow there as its our winter at the moment....

Cheers.
Glen

tkchoy2003 said:

Do you have some photo on the compensation transistor and heatsink. I think my AU20000 have same fault before.

Click to expand...

Hi, no I don't have any but will try and take some photos of the area soon, as I'm listening to a different amp at the moment and the AU20000 is not hooked up at present.
Cheers!
Glen

Thank you for that thread! I am an AU20K owner ( ) and will most likely need to at least look into that issue this winter while going through the amp.

Thanks again.

AU20K said:

Thank you for that thread! I am an AU20K owner ( ) and will most likely need to at least look into that issue this winter while going through the amp.

Thanks again.

Click to expand...

Hi, no worries AU20K, I'm glad to have helped, thats what this site is all about, helping and sharing the hobby/interest and passion. Is your AU20000 going at present or has it got a similar problem?
Cheers
Glen

I experienced the exact problem on 2 Sansui 20000 on I worked on some years ago.
SAJ

Glenz75 said:

Hi, no worries AU20K, I'm glad to have helped, thats what this site is all about, helping and sharing the hobby/interest and passion. Is your AU20000 going at present or has it got a similar problem?
Cheers
Glen

Click to expand...

Right now, my AU20K is in storage. I plan to get it out this fall and work it over. The last time I played it, it had the loud crackle/hisssss at all volume levels (even at no volume) when in the phono modes. So I am looking forward to replacing some transistors. I am not sure which boards drive only the phono circuit, but I should be able to come up with it. I am sure I have some schematics/service manual for it somewhere. I am also going to replace caps. It should be an interesting project, that and a turntable that is also in the winter queue.

Cheers

Hi AU20K, yep it would definetly be transistors causing noise in the phono stage, I've just been trying to think from memory where the phono stage is in there, from memory its one of the modular boards in preamp section behind the front panel on the right hand side looking down from the front of the unit.

While you're doing the caps have a look at ones on the power supply board, they'll be in need of changing also, but you probably already know this anyway!
The AU20000 is a fine unit and worth the effort. Keep me posted if you like on your progress with it, I'd be happy to help if you get stuck.

They are a bit fiddly in some areas, I know, I had to rebuild mine from a sorry state, lets just say the power transformer was loose inside when I got it, the previous owner had a go at it and thought the transformer was faulty because it wouldn't turn on after replacing the mains fuse, so he took it out and tried to get inside it by hacking at the pitch on the bottom!!:saywhat:

Keep in touch

Cheers!
Glen

saj888 said:

I experienced the exact problem on 2 Sansui 20000 on I worked on some years ago.
SAJ

Click to expand...

Hi saj888 ah, so I'm not only one who has struck this problem! This is good to know and to other Sansui AU-20000 owners! :yes:
Cheers!
Glen

Great thread.

Quintessential.

Thanks for the nice comments guys! I always like to write a good story....

Cheers
Glen

AU20K said:

Right now, my AU20K is in storage. I plan to get it out this fall and work it over. The last time I played it, it had the loud crackle/hisssss at all volume levels (even at no volume) when in the phono modes. So I am looking forward to replacing some transistors. I am not sure which boards drive only the phono circuit, but I should be able to come up with it. I am sure I have some schematics/service manual for it somewhere. I am also going to replace caps. It should be an interesting project, that and a turntable that is also in the winter queue.

Cheers

Click to expand...

Hello all,

i have purchase recently one AU20000 from USA (i'am in France).
I repair it, resistor R01 if f2511 board are broken after modification for 240v.

Amplifier works fine but, after 5/10 min left chanel "crackle/hisssss" are present, and left chanel no working propely. :sigh:
if i up the volume knob, le left chanel working fine after 3sec . But, after 5/10 min the problem are renew.

I reajust bias in left and right chanel, but, in left chanel, bias are not stable.... For 35mV, every 2/3 sec the voltage vary (from 0mV to 40mV) For me it's one capacitor in f2507 driver board. And for you ? any resistor in dirver board are open ? what is this resistance ?

I clean all contact and knob with contact cleaner.

Sorry for my poor English, but this forum is for me the last chance to repair it.

Thanks in advance for your assitance

David

Hi David, I will try and explain this simple as you said your english is not too good
I don't think it would be any capacitors on the F2507 driver board, it is probably a faulty transistor on the driver board, I had a very similar problem on my AU20000 while I was fixing it and it was one of the driver transistors Tr10 and Tr11 but it could be another transistor also.
The problem could also be a bad connection or bad solder joint somewhere. Try moving the board in place when the problem shows and see if it goes away. Hope this helps you.

Cheers
Glen.

Hello,

thanks for this information ! :thmbsp:

The problem:
after 5 minutes of use, the left chanel begins to saturate.... I climbs sound, crackle disappears for 5 minutes and again. In sum sound, it disappears again and then 5 minutes after etc etc etc.

Very difficult to fix this problem.

the next step is to control all parts in driver board !

thanks for your help

David

Hi David, thanks I'm happy to help you out.

Some other things to check, make sure you have the correct power voltages to the driver board, you can measure +65 volts and -65 volts on the socket pins that the driver board plugs into underneath, but be careful not to short the pins out otherwise you will cause damage, if those voltage readings are correct, then the power supply is working ok. I think the power supply is fine, but it pays to check just to be sure.
Also check the +35 volt and -35 volt voltages from the power supply board F2508 you can measure them on the socket pins underneath as well.

The output transistor heatsink has a green circuit board in between, that could also have bad solder joints, could be causing the problem too, try moving that board when the sound begins to saturate and see if it changes.

Just had an idea, you could try cooling parts on the driver board with Freeze in an aerosol can, your local electronics store should have this stuff, that might make the problem show up. The Bias adjustment VR02 maybe faulty or dirty.

If you do have to take the driver boards out, and they are not easy to remove, change the electrolytic capacitors on the board as well, they will be needing replacing by now.

There are some other people who may be able to give you some advice also on here.

Let me know your progress and good luck!

Cheers!
Glen

Hello all,

i'am change all capacitor in driver board, and i'am replace the 2sc1669, and amplifier working fine !

Thanks all for your help !

David.

Great stuff David!:thmbsp: Good to know you have fixed the problem. Now sit back and enjoy it!

Cheers
Glen