SANSUI AU 7900 WOES...

STR-6046A

STR-6046A

Member
I got this AU7900.
when I turned the thing on it just has a light that comes on , no clicking or anything. The PO told me that he 'took the lid of the relay and jumped it to see if it's the relay. there was sound in one channel and then later there was none'.
So far I replaced all the caps on the main board , the caps on the equalizer board. I checked the output transistors that seem ok. I took them off the circuit and tested them. and then I put them back on the board. Right now I'm in the process of replacing the caps on the tone control and filter unit circuit board. then after that I don't know, if it doesn't work.any hints?mess.jpg mess1.jpg
 
Thank you,
as for now I am having not just trouble getting it to run, but also rouble getting the parts here. I can't believe that with EVERY order from mouser there is something back ordered, I am waiting on some parts that I have ordered a month ago, and the lead time is another month. It's very annoying to have 10 ordered items still floating around somewhere, not just the fact that my projects are stalled until then.Anyone else had that experience?
 
The AU-7900 is a great amp. Well worth restoring. If the protection relay is not turning on, jumping it is a bad idea. 90% of the time, if the protection relay is not turning on it's because the protection circuit is working perfectly and it needs to be in protection. If it is in protection, the fault is in the amplifier and it's unlikely to be caused by a capacitor. With it apart like this, you are not going to be able to do much diagnosis.

While you are waiting for parts, please post the serial number for the database. Click on the link in my signature block.

- Pete
 
When I had the same problem with mine it was a bad 10D1 diode in the rectifier bridge. Upon close visual inspection it looked burnt. I always replace 10D1/10D2s on sight with 1N5393 now. Cheap and easy piece of mind.
 
OMG.

Where to start?

We fix before we "restore"

There was a problem in the amp or the protection circuit, which may be part of the amp.
Now? Who knows?

Since you now have trouble even getting it to run, something you did has made things "worse".

The previous owner, hacking the relay wasn't a good move, but the relay is there to protect the speakers when the amp blows.

Mouser may be out of a particular part number, but there are always many other parts available, especially for e-caps. Bulk versus tape, Nichicon versus Elan versus Panasonic versus etc.


I guess while you are waiting for parts, you should fix the amp.
Find the parts that are broken, replace them with available new parts.
The smart move is to test all the transistors and diodes on the amp. That is usually the first parts that fail.
Then checking all the resistors around the failed parts found is smart.


If all the other boards are in working configuration (all the parts installed), hang everything back together and see what is working and what isn't working.
 
I agree with you blhagstrom . I went into the amp thinking the capacitors were the issue. It was afterward that I learned from the PO what he did with the protection relay.I got it not running to begin with , so I couldn't have made anything worse.

I replaced many of the capacitors , replaced the VD1212 with a series pair of 1N4148 and replaced four 220 ohm fuse resistors.
 
Assuming your work is proper and well done, work towards getting things all hung back together.

I would work through the amp transistors.
You checked the outputs, now go for the drivers, pre-drivers, etc....
The service manual diagram should indicate who is what. Working from outputs back through the signal path is my method, but I usually end up checking them all anyway.
Usually, amp problems that cause relay protection are in the final stages of the amp, so I tend to start hunting there.

You can do a diode test in circuit and only pull those that seem to fail.
An "octopus" on an o-scope is much handier, but if you had that, you'd be done by now.

You can diode test the diodes and only lift one leg on those that seem to fail.

Running through the resisters on the amps in circuit is a great way to pass the time.
Comparing side to side on ones that are way off the mark helps against in circuit influence.
Actually, comparing side to side is quick and dirty and only lift a leg on those that don't match. Blowing the same resistor on both sides would be rare.
IF you find a pair the don't match in circuit but are both fine out of circuit, there is something nearby on the circuit that is not matching. It's a hint.
 
STR-6046A said:
I agree with you blhagstrom . I went into the amp thinking the capacitors were the issue. It was afterward that I learned from the PO what he did with the protection relay.I got it not running to begin with , so I couldn't have made anything worse.

I replaced many of the capacitors , replaced the VD1212 with a series pair of 1N4148 and replaced four 220 ohm fuse resistors.
Click to expand...


Capacitors rarely cause "not working" conditions.

To ANYONE reading this. Re-capping is NOT a repair technique.
I am hearing from techs that say they are seeing units where a re-capping was done to try and fix things.
There are a handful of things we directly suspect bad (poor) caps of causing but its more than likely that units not working, at all, is not a capacitor problem.

That's said, on a situation like this. WHILE working on the amp to determine where a problem really is, IF you have access to the board anyway, replacing the e-caps is fine while repairing.

We really prefer to find the failure before making changes.
Even the best make mistakes and adding a mistake to a problem can really cause headaches.
 
Interesting discussion as I have an AU 7900 I was thinking of recapping. It has developed a low level hum in both channels (volume at max before noticeable) that has started me down the road before fully understanding the source of the problem. Maybe time to rethink the situation before tearing the beast apart. It is a wonderful amp that I would hate to destroy.
 
STR-6046A said:
I can't believe that with EVERY order from mouser there is something back ordered
Click to expand...
I have this with element 14 - and you need to pay extra for them to ship to NZ. When you order items they DONT EVEN TELL YOU that the item is backordered! Pretty annoying.

Agree with blhagstrom here - you definitely want to go "softly softly" when fixing an amp. Diagnose and fix the fault first, then restore once thats done.
 
Childof60 said:
Interesting discussion as I have an AU 7900 I was thinking of recapping. It has developed a low level hum in both channels (volume at max before noticeable) that has started me down the road before fully understanding the source of the problem. Maybe time to rethink the situation before tearing the beast apart. It is a wonderful amp that I would hate to destroy.
Click to expand...

THIS sort of issue is more likely to be old capacitors.
Namely filter capacitors and or a tired bridge rectifier.

Not to hijack the thread, but...
DO you have any phono shorting plugs?
Plug those into AUX, set the unit to AUX. Try max volume.
IF no hum, no real problem, just too much gain.
 
@Childof60 you'r problem has nothing to do with mine.

Everyone my problem has nothing to do with caps. The amp wasn't working before I got it. We got that figured out already.
This thread is NOT about wether or not you should recap, it is about what could be wrong here.

every transistor , diode, resistor I took out checked out ok... at least not broken.
powerfuse f701
ac fuse 601 to 604
TR01 to TR04 TR 604 TR605 TR9 TR11 Tr13 are all good
I can't find TR16
 
STR-6046A said:
it is about what could be wrong here
Click to expand...
Is kind of hard given the amount of info.
Lets forget the recap, unless you see an obvious damaged capacitor.
Try to focus on one section at time.

1. Read carefully the page 11 of the service manual.
2. Check all semi conductors first on F-2596.
3. Report, don't worry you are in good hands here (a lot of kind people willing to help)

I forgot... use a DBT, and check D603 to D608
 
Last edited:
Hi, just my two cents worth, I received an AU-6900 stuck on protection circuit, followed this guide: https://www.audiovintage.fr/2015/08/09/sansui-au-6900/

and boom unit came to life. I hope that helps, however, if anyone has thoughts on my current issue:

I was given this amp missing 3 x NEC B545A output transistors. (I assume the previous owner thought they were faulty).
With the knowledge I had I replaced those with NTE88’s.

My current issue is that after all of the parts replaced and the unit now working, the output, even through the headphone jack is distorted. (Both channels but more on one then the other)

Many thanks
 
Leaving recapping out of the equation for a moment, how about re-transistoring? These old units often have problems with transistors having the black leg syndrome. The leads of the transistors get corroded, and that corrosion creeps up inside the transistor and makes it noisy. Unless it fails completely you're left with a unit that pops and crackles, but the transistor may test okay.

My own AU-5900 had this problem, causing it to go in and out of protection. Replacement small transistors are very cheap, especially if you buy a hundred if you plan on doing this kind of work for a while.

Lee.
 
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