Got it working...kinda

Looks like you skipped step 1.. WHOOPS.
1) From your audio source.( start with the volume way down ) . plug that in to AUX-1. Should have made it clearer turn the COMPUTER source WAY DOWN...
And this PROVES Many Moons point…. Something is fubar on the AUX ( and or other ) jacks on the back…..
 
Remember this?

I have this AU-7900 for repair now, sent to me by its owner - the reason for the extremely low output from one channel is a mysterious ~15Ω resistance appearing across the signal input when certain inputs are selected.

I haven't even powered up the amp yet, I don't need to with this fault :)

Investigations continuing...
 
Watching with interest.......

I have a 7900 here at the moment, let me know if you need any comparisons done or photos etc

Cheers

John
 
That's OOOOODDD ?

Indeed - I was measuring 'in line' with the signal, expecting a 'break' somewhere and finding no problem. So imagine my surprise when measuring across the signal path to find one channel measuring 40-50K, and the other ~15Ω.

There is no damage to the input sockets, although I am wary of what looks and feels like like Sansui glue present in the phono socket holes in the PCB. I have isolated the 'inner' on 2 of the sockets and checked inner/outer resistance - all good.

Watching with interest.......

I have a 7900 here at the moment, let me know if you need any comparisons done or photos etc

Cheers

John

Thanks John - I might need some assist like that :)

I have determined that the 2 rotary input switches influence the appearance of the fault, but no other switch or control makes any difference to it. Movement of the volume and/or balance control affects the reading slightly on the good channel.

This is going to be a LULU... :D
 
Yes it does look completely untouched, it's actually in very nice condition - peachy.

Pins 31/32 is where I have been monitoring - I forget which one - but that's where the 15 ohms appears.

I'll be having another look tonight, I will isolate that part of the circuit (switching & controls) and start lifting connections.

Gut feeling at the moment is a (partially) shorted signal cable... but it may turn out to be stranger than that ;)

Thanks Mike - (it is Mike isn't it?)
 
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Yes John… it's Mike….
So your looking at the system from the tail back of the 2599….15 ohms that is so strange.. just like the issue the person TRIED to fix on my BA-2000…..

Mike
 
Well at least I have it now and feel I have the necessary skills to find it, and fix it properly :) :yes:

That 15 ohms varies a little +/- 0.5 ohm - and a couple of times I thought I detected it disappearing momentarily as I touched and pressed things, we'll see :yes:
 
Tape Rec 1 socket ! Had something (tin whisker?) shorting the inner and the outer, after removal and cleaning we are good.

I checked all the other sockets - nothing.

Now to put it back together again set it up, and have a listen. :)
 
I got it all back together again, I changed the mains lead at the request of the owner, then powered it up on the DBT - all good.

I adjusted the bias and watched it track - also monitored the DC offset and PSU voltages - all good. Powered it off and let it cool down, then powered on again and watched the bias again - just to see if the setting was correct.

It does have a good dose of 'the glue' on the main board, so I have the go-ahead from the owner to get rid of the glue and perform the usual maintenance tasks, and include a re-cap as well. I have just ordered the parts which should be with me early next week.

Listening tests confirmed what a really nice amplifier this is, and it looks fantastic too. The owner very much wants to keep it, and wants it to be insured as much as possible against future failure.

That's all for now. :thmbsp:
 
I have had this AU-7900 for a while now, and the recondition is progressing.

The thing I was most concerned about was de-soldering the precious original output transistors to get some thermal compound underneath them. This went well, and to help alleviate the heat I used LMP silver solder to limit the heat as much as possible when re-soldering afterwards. I reassembled and tested it with no problems at all.

But disaster struck as I was manipulating the chassis of the amp, during re-capping the tone control board, I cracked one of the treble switch wafers :thumbsdn:

Anybody who has worked on one of these will know what a disaster this is, as there are tracks and resistors laid down on the switch wafer, for the multi-position switch to make contact with to alter the tone in steps. The switches seems to be unobtanium, except from a unit being parted out (rare) and they do not seem to be commonly used across the range.

A glimmer of hope
The part number seems to be 1090060 - and I can find this P/No at Sansui Parts Shop but it doesn't look like the ones I have in this unit - any information on this anybody? The ones in this unit have have 'open' wafers - the one at Sansui Parts Shop is a closed type - is it the same?

I have informed the owner who is understandably very concerned, and I can't tell you how annoyed with myself I am for making this dreadful mistake.

Any thoughts anyone?
 
Have you tried sending Nicowico (Nies) a P.M.? He can likely confirm if the picture is accurate.
 
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