[Sansui AU 101] Loud crackle & low volume on right channel

[Insomniac94]

Hi, hope you’re all well. I’m new here & would really appreciate some help here as a novice to vintage amps.

My lovely Sansui receiver was working wonderfully the past 3 years until the right channel suddenly developed loud static & low volume. Left channel is working fine. I swapped the speakers around to confirm the issue lies within the amp.

I’ve opened up the case & noticed nothing out of the ordinary (no leaking/bulging caps, no damage to boards which are also clean, etc. All original components are still in place.

I’ve tried:

• Deoxit on the pots & switches, working them back and forth.

• Swapping the left & right input transistors (TR601/602, TR603/604, TR 701/702)

• Swapping the signal-path capacitors (C705/706, C711/712, C801/802)

This had no effect whatsoever.

Waiting for my multimeter to arrive to start diagnosing. Where should I begin look? Should I try swapping other components between the left/right channels?

A quick google search shows either to replace input transistors or recap the entire thing. I’d like to diagnose it properly rather than throwing random parts at it.

Cheers!

Here’s a schematic if it helps.

 

[idf.bts]

Even though a component may visually look ok, it may not be.

I would avoid doing any more intervention until you understand the problem better. This sounds like dirty contacts on the input or one of the face plate pots. You may go down a rabbit hole diagnosing a problem and end up damaging the amp with the tinkering. If you have swapped caps, then it could be a signal path transistor, or an op amp has gone bad.

Did you clean the contacts on the back panel? I didn't see you mention it, that could be the problem.

Mostly, the advice out there eliminates 80% of the problem points, and is routine maintenance anyway. Also, a meter hits its limits and you will need an oscilloscope. You will have little chance of diagnosing a cap properly with a standard handheld meter, and those little transistor test tools are neat, but are useful more to confirm a problem than find one.

 

[cademan]

Most likely a pre driver preamp transistor on the bad channel main amp board.

 

[Hyperion]

Swapping the left & right input transistors (TR601/602, TR603/604, TR 701/702)

Don't bother to swap them, just replace them, with approved replacements. You are doubling the amount of 'PCB track trauma' by swapping rather than replacing.

It will very likely be noisy transistors however.

 

[Insomniac94]

Don't bother to swap them, just replace them, with approved replacements. You are doubling the amount of 'PCB track trauma' by doing this.

It will very likely be noisy transistors however.

I’d expect the issue to transfer over to the left channel after swapping the transistors over. However, I’ll take your advice and replace them as they’re pretty cheap & see how that goes.

 

[idf.bts]

I’d expect the issue to transfer over to the left channel after swapping the transistors over. However, I’ll take your advice and replace them as they’re pretty cheap & see how that goes.

You're correct, but I think you swapped power transistors only. There are transistors in the signal path that are the likely problem you have not touched yet.

 

[Insomniac94]

You're correct, but I think you swapped power transistors only. There are transistors in the signal path that are the likely problem you have not touched yet.

I’ll try replacing them too and see if has any effect.

Also, I’ve cleaned all the contacts on the back panel too which unfortunately had no effect.

 

[idf.bts]

I’ll try replacing them too and see if has any effect.

Also, I’ve cleaned all the contacts on the back panel too which unfortunately had no effect.

I want to emphasize being careful not to touch too many things. It's really easy to damage a trace and make all measure of extra work for yourself. Even worse is introduce a new problem you can't see with the naked eye then all your results make no sense now. With a 2 channel scope, you can probe the left and right equivalent leads off the transistors and compare them. This process is faster and less intrusive, and also a lot more learning is done this way. Its not for everyone to own a scope but you seem to have the right level of interest. Get an old analog Tektronix, it's fun when it works.

 

[Insomniac94]

I want to emphasize being careful not to touch too many things. It's really easy to damage a trace and make all measure of extra work for yourself. Even worse is introduce a new problem you can't see with the naked eye then all your results make no sense now. With a 2 channel scope, you can probe the left and right equivalent leads off the transistors and compare them. This process is faster and less intrusive, and also a lot more learning is done this way. Its not for everyone to own a scope but you seem to have the right level of interest. Get an old analog Tektronix, it's fun when it works.

I’m not new to soldering as I’ve built quite a few custom consoles, keyboards & have redone the whole wiring system on my old Austin. Leaded solder used throughout. Use of a desoldering gun sure makes easy work of it

However, I understand where you’re coming from. If replacing the transistors doesn’t work, I’ll definitely leave it aside until I can get my hands on an oscilloscope to diagnose and carry out the proper repair.

 

[Insomniac94]

Found the the culprit! Issue lied within a 2SC1364 transistor, swapping TR803/804 transferred the issue over to the left channel.

Can anyone recommend a decent equivalent for 2SC1364 & 2SC871 (might as well replace them all) which I may be able to find in the UK?

I have some 2SC1815 laying around, would they be up to the job?

Cheers!

 

[idf.bts]

Seems like there are originals still out there, here is an example.

https://www.ebay.com/p/694053687


An old thread here discusses replacements. I have not read any datasheets mentioned, but this looks like a pretty easy to match.

https://audiokarma.org/forums/index.php?threads/looking-for-2sc1364-transistor-substitute.498346/

 

[Insomniac94]

Seems like there are originals still out there, here is an example.

https://www.ebay.com/p/694053687


An old thread here discusses replacements. I have not read any datasheets mentioned, but this looks like a pretty easy to match.

https://audiokarma.org/forums/index.php?threads/looking-for-2sc1364-transistor-substitute.498346/

It’s fixed! I’m getting clear even audio on both channels now. Replacing with 2SC1815 sorted it out.

While I was in there, I also readjusted the dc bias offset to just under 20ma on both channels.