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My Sansui playing up?


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I turned on my AU3900 early this morning for some easy listening. There was a strange crackle coming from my ARs. So I investigated the signal chain. switching them on 1 at a time but crackled every time. Finally turned on just the amp alone and still there. No signal at all going through but some kind of voltage at the output I suppose or the speakers. Wired the ARs to my quad, not the speakers. The poor Sansui is playing up. Damn. Does anyone know what it could be? It did have audible hiss since I had it but I never used it for critical listening just easy leisure so noise floor and dynamic range really wasn't an issue at the time.

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You called.......

Sorry to hear about your troubles with your AU-3900. Crackles, pops and hiss are common problems with a lot of vintage gear, and Sansui is no exception. It could be just that the controls and swtiches could do with a clean, but it is more likely that the are one or more small signal transistors have gone noisy. If so then the only recourse is to replace these, and there are modern equivalents available that do the job nicely. These are full of 2SA726 and 2SC1313 transistors which are know noise makers, and it could also be the differential input pair at the input of the main amp - 2SA798 two transistors in the one 5 leg package.

Are you handy with a soldering iron?

Cheers

John

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You would not believe it. I opened the chassis. Visually looked good apart from some dust. There are no burns and no smell. I removed the front fascia and removed the 2555 and 2554 PCBs. Bagged and tagged all the screws. Called it a night and while I had my glasses on, for the first time I noticed a small amount of Gaff tape on the power cord. I never saw it before. It was really neat as neatly wrapped. I un wrapped it and under the gaff was white lecky tape. I thought, here we go! Yep, it was twisted and joined together in an attempt to replace the plug I guess. No solder or at least heat shrink! The ground part was loose. Now I recon it was the problem. "Some stupid with the gaff tape could have burned the place to the ground" or killed me. It suffered a little hiss so while I have it apart, I might as well attend to that as well. Holy sh&T BATMAN! How did I miss it? Going to bed with SMOKE ON THE WATER on my mind. At least it aint in my lungs. I'm a bit ADHD tonight.

Edited by Quadmad
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Hi  Jessie

 

this is my first ever , post on  this  site     , FYI   , I accidentally   discovered  your post  .. looking for preamps

 

  I hope this helps

 

I have a soft spot  for  early model  Sansui Amplifiers ( Sansui  new how to design amps )  ,  I have owned  used  this model in my Work shop    and serviced  a lot of   amps  in the past .. when I had a service center /Hi Fi Shop  on the Gold Coast  for 15years

 

What john said above   is  correct  and good advise

 

I hope this  information  also  helps   ...    from  many personal experience  in the Service Game   , regardless  of what other faults you may find ( noisy switches , dry joints    caps   etc etc  )   ,  I   recommend   replace  the 2SA798 two transistors in the one 5 leg  transistor package  ,   do  this  in both   L&R channels   as a no brainer   preventative  ..  . this will elimitate a lot of  misleading faults

 

2SA798  can and will break down internally     , causing   intermittent     crackling noise  ,  , intermitt , DC offset  in the o/p transistors  , activating the protection network   , when  unit  ( amp ) is either  hot or cold  

 

2SA798   are very cheap  on Ebay   ..  and worth the exercise replacing    this will  prevent accidental damage to your speakers , if the  Protection network  fails or is not quick enough .

 

Dont expect a quick response to this as i don't normally  go near this site very often  . Email me if you need more help 

 

Kind Regards

Russell Storey

 

Stones Sound Studio   (Australia)      

Audio & Acoustic Consultant Engineers

http://www.stonessoundstudio.com.au/

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Hi Jesse,

Russel gives some good advice re replacing the 2SA798's in any case as a matter of preventative maintenance. Re buying these from EBay, you have to be careful where/who you get them from. China and Hong Kong suppliers can be a bit hit and miss, and they are available in Sydney cheaper than you will buy on eBay, and I have some stock of these.

There is another thing that should be attended to whilst you are in there. Depending on the build date of your particular unit, it may have what are called fuse resistors (theses are fault protection resistors that fail gracefully if there is a fault in the driver section of the main amplifier, and these are known to drift high in value, and even going open circuit. The amp will still operate with these open circuit, but out will have low to no bias current with potential crossover distortion. The resistors in question are R29 through R32 (220 ohm) on the main amp board. These should be replaced as a matter of course as the originals will go faulty if not already on the way out. You can get suitable resistors in Sydney.

Edit; there is some info on fuse resistors in a thread I started over on Audiokarma

http://audiokarma.org/forums/index.php?threads/fuse-resistors-the-wheres-whys-and-hows.430439/

And lastly, if you want to make life easier when you are adjusting the bias current, you can fit multi turn trim pots, it makes the adjustment much easier, and more stable.

Cheers

John

Edited by skippy124
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