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2SA798

mar7in

mar7in

New Member
Hi everyone,

This is my first post. I've recently picked up a Sansui AU 3900 Amplifier from marketplace which has a distortion / crackling sound on the left channel only.
After reading a few posts here, this is usally related to the 2SA798 that I wanted to try to replace.

Now, I've found a replacement for it on eBay: https://www.ebay.com/itm/115598527727?hash=item1aea358cef:g:VJQAAOSw2dJjbBeA

1. The seller states it has been designed by some fine people from this forum, I hope this is true :)
2. Anyone has experience with these ?
3. Would I need to replace both of them, or is it enough to replace just the noisy one for now ?

Thanks in advance,
Martin
 
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I can get originals from my supplier in Tokyo if you prefer. About $30/pair + postage.
 
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Seller is a member here. Great resource for hard to find components. Have purchased from him many times and have nothing but praise for his parts and fast delivery. I would replace both 2SA798 for symetry.
 
You could also replace those with 2 KSA992 transistors (for each 2SA798) but it's probably necessary to match them and probably a good idea to thermal glue them together. That's a lot cheaper but if you don't have the equipment to match transistors or don't want to mess with gluing and heatshrinking them, those boards you found should work fine. I'd replace both just because, you know, symmetry, and because if one went bad the other might too.

Here's a thread on this for reference - https://audiokarma.org/forums/index.php?threads/2sa798-dual-transistor-replacement.924223/
 
thanks guys! would there be any difference in the sound between those and the originals? i guess the ones from ebay will last longer ?
 
mar7in said:
thanks guys! would there be any difference in the sound between those and the originals? i guess the ones from ebay will last longer ?
Click to expand...
The original parts are failure prone, the eBay parts are sure to be a good product from a solid seller.
I just gain match a pair of KSA992s and haven't had a problem yet.
 
Martin, did you try to interchange the 2SA798 between the left and right channels to confirm that the 2SA798 is the root cause of your problem?
 
I've also "made" them from some hand matched transistors but honestly this looks like a nicer solution. Doesn't seem like an unreasonable price for 2 of them either IMO.

redk9258 said:
Many amps use 2 matched transistors for the differential pair that are separate and not bonded together.
Click to expand...

true, but if you can keep them the same temp it won't be prone to wandering offset as the parts heat up. Someone on here had an amp with an absolutely bonkers layout that put the two transistors of the diff pair significantly apart on the board. One was near something that got hot. Power resistor maybe? Either way the offset would shift considerably because the transistors heated differently. Thats probably the worst example I can think of for that but having them in one common blob isn't a terrible idea.

I'm sure the mfg original goal was to just use a single part and eliminate having to match or adjust things to get within spec. Anyplace you can save time means better profit. We're just dealing with it many decades after they probably expected this stuff to be in the bottom of a landfill.
 
ecluser said:
Martin, did you try to interchange the 2SA798 between the left and right channels to confirm that the 2SA798 is the root cause of your problem?
Click to expand...

Exactly, how do you know its not a dirty control job or switch?

I always first clean all the contacts etc to eliminate that as a possible cause for scratchy sound.

Nashou
 
mar7in said:
Martin, did you try to interchange the 2SA798 between the left and right channels to confirm that the 2SA798 is the root cause of your problem?
Click to expand...
I've just interchanged the two and can confirm the crackling has dissapeared on the left channel.
After testing the left one, I tested the right one again, there was no sound coming throuhg (it did before the interchange) and the crackling started.
Then all of a sudden it started to sparkle around the red circled area and I quickly switched off.

I hope I did not destroy some other components.
 

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A neat idea but being in Canada, if I were to order those fabricated replacements right now the landed cost per pair, including currency exchange, would be $52 CAD.

Twice I've needed to replace 2SA798's. The first instance was a few years ago where I learned of subbing two KSA992's so I got a bunch. That happened to involve a Sansui integrated. Model escapes me but it was a low power unit.

My second need to substitute for 2SA798's was in a Pioneer integrated - one in the 40/45 watt range.

Neither were mine but as far as I know, a pair of KSA992 worked fine and were likely less than $1 per transistor.

The only extra work was enlarging the common emitter hole to accept 2 legs and shrink tubing the transistor bodies together. As near to being drop in replacements as I could hope for.
 
Nice fix for putting them both together. And keeps the pair together for placement on the pcb. Got to keep this in mind. Thanks kelontong.
 
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