Barn find Sansui AU-7900
- Thread starter Kylek1443
- Start date
GrisPato
Super Member
That are them yes. Just wondering if you did measure them out of circuit and checked the mica insulators.
I have not measured them out of circuit. And here is my reasoning for it. They are soldered in, not socketed in. My thinking is that IF the OP transistors are the issue, I will replace them all in a restore anyway. So I personally would much rather test them, when I have replacements on hand ( I can always return or keep spare parts). I figure they are going to get replaced anyway. But what you’re asking, and what we need to know, is if the OP Trans are the problem, or if it is deeper in the board. I’m praying it’s a short in an OP Tran. To the owner of the amp money doesn’t seem to be an issue. So if I replace the OP trans, and it DOESNT work, what would be the next step after the OP tansistors?
GrisPato
Super Member
Did you do this? Takes two minutes.
GrisPato
Super Member
If that would work!
The thing is that where this amplifier was stored could mean that the problem can be anything. For sure, one or the output transistors is a good possibility. My problem is that it's so easy to check and you know it. Don't understand the resistance to that.
But as said before it's your amplifier to fix.
In a way I would say, just restore it, recap, change the transistors, trimpots and zeners and then see if it works.
Only problem with that is that you could have introduced your own faults and it will get more complex.
I’m not totally sure what you mean by mica insulators. I’m so sorry for sounding ignorant. Those clear plastic “gasket” looking things in between the transistors? Each of the 12 transistors on the back of the heatsink has one. But it looks like Sansui used some kind of ultra clear thermal paste. All of which is almost non existent on all these transistors due to drying up over time. I scraped some out of a hole on the heatsink when I replaced the other transistors. It looked like Vaseline.
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I would love to just go “ham” on this board and restore it. I have no doubts in my ability to solder and replace parts. I just wish I could get the damn thing to work before I do that. I’m just not the best with the MM and I’m limited with tools I have to work with. The DBT was the best thing I’ve ever had and YOU guys taught me. I’ve learned more on here in less than a months than I have in 10 years.
GrisPato
Super Member
Ah, don't hesitate to ask more information if you don't know what we talk about. They are there in between the transistor and the heatsink to isolate. They can have cracks and in the case of your amplifier there could even be moisture. It's a small change but a realistic change.
How important are they? If they are cracked do I need replacements? And if so, can I get them?
GrisPato
Super Member
I would advice you to change them.
Are they important? Mhh, yes they are, they provide isolation between your rail voltage that is present at the collector ( with TO3 the whole case) and the heatsink which can be connected to the case. Or in other words, when they are not providing isolation you have a short circuit. ( In some exceptions not )
so from the metal heatsink it should be heatsink> Mica insulation pad> thermal paste>transistor.
thermal paste both sides of the mica insulator..
The Vaseline like stuff is Silicone grease - withstands high temperatures and is almost indestructable. There is a better purpose designed product now called 'heatsink compound' (or thermal paste 
), widely available - that's the stuff to use after you have cleaned (wiped) off all the Silicone grease.
), widely available - that's the stuff to use after you have cleaned (wiped) off all the Silicone grease. Hey guys! Sorry there hasn’t been updates lately I’ve been waiting on parts. Got the replacement OP transistors in yesterday. I just wanted to post a few pics of them and make sure I have them in the correct order. Let me know if everything looks good to you guys. I won’t attempt the swap until I hear the hopefully good news.
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The Vaseline like stuff is Silicone grease - withstands high temperatures and is almost indestructable. There is a better purpose designed product now called 'heatsink compound' (or thermal paste
not sure if you’ll see this but you’re my go to guy. Are these the correct replacements? MJ21193G replaces D388A and MJ21194G replaces B541A?
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Other way round...
2SD388 is NPN, replaced with MJ21194G is NPN
2SB541A is PNP, replaced with MJ21193G is PNP
THIS IS WHY YOU ARE MY GUY! Had to check with you before I did anything. 94G is black and 93G is red. Thank you!!! I’ll keep you updated. Going to thermal paste both sides of mica pads.
Other way round...
2SD388 is NPN, replaced with MJ21194G is NPN
2SB541A is PNP, replaced with MJ21193G is PNP
May have found the problem!!!! I didn’t notice until now that this OP transistor was missing its mica insulation pad. Pulled it out, and look at that!!! This could be it!
Other way round...
2SD388 is NPN, replaced with MJ21194G is NPN
2SB541A is PNP, replaced with MJ21193G is PNP
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I just ordered new pads on digikey. I’m holding off on doing anything else until I get them. I’ll keep you posted, it just might take a week or so. I have all the new OP transistors to replace. Let’s hope and pray this is the fix and this damned Sansui can keep on living.That looks nasty, - yes it could be 'it'
More for the missing mica washer (cause) than the bubble of black stuff next to the transistor leg (effect).