• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Barn find Sansui AU-7900

K

Kylek1443

Active Member
Hey guys, this is my first post and I'm not really all that great at rebuilding amps yet. I was wondering if anyone could help me with my problem. I have no real history of the amp other than it was in a barn with its top and bottom off, and had a mouse nest that had mouse chewed wires in it.

I repaired all the wires that were chewed, carefully looking over them, and I think I have the damage repaired.

The main problem seems to be the amplifier board (Sansui F-2596). The 2 relay fuses 8A 250V on the board, blow immediately when the unit is turned on.

The 2 1A 250V fuses do not blow. The main power fuse doesnt blow either, the unit still stays powered on.

My first thought after doing some research was to replace the 220ohm fuse resisters at R31 R32 R41 R42, but the new fuses blew instantly still.

Does anyone have any ideas? I want to try and fix the problem before recapping, because I know caps most likely arent the problem, even though there are some that look pretty bad. Thank you guys for any help.
 
Last edited:
Register to hide this ad
Welcome to AK!
First, check the output transistors, some of them might be shorted. Do you have DBT? If not buy or make one.
 
DBT: Dim Bulb Tester. If you are planning to fix or work on the amp it is a good idea to have one.
 
Sorry if I’m asking too many questions. This is the only amp I want to work on. I’ll probably be done fixing them if I can get this one done.
 
Kylek1443 said:
Sorry if I’m asking too many questions. This is the only amp I want to work on. I’ll probably be done fixing them if I can get this one done.
Click to expand...
With the greatest respect this particular amplifier is not one you can just work on and fix without any prior experience, this is your first correct? If it really is a 'barn-find' and chewed by mice, and presumably pee'd on by mice. However, you may have hidden aptitudes that I am not aware of, without that information I am just trying to advise you of what you are facing.

It will be particularly in need of an expert touch and much TLC before it will work again reliably and well. Add to that, it is a particularly nice model in Sansui's range - desirable and loved by many.
 
Last edited:
Hyperion said:
With the greatest respect this particular amplifier is not one you can just work on and fix without any prior experience, this is your first correct? If it really is a 'barn-find' and chewed by mice, and presumably pee'd on by mice. However, you may have hidden aptitudes that I am not aware of, without that information I am just trying to advise you of what you are facing.

It will be particularly in need of an expert touch and much TLC before it will work again reliably and well. Add to that, it is a particularly nice model in Sansui's range - desirable and loved by many.
Click to expand...
. It gets power without the main fuse blowing, I’m not entirely ignorant of electronic repair. I’m just asking if anyone who has worked on this particular amp has encountered this problem. The amp blew fuses with the damage. The mouse damage was contained to the top of the amp. A few wires had been chewed on, all of that has been repaired. The amp did the same thing after the damage was repaired. There’s obviously a short somewhere on the amplifier board that is blowing the two relay fuses. I’m nearly certain the problem is the amplifier board. The power transistors are corroded and I’m sure there is similar unseen damage to the output transistors. No disrespect, but there’s only so many things it can be.
 
Last edited:
Kylek1443 said:
. It gets power without the main fuse blowing, I’m not entirely ignorant of electronic repair. I’m just asking if anyone who has worked on this particular amp has encountered this problem. There’s obviously a short somewhere on the amplifier board that is blowing the two relay fuses. No disrespect, but it’s not rocket science. It’s a 60 year old amp that has an issue with one board. There’s only so many things it can be.
Click to expand...
Do you want to try to diagnose and repair this amp by yourself? If yes, I would suggest a couple of first steps: 1) Download the service manual and check the schematic. 2) Buy or make a DBT that will save you further blowing of components when you check the repair, 3) A good DMM and a transistor tester would be very helpful.
Here you can find the use of DBT
https://antiqueradio.org/dimbulb.htm#:~:text=What Is a Dim-bulb,built about 25 years ago.
 
Kylek1443 said:
. It gets power without the main fuse blowing, I’m not entirely ignorant of electronic repair. I’m just asking if anyone who has worked on this particular amp has encountered this problem. The amp blew fuses with the damage. The mouse damage was contained to the top of the amp. A few wires had been chewed on, all of that has been repaired. The amp did the same thing after the damage was repaired. There’s obviously a short somewhere on the amplifier board that is blowing the two relay fuses. I’m nearly certain the problem is the amplifier board. The power transistors are corroded and I’m sure there is similar unseen damage to the output transistors. No disrespect, but there’s only so many things it can be.
Click to expand...
 

Attachments

  • A680BAC3-C637-4661-9A53-AC53C979B8A9.jpeg
    A680BAC3-C637-4661-9A53-AC53C979B8A9.jpeg
    73.2 KB · Views: 135
  • 09F2E636-C50D-4DE8-A065-AA733480121C.jpeg
    09F2E636-C50D-4DE8-A065-AA733480121C.jpeg
    75.5 KB · Views: 126
  • E145C4C0-A869-4639-A9D3-FE665E726EC9.jpeg
    E145C4C0-A869-4639-A9D3-FE665E726EC9.jpeg
    100.1 KB · Views: 128
  • 95A9DC94-0F60-4A34-B032-1313AF59F967.jpeg
    95A9DC94-0F60-4A34-B032-1313AF59F967.jpeg
    102.6 KB · Views: 126
sanjib said:
Do you want to try to diagnose and repair this amp by yourself? If yes, I would suggest a couple of first steps: 1) Download the service manual and check the schematic. 2) Buy or make a DBT that will save you further blowing of components when you check the repair, 3) A good DMM and a transistor tester would be very helpful.
Here you can find the use of DBT
https://antiqueradio.org/dimbulb.htm#:~:text=What Is a Dim-bulb,built about 25 years ago.
Click to expand...
Thank you. Yes I’d like to fix it myself. I have a good MM and the service manual for the 7900. It’s a rough photocopy but that’s all that’s available online. Thanks for the advice on the DBT. Never used one but I’ll look more into that. You’ve been a great help. I’m hoping the issue is just replacing the power and input transistors. The rest of the unit looks solid. I just know I want to repair before I restore. I’ve not touched caps or anything. Just replaced the 220ohm fuse resistors because they were toasted.
 
That will be my next move. I already ordered new output and power Ts because the only place I could find them was on eBay from China. I’m assuming they should be replaced regardless because of the age and the corrosion on them. Here is a picture of the 220ohm resistors I replaced. The old ones were dry and flaking so they obviously needed swapped out.
 

Attachments

  • C79B5CB3-68F2-483D-AB7E-EB47D4BB334F.jpeg
    C79B5CB3-68F2-483D-AB7E-EB47D4BB334F.jpeg
    72.8 KB · Views: 72
Don't buy old transistors from eBay. They are mostly fake Chinese except from very few US sellers. You can replace the output transistors with MJ21193/MJ21194G pair and for the driver transistors, you can use MJE15030/MJE15031 pair. You can buy them from a reputed source like Mouser or DigiKey.
 
Kylek1443 said:
That will be my next move. I already ordered new output and power Ts because the only place I could find them was on eBay from China. I’m assuming they should be replaced regardless because of the age and the corrosion on them. Here is a picture of the 220ohm resistors I replaced. The old ones were dry and flaking so they obviously needed swapped out.
Click to expand...
Cancel those orders.....
Yes, replace all the fusable resistors.
 
Hyperion said:
With the greatest respect this particular amplifier is not one you can just work on and fix without any prior experience, this is your first correct? If it really is a 'barn-find' and chewed by mice, and presumably pee'd on by mice. However, you may have hidden aptitudes that I am not aware of, without that information I am just trying to advise you of what you are facing.

It will be particularly in need of an expert touch and much TLC before it will work again reliably and well. Add to that, it is a particularly nice model in Sansui's range - desirable and loved by many.
Click to expand...

I also have to say. I love this Sansui. This isn’t my first amp I’ve repaired. I actually have a Bose 550 Spatial that was the most complicated SOB I’ve ever seen. I’m sure there’s much more complex out there, but this Sansui is fun, that Spatial was a nightmare.
 
sanjib said:
Don't buy old transistors from eBay. They are mostly fake Chinese except from very few US sellers. You can replace the output transistors with MJ21193/MJ21194G pair and for the driver transistors, you can use MJE15030/MJE15031 pair. You can buy them from a reputed source like Mouser or DigiKey.
Click to expand...

I was honestly nervous about swapping those transistors with another kind. That’s why I was wanting to stick to the original NEC D388A/B541A. Thank you for that information. Can you tell me which swaps with which?
 
ivandezande said:
Cancel those orders.....
Yes, replace all the fusable resistors.
Click to expand...

are the only fuse resistors R31,R32,R41,R42? Are you familiar with this amp? You guys are awesome with your responses. My experience is more with computer motherboards and game consoles. Vintage electronics, specifically amps, are somewhat new to me. I have a few things under my belt but I’m not an expert.
 
It looks like MJ21194G is not available now both on mouser or Digikey but you can check on Newark Electronics or Bdent.
2SD388 (NPN) - MJ21194G, (or MJ21196G)
2SB541 (PNP) - MJ21193G (or MJ21195G)
2SD382 (NPN) - MJE15030
2SB537 - MJE15031
All of the fuse resistors are mentioned on the parts list in the service manual.
 
You must log in or register to reply here.
Back
Top Bottom