Sansui AU-5900 Amplifier
I recently acquired this beefy little amp as a new restoration project. After taking it home, I found out that it required slightly more repair work than I was used to performing. I ended up having to replace several resistors and diodes on the main amp board.
After recently moving and becoming a new parent, I've been seriously neglecting my hobbies. Last month I decided to put an end to that by looking for a new piece of stereo equipment to restore—and what I found was an extremely dirty Sansui AU-5900 amplifier. Apart from a thick layer of dirt and a missing knob, it seemed to be fully functional.
After bringing it home, giving it a good cleaning inside and out, I hooked it up to give it a more thorough test drive. It worked great for about three minutes until I heard a loud pop and it went completely dead. After a little rest period, I tried again, but the same thing happened.
I spent some time looking into possible solutions, assuming I would probably have to replace some bad capacitors. But after asking around on Audiokarma.org, a helpful forums user pointed me in a very specific direction. He told me that I should replace resistors R31, R32, R41, R42 and diodes D01, D02, D609, D612 on the main amp board—a common issue with the Sansui AU-x900 series of amplifiers. His expertise was backed up by other users, so I figured it wouldn't hurt to give it a try. The replacement resistors had to be 220 ohms, and I went with Digikey part number 220ADCT-ND. For the diodes, I went with part number 1N4148VSCT-ND and I was told that I would need two, installed in series (eight total for the repair).
One day and $12 including next-day shipping later, I was ready to work. I had never done this kind of intricate repair before, so I asked my cousin to give me a hand, as he has a lot more experience in this area than I do. The repair went easier than expected, and I learned a lot of useful techniques which I'll be sure to take advantage of in the future.
So now it was the moment of truth. I re-assambled the amp, hooked it up, and waited. Several minutes passed and it was still going strong. Five minutes turned into 10 minutes, and then 15. It appeared that the repair was completely successful!
Despite the success, another small problem became apparent—After 10 to 15 minutes of use, one of the audio channels began to fade and then eventually cut out. If I adjusted the volume knob, the sound returned to normal. After some brief research, I found out that a common cause might be dirty contacts in the circuit protection relay. A quick cleaning of the relay contacts solved the issue entirely. The relay is housed inside a clear plastic box, which can be removed with some very careful prying. You can see it in the first photo of the dismantled amplifier board in the top left section.
With the main issues fixed, my cousin also helped me adjust the DC offset as per the service manual, returning the amp to good-as-new condition! I still had to track down a replacement for the missing knob, but after finding no suitable replacements on eBay, I decided to drop my asking price a little bit and sell it as-is. It didn't take long to find a new owner.
For more technical specifications, visit the Sansui AU-5900 page on Hifi Engine.