Sansui AU-517 Preamp Distortion

[Common Beat]

Hello. I've been fussing with this AU-517 back and forth I've kind of hit a wall. I'm getting a very anemic sound through the left channel only and it's 100% happening in the preamp somewhere. I've already cleaned all the pots, switches, and jacks front and back. I've replaced the fuse resistors on the power amp board and reset the DC offset and bias, and I've resoldered the header pins that attach the various boards together. For a while these things made it work perfectly, but then it went back to a distorting and anemic left channel. I've patched signal straight into the power amp and it's clean and clear as a bell so I know the problem is focused on the preamp board and I've focused my efforts there. On that board I have tested resistors, transistors, caps and even reflowed all the connections on that board and it's still distorted. Quite a strange problem as the right is totally fine.

Looking for any advice on how to get this thing working correctly because it's starting to drive me nuts!

 

[Aud10file]

The preamp-poweramp switch on the back of the 517 and 717 is notorious for symptoms very similar to yours, The post did not mention which cleaner was used. It can’t hurt to try a Deoxit treatment on the switch if you haven’t already, or even if you have.

It’s an easy fix if it works. If it doesn’t, then no harm done and the hunt continues.

 

[MIJ_Pioneer]

How does the voltage look on the tone board. Carefully compare the working channel to the non working. This can (and should) should also be done on the PSU.

I had an issue similar to yours on my 717 where the tone section was not getting the 33v to operate due to an error on the PSU. This is from memory, not sure the voltage for the 517.

 

[Bryan Farmer]

This is how I found the low chanel output issue on my daughter AU-517 by using a signal generator and bringing it on the left and oscilloscope on the preamp output and providing a + and - voltage rails for preamp. Even though I had cleaned those lever switches I ended up having to desolder them and take the bottom part of the switches off, as well the slider tabs and use silver jewelry cleaner to get the tarnish to remove. My low output was from the middle lever switch, but I cleaned them all that way and it returned to normal output. Hope that helps. Picture of the final amp before her amp was boxed up for Christmas.

 

[Common Beat]

I need to check the voltages again. Each channel basically looked the same if I recall voltage wise. I may pursue that switch disassembly and cleaning solution as well.

 

[Bryan Farmer]

I forgot one other thing if you replaced the transistors on the preamp board TR05 was screen printed wrong on my board. Not to say that yours is not correct cuz sansui liked to keep things interesting like that. The front face of TR05 which is all the way to right should face toward that right most lever switch. Just some memory coming back on that amplifier.

 

[Common Beat]

This is how I found the low chanel output issue on my daughter AU-517 by using a signal generator and bringing it on the left and oscilloscope on the preamp output and providing a + and - voltage rails for preamp. Even though I had cleaned those lever switches I ended up having to desolder them and take the bottom part of the switches off, as well the slider tabs and use silver jewelry cleaner to get the tarnish to remove. My low output was from the middle lever switch, but I cleaned them all that way and it returned to normal output. Hope that helps. Picture of the final amp before her amp was boxed up for Christmas.

I disassembled the switches on the preamp board as you suggested and buffed up the pins with 2000 grit sandpaper and a little WD40 to remove the tarnish. I reassembled everything and wouldn't you know it, the left channel is back strong and matches the right in volume and tone. Thanks so much for the suggestion, I'll remember this one for the next time I get a Sansui AU series amplifier on my bench!

 

[Bryan Farmer]

Be careful using sand paper as you can take the plating off, but glad you were able to get it cleaned and working again. It's a very good sounding amplifier. My daughter absolutely loves it.

 

[kevzep]

Ooooooo yeah don't do that!! Not sandpaper!! As already stated, the switches have a very light silver plating on them, you'll take it right off with 2000 grit, and then they will work for a while, but they will oxidise very quickly and you'll actually be worse off than you were.

 

[usblues101]

We use lighter fluid and lacquer thinner in the Harley shop for gunk and what not but I'm not sure about your deal......used 3000 grit once on a ladys fingernail.Looked like clear polish was applied......cheers men...