Sansui AU-719 Tone Problem

Rotoflex4

Active Member
I've been recapping my AU-719 in stages, and everything had been going well. I did the boards one at a time and tested the amp after each stage. All good.....sounded great. Lastly, I replaced the filter caps and recapped the F-3101 board (Power Supply and Protector Circuit Board). It's the one wedged between the big filter caps and the transformers. So....I test it and the amp sounds wonderful......except.....when I flip the tone switch "on" the amp belts out a horrendous loud hum that completely obliterates the music. I haven't touched the Tone Control Board at any point. A lot of wires had to be detached and soldered to complete this stage, but I'm pretty meticulous about notes/photos and am pretty sure I have things connected OK. There was a tiny wisp of smoke from the middle of the F-3101 board for a few seconds after I turned the amp on. It was quite small though - I don't think I would have noticed it if I wasn't staring at it looking for potential problems. I thought that maybe a little dust or something was just burning off - maybe significant...maybe not....I don't know. I'm pretty methodical and careful but am a relative novice at this. It pains me to pull the F-3101 board again, especially if it isn't necessary. But with no obvious alternatives I may just have to do that. I've looked over the board with a flashlight and nothing appears fried. Anyone have any ideas where the problem might lie?

Thanks.

-Will
 
I seem to remember there is a capacitor on the PSU/Protector board which is marked with the wrong polarity, it might be C614 or C615 ? - if you took pictures it should show up. With the capacitor the wrong way round it will effectively short out that supply line, which will burn current limiting resistors, and maybe damage semiconductors too. Check the voltages coming out of the regulated supplies on the PSU protection board, this should lead you to the problem.

Above all don't turn anything round without checking thoroughly first. And if it is that capacitor, replace it as it will be damaged now, check for any collateral damage - something burnt when you saw that smoke, the current limiting resistors are 2 x 68ohm resistors and 2 x 6.8 ohm resistors - you need to investigate these and replace damaged components as necessary.

This thread might help (see post #11 pic 10, and post #13)
http://audiokarma.org/forums/index.php?threads/sansui-au-719-buzzing.359597/#post-4999387

Good luck
 
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Thank you very much. I thought I was being so careful, but it simply never crossed my mind that the labeling on the board might be inaccurate. I do have one photo showing C615 prior to recapping. And it appears that C615 should be oriented with the positive side TOWARD C614. I have it the other way around.

I really appreciate your help. I don't think I would have spotted that on my own.

So.....just to confirm.....the PCB markings for C614 are correct, and I should leave it alone. The two caps should be in series so that the negative side of C614 is lined up against the positive side of C615?
 
o.....just to confirm.....the PCB markings for C614 are correct, and I should leave it alone. The two caps should be in series so that the negative side of C614 is lined up against the positive side of C615?

Sounds right.
 
Just a follow-up, in case this info is useful to someone else. I put a new cap in C615 and had to replace one 68Ω resistor that was fried due to this mistake. The amp now runs beautifully. Both the capacitor and the resistor could be swapped out without removing the board, which was kinda nice.
 
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