Help with AU-717

[Eldin]

So a really strange problem is going on. I just fixed up this amp and everything was alright, but the volume pot was really scratchy at low volumes on the right channel. I gently opened up the pot and tried to clean everything up inside, and closed it back up. Now the right channel doesn't work at all, but i can very faintly hear it if i put the volume all the way up to max. Any ideas? Did I blow something?

 

[Robisme]

Did you clean the switch on the rear of the amp. Try moving it around.

It's possible you damaged the volume control. Start with the switch and we'll go from there.

Rob

 

[Eldin]

Yes, that's the first thing I did before even powering on the amp. I took the switch and cleaned it with a generous amount of deoxit and after cleaning it the amp was working. I did just clean it again to be sure and it still has the same problem.

The right channel (when the volume pot is maxed out) plays music still, but incredibly faint. Thanks for the help

 

[Leestereo]

Test the main amplifier stage separately (e.g., with ipod) to confirm that the problem is indeed in the preamplifier stage if you suspect the volume control. And if possible, test the preamp stage by hooking up to another power amp.

 

[Eldin]

I plugged in my phone to the power amp in and the "dead" channel did work. This means the problem is in the preamp stage right? I'll try testing the preamp stage later once im home again. Thanks for the help

 

[Eldin]

Just an update. I plugged in the pre-amp out from the AU717 into my SX980. The good channel is fine and the bad channel still doesn't work. Is there a way I can test the volume pot to see if it works? Also is there a way (that anyone knows of) I can open up the pot and try and repair it? In the meantime i'm going to take out f-2720 and see if something may have gotten fried and not the volume pot.

 

[Eldin]

Just another update. I was poking around a bit and i'm fairly sure one of the balance pots went out. I'm not entirely sure how this would even happen, but there it seems is the problem. I'm going to try and take off the pot and open it up to try and clean it. In a perfect world this will fix it, but who knows.

 

[Eldin]

Just something i forgot to mention. The left channel is the one that's out. Turning the balance pot left was met with some resistance almost like it was badly oxidized (was good prior). I tested the balance pot out of circuit and it seems to be fine. Also tested the volume pot and from my quick measurements it seems to be functional. I'm at a loss

 

[Leestereo]

Have you confirmed that the power supply voltages to the preamp board are approx. +35v and -35V for each channel? If the measured voltages are significantly different, I would suspect the regulated voltage circuit on the F-2663 board.

 

[Eldin]

So on the F-2663 board i measured pin 23, and the voltage came out around 32V. Pin 24 however sat at around 3V. Are there better/different points I should have tested? I'm still pretty new to this whole hobby and any pointers would be a huge help

 

[Leestereo]

On F-2663, pins 23/24 should both measure approx. -33V and pins 17/18 should both measure approx. +33V. Your measurement of 3V on pin 24 indicates that you have a problem on this board; it should be rebuilt. See this post: Sansui AU-717 F-2663 Restoration

 

[Eldin]

I probably shorted something from the volume pot then?
Thanks a bunch for the help by the way

Pin 19 measures 32
Pin 23 measures -32

Would the low voltage on 24 cause the left channel to be very weak unless the volume pot is maxed out?

 

[Eldin]

Could anyone by chance let me know what to look for when testing the volume pot? I may just be looking for the wrong thing, and not know it. I'm comparing the values between both channels as one channel works and the other doesn't, and comparing the resistance at max and min volume.

 

[Leestereo]

...Would the low voltage on 24 cause the left channel to be very weak unless the volume pot is maxed out?

The loss of the B+ and B- voltages for a channel's preamp stage would definitely result in very weak signal for that channel.

 

[Eldin]

Sweet. Thanks helping me narrow down which board is at fault. I probably shorted something while removing the volume pot, i'm just glad the pot itself isn't bad (they are unobtainium). I just wanted to avoid replacing a bunch of parts and "shotgunning" what the problem could be. I'll follow your guide on how to make this amp great, and post back the results

 

[Eldin]

The glue on the power supply board REALLY ate the components. This channel is out because the resistor seems to have finally given up. Measures 1Mohm should me 10k

 

[Overundr1]

Now you are heading in the right direction. Lots of good threads on rebuilding (and measuring) the power supply. You mentioned the balance control not feeling smooth in both directions. rather than opening it up check to ensure it has not been over tightened against the front frame, give it a good deoxit bath then a shot of faderlube. Take your time on the power supply section, usually you end up having to remove and replace a bunch of components that the glue has come into contact with. That board is kind of a pain to get out and careful work now will save having to take it apart again.

In case you accidentally broke a wire off the volume pot---

http://www.audiokarma.org/forums/index.php?threads/au-717-volume-potentiometer-issue.802783/

And more pictures and power supply measuring tips---

http://audiokarma.org/forums/index.php?threads/au-717-the-leestereo-way.770991/

-Lee

 

[Eldin]

A good deoxit bath with it out of circuit did help it come back to feeling smooth again and I tested it to be sure, and it is fully functional.

The power supply board being a pain to get out is an understatement hahah. The one thing that just confuses me is that both channels were working. I'm just going to compare channel to channel and see if there are any differences in measurements. Honestly with the glue aftermath, it's a wonder either channel worked at all!

Thanks all for the help, hopefully i'll be back thrilled about the sound