Troubleshooting Sansui AU-101 Amplifier

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trashcanman

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Joined
Jan 26, 2016
Messages
104
I have a Sansui AU-101 amp that I bought cheap as one channel was dead. Opening it up showed that one of the fuses was blown and it blew immediately again on replacement. I managed to trace  down  a few dead/shorted transistors and got it going again but the output transistors on one channel were still getting hot so I thought it would be just a matter of time before they died again and went further solving the issue.
The voltage from the power supply was quite a bit off and pretty basic so I replaced it with a diy version, basically that same but with a voltage regulator after the rectifier diodes and a few more smoothing caps.  Where I'm at now is one channel works fine and is nice and quiet thanks to the new psu I assume, the other channel however causes the psu to crap out when I have the fuse in and my measurements indicate differences in resistance to ground when comparing channel to channel.
I'm pretty sure the problem is still somewhere into the output section but all transistors and caps test fine. My knowledge is pretty limited so if anyone has any advice about where to look I'd appreciate it. Below is the schematic.
 
2zI6G15.jpg
 
try this on the affected channel.

regulated power supply,
for a power amplifier?
 

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Are you suggesting I short those two points?  I don't know much about electronics so what's the disadvantage of a regulated psu in an amp like this? I only put one in because the voltage from the original was way off and there was a lot of noise so I thought it could do with cleaning up.
Also I've just found that TR812 has a short collector-emitter so I'll replace that and see if that fixes it.
 
Check emitter resistors R835-838, they often fail when the output devices fail.
Check DC conditions between the 2 channels.
Switch off and measure all resistors and caps in circuit, look for way out of spec readings.

When you switch on is there a DC voltage on the output terminal?

http://audiokarma.org/forums/index.php?forums/exclusively-sansui.39/
 
trashcanman said:
I have a Sansui AU-101 amp that I bought cheap as one channel was dead. Opening it up showed that one of the fuses was blown and it blew immediately again on replacement. I managed to trace  down  a few dead/shorted transistors and got it going again but the output transistors on one channel were still getting hot so I thought it would be just a matter of time before they died again and went further solving the issue.
The voltage from the power supply was quite a bit off and pretty basic so I replaced it with a diy version, basically that same but with a voltage regulator after the rectifier diodes and a few more smoothing caps.  Where I'm at now is one channel works fine and is nice and quiet thanks to the new psu I assume, the other channel however causes the psu to crap out when I have the fuse in and my measurements indicate differences in resistance to ground when comparing channel to channel.
I'm pretty sure the problem is still somewhere into the output section but all transistors and caps test fine. My knowledge is pretty limited so if anyone has any advice about where to look I'd appreciate it. Below is the schematic.
When you have a stereo anything with one channel working and one channel not, you can probe voltages on the working side and the questionable side to look for differences. The differences will eventually lead you to the remaining problems. Of course be careful probing around with power on so you do not break something else with a slipped probe. (it happens).

There is a trimpot for setting output class A bias. (VR801). Set that for minimum R for minimum class A current and heat at idle. (BTW that is not a very clever design, since trimpots are known to fail open circuit so, output stage goes to full bias if trimpot fails.  A safer design would use a fixed resistor with trimpot in parallel so a bad trimpot doesn't kill the amp.

JR

 
Replaced that last dead transistor and it's working smooth now, no heat and psu is holding up, though I don't plan on driving any huge speakers but that might be a possible problem.
 
Nice that it's working, but before you close it down measure all the voltages at each transistor leg, write it dowm, and compare the readings with the other channel.

Like JR said, do it slow and be careful with the DMM probes to avoid doing a short somewhere and damage the transistors again.
Normally the output transistors don't die alone.

I did that mistake last week, I've sent a couple of days trying to fix a NAD 302 amplifier.
When I finally manage to make it work again and stable, I went on to the the last measurements and shorted a transistor leg.
Back to square one.
It toasted the output transistors and some resistors
 
Whoops said:
Like JR said, do it slow and be careful with the DMM probes to avoid doing a short somewhere and damage the transistors again.
Might I suggest heatshrink over the entire probe tip, leaving only the tiniest conducting point sticking out. If you can't short two pins of a chip together with it, you should be safe.
Gene
 
That trick Gene mentioned about insulating all but the very last mm of the probe tip is well worth doing . I normally use a spring loaded retractable hook type probe  for high voltage rock n roll ,but of course I still get the odd lighting flash accompanied by flying molten metal  ;D
Valves tend to survive these kinds of momentary overloads extremely well ,transistors pffft they go up in a puff of noxious smoke .
Carefull out there guys. 
 
Gene Pink said:
Might I suggest heatshrink over the entire probe tip, leaving only the tiniest conducting point sticking out. If you can't short two pins of a chip together with it, you should be safe.
Gene

Thanks I will do that next time.
Also dont measuring right at the transistors legs is also a good method. Follow the track and measure at the next solder spot
 
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