Hi,
I'm having trouble with my Sansui AU-777 solid state amplifier.
TLDR:
The amplifier has a protection circuit with two current sensing resistors on the emitters of the output transistors. When the voltage across one of these resistors is too high, it turns on an SCR that switches on the "protector" light, and mutes the main amp. However, the SCR switches on all the time, pretty randomly, without high voltages across the resistors.
Could the SCR be broken?
History:
A couple of weeks ago, the protection circuit randomly kicked in while the amplifier was idle (my computer was connected, but wasn't playing any sound). After a few hours, I tried to turn it on again, and the protection kicked in again. I just unplugged it, and put it away until a few days ago.
I didn't see any visible damage, so I plugged it into my dim bulb tester to measure some voltages. It worked again, and all voltages were normal, idle current of the main amp was normal, etc.
So I put everything back together, connected some speakers, all fine, no distortion, tried a stress test at high volume, no problems whatsoever.
However, later that day, while playing at low volume, the protection circuit kicked in again.
Schematic:
The protection circuit works by connecting B6 to ground, taking away the power from one of the first stages of the amplifier, basically muting the main output stages.
The official schematic is attached as a PDF file.
What I've done so far:
I've measured the voltages of the power supplies, and they all are pretty close to what they should be according to the service manual.
I've adjusted the biasing voltages and main amp current, as explained in the service manual.
I've hooked up my scope to the gate of the protection circuit SCR, and waited for it to trigger. This is what that looks like:
The SCR triggers when the gate voltage is around 0.6V, then it turns on and the gate voltage rises to 0.9V (within a couple of microseconds).
However, if I look at the voltages right before the two diodes (at the emitters of the output transistors), there is nothing there to trigger the SCR. Sometimes when turning on the amplifier, there's a small peak of 400mV at most, but nothing as high as 0.8V (0.6V trigger voltage + 0.2V diode forward voltage). Sometimes the SCR just triggers a couple of minutes after turning on, without anything before the diodes.
Even when I disconnect the power to the main amp (Vaa), the protection circuit often kicks in.
I have absolutely no idea why this happens. I've tried adjusting VR901. That seemed to work at first, but eventually, it triggered again. Triggering seems to be independent of the position of VR901, except when turning it down completely (i.e. tying the gate directly to ground).
Right now, it triggers pretty much every time I turn it on, about an hour ago, it worked just fine.
What could be the cause for this problem? Is the SCR broken? Should I look for another culprit?
Thanks in advance,
Pieter
I'm having trouble with my Sansui AU-777 solid state amplifier.
TLDR:
The amplifier has a protection circuit with two current sensing resistors on the emitters of the output transistors. When the voltage across one of these resistors is too high, it turns on an SCR that switches on the "protector" light, and mutes the main amp. However, the SCR switches on all the time, pretty randomly, without high voltages across the resistors.
Could the SCR be broken?
History:
A couple of weeks ago, the protection circuit randomly kicked in while the amplifier was idle (my computer was connected, but wasn't playing any sound). After a few hours, I tried to turn it on again, and the protection kicked in again. I just unplugged it, and put it away until a few days ago.
I didn't see any visible damage, so I plugged it into my dim bulb tester to measure some voltages. It worked again, and all voltages were normal, idle current of the main amp was normal, etc.
So I put everything back together, connected some speakers, all fine, no distortion, tried a stress test at high volume, no problems whatsoever.
However, later that day, while playing at low volume, the protection circuit kicked in again.
Schematic:
The protection circuit works by connecting B6 to ground, taking away the power from one of the first stages of the amplifier, basically muting the main output stages.
The official schematic is attached as a PDF file.
What I've done so far:
I've measured the voltages of the power supplies, and they all are pretty close to what they should be according to the service manual.
I've adjusted the biasing voltages and main amp current, as explained in the service manual.
I've hooked up my scope to the gate of the protection circuit SCR, and waited for it to trigger. This is what that looks like:
The SCR triggers when the gate voltage is around 0.6V, then it turns on and the gate voltage rises to 0.9V (within a couple of microseconds).
However, if I look at the voltages right before the two diodes (at the emitters of the output transistors), there is nothing there to trigger the SCR. Sometimes when turning on the amplifier, there's a small peak of 400mV at most, but nothing as high as 0.8V (0.6V trigger voltage + 0.2V diode forward voltage). Sometimes the SCR just triggers a couple of minutes after turning on, without anything before the diodes.
Even when I disconnect the power to the main amp (Vaa), the protection circuit often kicks in.
I have absolutely no idea why this happens. I've tried adjusting VR901. That seemed to work at first, but eventually, it triggered again. Triggering seems to be independent of the position of VR901, except when turning it down completely (i.e. tying the gate directly to ground).
Right now, it triggers pretty much every time I turn it on, about an hour ago, it worked just fine.
What could be the cause for this problem? Is the SCR broken? Should I look for another culprit?
Thanks in advance,
Pieter
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