Ampeg SVT III Pro

[Yurka]

Im looking at a SVT III pro with some overheating issues. Plays perfectly fine for about 20-30 minutes, then output dies. Iv read through all the other threads about the same amp to see if anyone else has had this problem. So far I have pulled the board to see if I could find any obvious issues, bad solder joints, etc. The circuit has an "Over-Temp Shutdown", which I am guessing would be a good place to start. Any suggestions on what I can possibly be looking for would be appreciated.

42601h15.pdf

[MarkusBass]

What do you mean by "output dies"? Speaker is disconnected or output transistors are shorted? If this is overheating, a good place to start is to measure output transistors' current (on ballast resistors).

Mark

[Yurka]

Great question. Ill provide more details about whats going on before and after the output dies, thanks!

[Enzo]

Isolate the problem. The symptom is that the sound goes away. If I understand you , it is only a guess that it is a thermal shutdown. A thermal shutdown is a specific condition. Do the indicator lights indicate a thermal shutdown, or a shutdown of any kind? I differentiate between a thermal shutdown and a thermal problem not related to the shutdown circuit. The shutdown circuit turns the speakers OFF when it detects a thermal problem. But a bad drive tube could fail when IT gets hot without involving the thermal circuits at all.

And just waiting 20-30 minutes is not itself necessarily indicating a thermal issue. A leaky cap might take that long to quit working. If the amp is shutting down thermally, then the heat sink ought to be very hot. Is it?

Does the fan work? If not, fix that first. If it does, does it speed up before the amp fades out? The thermal sensor that runs the shutdown also controls the fan, so if it is shutting down, it will speed up the fan trying to save itself.

Does the sound click off in an instant? Or does it fade away? If it clicks off, is the speaker relay dropping out?

There are a couple tubes in the power amp, make sure the tube V2 in socket J12 is a 12AU7 and the other tube a 12AX7. Bad 12AU7s are the most common thing I have to replace in SVT series amps.