Overheating amp help

Thread Starter


Joined Aug 3, 2009
Hello to all- I am repairing a Yamaha M2 amp that I've had for years. One channel went out, and I replaced the outputs, drivers, and several diodes plus 2 shorted caps in the driver and predriver circuit( see attached schematics)-

I replaced the fuse resistors as well, most of them blew out, but I cant find exact replacements anywhere, so I just installed 2% 1 watt resistors instead. Well, now the amp is working BUT the repaired channel gets hot enough to fry an egg on the heat sinks within 5 minutes with no load.

I checked all my power supply voltages and am fine, but I cant seem to locate the cause of the overheating, and although I am capable of repairing certain kinds of problems, my knowledge level is a bit lacking of tracking down the source of the overheating. Common sense is telling me that the 6 output transistors are simply conducting excessive current with no load applied, but I am not sure where in the schematic to start looking. As I mentioned, if I hook up an input source, it actually plays music just fine, it just is getting too darn HOT- ( as I mentioned, only the repaired channel is overheating)

One last thing- the output transistors in the schematic are no longer available, so I had to use NTE284/285 comp pairs instead of the 2Sa1051 & 2SC2461. Spec sheets are essentially a perfect match.

Any ideas?

Thanks for any help
File #1 is the predriver board
File #2 is the output board



Joined Feb 17, 2009
Check TR401, TR402 and pot. VR401 -regulates output stage idel current.
And connect oscilloscope because overheating maybe caused by oscillation.

Thread Starter


Joined Aug 3, 2009
Service manual states that I should have 50-55 mv at my bias adjust test point, which I can attain and hold, as well as dc offset of less than 5 mv, which I can get-
So far I have not found any erroneous voltages, but I have not checked every possible location.

If the problem is oscillating, where would the likely culprit be? I have not yet tried the scope to see what the output looks like, but I will certainly do that- Thanks for the tip