Yamaha M85 - should I stop at this point?

lixaudio

Active Member
Hi everybody,

Few months ago I received a Yamaha M85 in really bad condition - burned right channel and physically damaged (bad package). I've found corroded ground rails on the PCB,because of the fact that some glue change it's chemical stability with age, I presume the lost of ground burned one channel. Found original transistors on the known e-site in UK (yes, they were all originals), replaced some resistors, repaired the chassis and connectors and finally with the help of my much experienced technician fired it on with the variac. Click and apparently with no problems started at the first attempt. Left channel perfect, right channel 22mV of offset (no trimmer for offset). Ok, I can live with that. Set bias on both channels - perfect. Put the dummy load on the right channel - up, up, up, went just below clipping and the relay disconnected the right channel - WTF. On the left channel we've measured the same power, but could clearly see the clipping and no disconnection of the channel. Did I miss something? Probably. Is it possible that just below clipping that offset makes problems (we have measured more than 300W)? Should I leave it so? It works fine and I'm not going to push it to clipping limits.

Thanks,
LIX
 
What does this unit use for a differential pair ? upa75V/74V. or Dual FET's like a 2SK389.
 
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I think you have to make the call whether you should. They can give experienced techs fits. Not for the faint of heart or for a kitchen table bench type of repair. The failures can go back far. I've had to replace diffs on M-80's and M-85's. ....whic is highly unusual for an output failure. If you've got a clipped waveform, don't forget about the diode string that clamps the driver bases.
 
I had a really squiffy cap in that glue corroded area that took down one channel on one of my M-80's. I'd be checking every component touched by that glue. I generally have to replace the 2 large and several smaller caps, the zener diodes, and many, many resistors in that area. I also remove the ground bus bars and polish the corrosion off to prevent further spread.

BTW, said cap recently measured dead, dead by my new blue ESR meter.
 
He he, I swapped the two 2SK389 between the channels, 20mV offset remained on the right channel, so it is somewhere else. I will continue next week, oh, next year :))
Thanks guys, I'll keep you informed if I find anything.

LIX
 
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