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Yamaha M-60 bias issue
Yamaha M-60 bias issue
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Old 12th May 2009, 12:36 AM   #1
FootFungus is offline FootFungus  United States
diyAudio Member
Join Date: Oct 2008
Default Yamaha M-60 bias issue

I bought a M-60 off a nice local fellow a few months back. It was sold to me stuck in protection mode. It had the common blown output pair with the extensive collateral damage to the driver stage. I replaced the 4 outputs, bias transistor, driver and input transistors, signal diodes, and zener diodes. Plus a few charred resistors. I went ahead and replaced all the small electrolytic caps with Nichicon PWs.

It works fine and sounds great. However, the auto-class A bias voltage does not hold. I measure the voltage across the 5w emitter resistor like the service manual states. Both channels hold 10mV fine in normal mode. If I hit the Auto-class A button, the repaired channel bias voltage increases up to 65mv over a few seconds. After peaking, it drops back down to about 30mv in about the same time. The original channel hits about 70mv and holds steady.

Any pointers on what to investigate for the bias voltage drop on the repaired channel?
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