Yamha M-80 no bias in one channel

Those 47 Ω base resistors on those 4 TO-126 transistors were added to limit the damage in the even of and output shorting out. There not in the M-80 schematic. But they are in the M-85 schematic M-80/85 is the same amplifier electronically. Same power transformers and all. Bought the only difference is the front and rear panel lettering.
 
And M-85 is and M-80 with all the service bulletins complied with.
 
The M-85 is advertised as 5 more watts per channel than the M-80. Both are very conservatively rated. Typically measure around 295-298 watts per channel before clipping @ 1 khz @ 8 Ω load.
 
I tried connecting a meter to TR115 and it does appear to increase in voltage when turning the bias pot. It goes from 2.8v to about 3.6v, this is through the dim bulb.
 
When the unit is powered up does the bulb start bright , then dim down ?
 
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Do you have an o'scope and signal generator? If you do, try passing a signal thru WITHOUT a load, if the amp passes a signal up to clipping in this condition the amp "should" be OK up to and including the drivers. Now load the channel with a large 8 Ohm resistor and try passing a signal again, if the output stage is the problem you shouldn't get too far without seeing a problem. Now if it passes this test up to clipping your problem is just low idle current and we need to concentrate on the voltage levels in and around the parts you replaced. You are positive all of your replacement resistor values are correct?

Craig
 
I have gone in and replaced all the TO-126 transistors in both channels. The left channel I can get a bit of an adjustment out of it now but it is weak. With the 150w bulb, I can only get a maximum of 36mv. It won't go any higher and it does not set off the protection. The right channel will go straight off and trigger the protection. I am getting about 10mv of offset in the right side and about 2mv in the left at the terminals. I don't have an oscilloscope or a signal generator, sorry.
 
Did you replace the 2SC1953. Its a small TO-126 plastic bodied transistor fastened to the large heatsink ? If yes. Where did you get it? Or did you sub it with something else ? If yes. What did you use ?
 
Did you replace the 2SC1953. Its a small TO-126 plastic bodied transistor fastened to the large heatsink ? If yes. Where did you get it? Or did you sub it with something else ? If yes. What did you use ?

Everything mounted to the heatsinks are new. I used KSC2690 as replacements for the 2SC1953's
 
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