Yamaha M65 Bias adjustment

gto127

Member
2007-05-09 2:51 am
Adjusting bias on a Yamaha M65. Manual states adjust to 10mv which is no problem then adds that when press "class a" button bias will be over 20mv.
When class a is pressed it's 80mv. I'm thinking this is too high. The unit gets very hot in only 5 minutes. It will play fine like this over an hour with no signs of cutting off but is sizzling hot. Can anyone confirm what class a bias on this unit usually runs and if high what could be issue.
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
There are two sets of voltages, the ones in () look to be for Class A? The voltage on the emitter resistors is 0 and (.1), that's 100mv so your 80mv looks to be OK. There is no mention of the () on the schematic that I could find. Does anyone know if the amp stays on the low voltage rail in Class A mode?

Craig
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
That's one of the things the service manual is lacking. How much Class A power does it produce? It's not biased hot enough to produce a lot of Class A power and then there's the rail switching which throws in another factor. If you have the test equipment you should be able to monitor the AC current draw at idle and then increase the signal until the current draw just starts to increase, that's the Class A power.

Craig
 

gto127

Member
2007-05-09 2:51 am
Although you already confirmed bias was OK I just received word from a shop that specializes in restoring these Yamahas that 80mv is completely normal for class a on this model so it's been double confirmed in case anyone has similar bias measurement question in future.
 
80-90v is about what they run. Had one a while back.That measured 83v on one channel and 60 volts on the other. Despite what the manual states-- 60v was abby normal. Found one of the emitter resistors was "open". Replacing the resistor brought it back up to 87v.
Boy.. I must have been drunk.. All the above voltages are suppose to be millivolts..