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.
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.
Looked at the service manual to find not too much info there. Both channels act the same? It is a rail switcher, could it be stuck on the high rail? Check the output transistor collector voltages at idle, +/- 33VDC?
Craig
Craig
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
Craig
Yes both channels act same. I was concearned since it gets very hot-too hot to touch lid very long after half an hour. It plays fine & sounds excellent-just didn't want to risk blowing a channel.
I just got it. I always check bias on any new amp since they are sometime too high. It was very dusty inside also.
Since it's class a will it pull close to it's maximum from the wall? I got a power meter I could check if that is the case.
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
Craig
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.
Just got an M-60 in for repair, both channels are fried. Anyway the service manual does have the Class A bias, 110mv across the emitter resistors, ouch!
Craig
Craig
That does seem like a lot especially for less powerful amp. BTW the heatsinks on mine gets super hot. Do you think computer fans would increase service life of these amps?
Wouldn't hurt. Although the SM states 110mv the highest I've ever seen is around 95mv. 110mv is nothing. Compared to a CA-1000 which is 890 - 990mv
Or 940 mv +/_ 50mv
Or 940 mv +/_ 50mv
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Boy.. I must have been drunk.. All the above voltages are suppose to be millivolts..
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