Yamaha M-80

[Tejaus]

I had originally posted this under EW's dc offset thread on Audiokarma but I thought it might be kinda buried there. Any help would be appreciated as these amps are chock full of components. I have a service manual and am planning on doing a recap since these amps double as space heaters.


I have adjusted the bias per the service manual to 15.5mV +/- 1mv and now have about 50mV dc offset on the left channel still. It will start out around 29mV then increase as the amp gets hotter. The left channel is also running about 10 degrees hotter than the right channel. The right heatsink measures about 147, the left about 157 degrees at idle. The manual also states for confirmation of Auto-A function using the same emitter resistors test points that the bias should increase to more than 31mV when you turn on the Auto A button. However, I get only a very slight increase, just to over 16mV then back down to 15.7mV. This happens on both channels so I'm not sure what's going on. Anyone have any ideas?

[Tube_Dude]

Originally posted by Tejaus



I have adjusted the bias per the service manual to 15.5mV +/- 1mv and now have about 50mV dc offset on the left channel still.

Hi Tejaus

-Have the offset begun only after the bias adjustement?

-Have the M80 a pot for adjusting the offset?

- What are the value of the emitters resistors (where you have measured the bias ).?

But anyway , 50mV offset voltage is nothing to worry about...

[Tejaus]

Jorge,

The offset was there before I did the bias adjustment. I was having issues with the protection circuit not releasing when I would first turn it on. I traced that problem to the adhesive used to glue the 1000uf 100v caps to the board. The right channel doesn't have any dc offset at all so I would like to get the left channel to match.