Rotel RB 980 BX bias question

[recorder]

Dear members,

I am trying to bias my beloved Rotel 980 Amp (10 mV according to the technical manual). Unfortunately, at the testpoint, my DMM is showing 0mV on both channels. Turning the pot does not change anything. Something seems to be wrong but I have no idea what to look for? Can anybody please help?

Thank you
recorder

 

[sgrossklass]

At the testpoint, singular? It's two each.

Can your multimeter reliably measure voltages that low?

If the amp really ran at zero quiescent current, it probably wouldn't sound too great. Did you notice anything unusual in that regard?

Assuming your equipment is OK, measure the voltage across R631 in the left channel (it's a 12k1 that connects to the bias adjust pot via R633). It should be approximately 2.7 V at correct bias (+1.7 V and -1.0 V when measuring to ground). For minimum bias it would be about 1.5 V or so.

 

[recorder]

Sorry, two testpoints .

Thanks a lot for your answer - about the sound I cannot tell 100%. I will measure R631 this evening and post the result.

recorder

 

[east electronics]

often people make the mistake to measure test points to ground .... to measure bias you measure between the two test points ....

 

[recorder]

@sakis

I see your point... there are so many possibilities do good things the wrong way - thank you for your reply!

@sgrossklass

It looks as if you were right on the money. I have measured R631 and it was showing about 2.2V. I turned the pot on both channels until the corresponding resistors showed 2.7V on the DMM. According to what you said that should be ok now. The sound has changed slightly. Cannot tell if it sounds "right" now, but it doesn't sound "wrong" either ...

Thank you very much !!!

 

[sgrossklass]

Do watch out for heat dissipation now. With such a ballpark adjustment, quiescent current may also differ between the two channels. Maybe you can measure a non-zero voltage drop across the testpoints now.

With only 2.2 V across R631 the output stage is definitely on the underbiased side of things... that's like 1.35 V for 3 B-E voltage drops. The difference you are hearing, assuming it is real, would be due to considerably reduced high-frequency distortion.

 

[recorder]

Good idea. I will try and measure again at the testpoints and see if there is a change.

The difference you are hearing, assuming it is real, would be due to considerably reduced high-frequency distortion.

It is hard for me to describe exactly how the sound has changed but it could be exactly what you said.