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
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
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.
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.
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
Thanks a lot for your answer - about the sound I cannot tell 100%. I will measure R631 this evening and post the result.
recorder
often people make the mistake to measure test points to ground .... to measure bias you measure between the two test points ....
@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 !!!
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 !!!
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.
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.
- Status
- Not open for further replies.