Okay, you may have more open resistors. Also the bias control transistors may be shorted. That will kill bias current. Stupidly, I see fuse resistors in series with the bias control. If that goes open, you will have excessive bias current. By excessive I mean blows fuse (at best) or outputs. If you have a serious fault in the output stage, the DC offset control may have little effect.
Edit: Yes, no DC offset controls. Your DC offset should be less than 50 mV, if it is greater you either have badly mis-matched diff pair transistors or a fault in the voltage amplification stage (Vas). The diff pair is Q701, a flat package with 5 leads. 2SCxxxx (something).
Edit: Yes, no DC offset controls. Your DC offset should be less than 50 mV, if it is greater you either have badly mis-matched diff pair transistors or a fault in the voltage amplification stage (Vas). The diff pair is Q701, a flat package with 5 leads. 2SCxxxx (something).
@anatdc offset is 6mV, 9mV surprisingly.
As for quiss current they recommend 6mV +-2mV , for 27mA +- 10mA , so around 10mA for 2mV? . I measured 1mV or so , and the pots would make it at best 3mV so something must be wrong, still.
I know how crossover distortion sounds like but even with this low quiss current I can't hear any. So i didn't bother. As he might burn the amp again, " abusing " it.
As for quiss current they recommend 6mV +-2mV , for 27mA +- 10mA , so around 10mA for 2mV? . I measured 1mV or so , and the pots would make it at best 3mV so something must be wrong, still.
I know how crossover distortion sounds like but even with this low quiss current I can't hear any. So i didn't bother. As he might burn the amp again, " abusing " it.
Okay, your DC offsets are fine. Who needs servos?
27 mA bias current is fine, I expect 20 mA to 30 mA normally. Some run as high as 50 mA for normal (BJT) outputs, normal class AB. Some amps are fine at 10 mA, but they adjust high in case you have bad matches. You don't actually need that much if everything is within normal parameters for the semi's.
Your controls ought to be able to adjust the bias current quite high and almost off or off completely. So you do have a problem. At this point I would check the protection transistors (Q704, Q705) and associated diodes. In fact, it is weird having two bias controls, very non-standard. They will probably interact because you have the voltage amp stage trying to maintain a zero output voltage. The main bias control is via TH701, TH702 and R711. Really non-standard, but if these have a problem your trim controls will not work properly at all. Q709 looks like a bias control transistor - check that especially.
You should have an oscilloscope and THD meter. You absolutely do need those in addition to a good DVM.
Tell me you have an actual transistor tester, not a check on a cheap DVM!
-Chris
27 mA bias current is fine, I expect 20 mA to 30 mA normally. Some run as high as 50 mA for normal (BJT) outputs, normal class AB. Some amps are fine at 10 mA, but they adjust high in case you have bad matches. You don't actually need that much if everything is within normal parameters for the semi's.
Your controls ought to be able to adjust the bias current quite high and almost off or off completely. So you do have a problem. At this point I would check the protection transistors (Q704, Q705) and associated diodes. In fact, it is weird having two bias controls, very non-standard. They will probably interact because you have the voltage amp stage trying to maintain a zero output voltage. The main bias control is via TH701, TH702 and R711. Really non-standard, but if these have a problem your trim controls will not work properly at all. Q709 looks like a bias control transistor - check that especially.
You should have an oscilloscope and THD meter. You absolutely do need those in addition to a good DVM.
Tell me you have an actual transistor tester, not a check on a cheap DVM!
-Chris