It sounds like you have a real fault here. This is a very complex design and there could be many reasons so will have to gather as much evidence as possible in the form of voltage checks. Check that the Zener stabilised supplies to the front end are constant even though there is a changing offset. There are four measurements per channel here.
Also check the voltage at the offset adjust trimmer is constant. If this is varying then try isolating the wiper (remove the 56k from wiper to input stage) and see if the offset is now constant.
Also check the voltage at the offset adjust trimmer is constant. If this is varying then try isolating the wiper (remove the 56k from wiper to input stage) and see if the offset is now constant.
Zener D403 and D404 measured -13.00 volts. D413 and D414 measured -23.4 volts, on par with the diagram.
C401 has 0 mV on the negative side and +16mV on the positive. C402 has -8 mV on the neg. side and +11 mV on the pos. side. I replaced the C402, it made no difference.
VR401 and VR402 measured about the same, as did the Q401 and Q402.
C401 has 0 mV on the negative side and +16mV on the positive. C402 has -8 mV on the neg. side and +11 mV on the pos. side. I replaced the C402, it made no difference.
VR401 and VR402 measured about the same, as did the Q401 and Q402.
Apologies for the late reply.
The voltage measured here should be constant and also if the resistor R419 is isolated (lifted) it should make little difference to the offset. All that would happen is that the normal small offset would not be trimmable. Lifting that resistor might be a quick check worth doing. If the high offset remains we know its nothing to do with the trim circuit:
One important question... does the amp have any hidden history? In other words has it been working while in your possession and this is a random fault that has occurred or could the amp have been worked on before by persons unknown?
Ideally I would at this point quickly look with an oscilloscope at the output to make sure the problem really is a pure DC issue and not related to any instability or oscillation.
Other possibilities could be something like a resistor going high as it heats. Freezer spray applied very sparingly can be really useful in situations like this. The offset in a good amp should be little affected by cooling transistors and show no change cooling resistors or caps.
Those sound fine but what about the positive rail:
The voltage measured here should be constant and also if the resistor R419 is isolated (lifted) it should make little difference to the offset. All that would happen is that the normal small offset would not be trimmable. Lifting that resistor might be a quick check worth doing. If the high offset remains we know its nothing to do with the trim circuit:
One important question... does the amp have any hidden history? In other words has it been working while in your possession and this is a random fault that has occurred or could the amp have been worked on before by persons unknown?
Ideally I would at this point quickly look with an oscilloscope at the output to make sure the problem really is a pure DC issue and not related to any instability or oscillation.
Other possibilities could be something like a resistor going high as it heats. Freezer spray applied very sparingly can be really useful in situations like this. The offset in a good amp should be little affected by cooling transistors and show no change cooling resistors or caps.
