My trusty QED A240SA, bought from new back in 1987, now has a hum (sounds like mains). I've spent a while trying to trace its source, replaced several capacitors, literally reversed engineered the whole circuit diagram (I downloaded a service sheet and found this to be helpful, but was incorrect and unclear due to photocopy, and missing information in places). Anyway, I have learned a lot about amplifier circuitry (Class A/B/AB...), but still not enough to solve the hum. The hum can be heard distinctly through the speakers and headphones. Separating the pre-amp and power-amp stages, the hum is still there, so getting in at the PA? The hum is common to both channels, so assumed a capacitor common to both, which have all been replaced (as far as I can deduce the correct type, i.e. ceramic, polyester film, etc), this is where I think experienced knowledge is now required: there is some ripple at the main PA output terminals at around 9mV, possibly at 100Hz (as a result of the push-pull design? Is this enough to be audible?). The main 4700uF caps haven't been replaced (I can't find a direct replacement yet, but they test OK with my cheap tester). There is a large amount of ripple on the +/-33V power lines, similar frequency, at around 800mV, but I think this is normal for this type of amplifier? There is a lot more to this, as I managed to blow up the right channel transistors, but now thankfully back to square one after replacing several of them. But the hum is still there! Any pointers, ideas most welcome please? All the best.
