Hi 55, 55N and 55N+ owners (and probably 100 series owners too),
I have built 5 or 6 of these and going back a few years where I worked with Hugh on the N and N+ upgrades I rebuilt/modified one amp about 8 times.
Recently one of the amps I built for someone was returned to me to repair. The owner stated that he had shorted a speaker connection and now one channel had about 1V of offset and bad hum on the output.
I eventually tracked this down to a blown (open circuit) R2, the 10 Ohm resistor which establishes the 0V reference for the input diff. amp. No fuses or semiconductors had been damaged. of-course I found that out after I had removed and tested every semiconductor in the amp
The reason for the post is that I have seen this exact same failure before. I was a bit annoyed that I did'nt recall the first incident until after I found the fault which was after a lot of un-necessary work. I think it happens when the "hot" speaker wire shorts to other grounded equipment (your preamp or similar).
So if you see this fault description, can I suggest you check R2 with a multimeter first, before ripping out semiconductors etc. You may save yourself a lot of time and aggravation.
Cheers,
Ian
EDIT: My theory is that the fault return current from the shorted speaker wire runs back into the AKSA via the shield of the input cable and then through the 10 Ohm resistor (R2) to local ground (speaker return). The 10 Ohms is large enough to limit the fault current to a value which can be handled by the output transistors without damage BUT is high enough to blow the resistor. That is the resistor is acting like a fuse. Say the fault current was 2 Amps. From Power = I squared R then that resistor would try to dissipate 2 x 2 x 10 = 40 Watts. The 0.6W rated resistor just blows up protecting everything else. We could almost think that it was designed this way on purpose although I think its probably just serendipity.
Todays Engineering "Wisdom":
The difference between practice and theory is that in theory there is no difference between practice and theory, whereas in practice there is.
