29-03-2013, 10:19 AM
The amp. will operate perfectly for hours at a time, then sometimes, when switched on, be totally dead. When this happens the voltage at the centre rail(C10+ve) is virtually at supply level(+17v), as it is at several other points in the circuit. I know that the o/p transistors(AC128 & 176) can suffer from the 'tin whisker' problem, but, unless I'm missing something, this would cause the voltage at C10+ to fall towards 0v, since the cases of the o/p pair are common with the negative line via the cooling clip & metal bracket. T1, T2, & T4 in this amp. are all 'Lockfit' types(2 x BC148 & 1 x BC159) which, I've read, can fail, possibly intermittently, though I've not found many to be faulty in over 40 years! All the electrolytic capacitors are originals, so maybe it's time for a re-cap. Anyone have any ideas as to the most likely suspects?