23-02-2015, 10:05 PM
Having fitted the new o/p transistors, I cautiously powered up the amplifier, and attempted to set up the DC offset for < +/- 30mV as per the service instructions, only to find that on both channels the pre-set pors (RV401 & 4020 needed to be set to max resistance. In addition, although I could get the offset down to zero as measured with a DMM across the speaker connections there was a problem in the LH channel in that the offset would drift to over 400mV. The cause turned out to be that both presets had gone high reasistance, in one case to about 13000 ohms!. I know this amp is maybe 30 years old, but didn't expect that! The original presets were open 'skeeton' types, but I'll be replacing them with enclosed Piher or similar components. The quiescent o/p collector current could, in both channels, be set to <30mA (<30mV across the 1 ohm resistor in each 2N3055's collector supply) so, with new, reliable pre-sets all should be OK.
