Headscratching time. On first receipt tried the amp out as owner had been using 1 channel only , after having blown the other by shorting it. Both LV ac fuses ok but one SAP15 p-type blown. Had to cross-over power Darlington terminals as mirror footprint from the usual BCE orientation. Cleared out and replaced with that fudge for the SAP and other blown
1/8W Rs and tried out at 50% mains. Variac meter swung right over to the right and both LV fuses blew. Replaced fuses and tried at 20% mains, half amp or so of mains draw. Diconnect mains and main +ve and main -ve rail measure 0 ohm to ground for 10 or 20 seconds and then goes high impedance. Tried at 10% mains and sometimes +ve rail would be at zero ohms for 20 seconds or so and sometimes -ve rail like that, on repeating this game. Cannot see anything in the way of a crowbar SCR anywhere or relay and circuit looks much as the A500 that is out there , including overlay numberings. There is conductive brown glue to the main caps but that measured 2 meg or so minimum, with close DVM probes. All output devices and my fudge cross-compare DVM-D cold fashion, fine still. When I say 0 ohms I do mean that , well 0.1 ohm or less resistance. Unless anyone has an idea what is going, the next will be to remove my fudge and power up again.- posted
7 years ago