> Result! Whilst photographing the board to trace out that circuit (it's > quicker for me than drawing it) I removed all the big capacitors. While they > were out of circuit I tested them and discovered that not only was one dead > in the -12V rail but also the through hole plating for another on the +12V > rail had failed to a trace on the back of the board so neither the unknown > 8-pin DIP or the LF351N were getting +12V.
Excellent! > > Fixed that and replaced the failed cap and now have -24V where it should be. > > If it helps with identification the unknown chip appears to be: > > 1 - GND > 2 - 56k resistor to pin 3 > 3 - Gate of IRF523 > 4 - +12V via 10k resistor > 5 - GND via decoupling cap > 6 - connected to 2 > 7 - NC > 8 - +12V > > That looks like an opamp to me, with bias pins connected to GND? Does it? It looks like a 555 timer to me at first glance. I would have expected another capacitor from pins 2/6 to ground though. 1 Ground 2 Trigger (linked to threshold, and to output via a timing resistor) 3 Output (driving the MOSFET) 4 Reset/ (pulled high via a resistor) 5 Control Voltage (decoupled to ground) 6 Threshold (see 2) 7 Discharge (not used here) 8 Vcc -tony
