22 Jan
2017
22 Jan
'17
12:10 p.m.
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