22 Jan
2017
22 Jan
'17
9:33 a.m.
On 22/01/2017 07:31, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
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.
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?
One more failed component off the list anyway.
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
The -24V
regulator can be seen just above the 4 blue caps, The regulator
that can be seen under the black wire is a 7812. The LF351N has -12V present
at its bias outputs, the opamp to its right has no markings helpfully.
What about that IRF523? That is a power MOSFET transistor and might well
be the chopper in the switching regulator circuit.