On 22/01/2017 07:31, "Tony Duell" <ard.p850...@gmail.com> wrote:
>> 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. 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?