tracing out schematics

From: Tony Duell <>
Date: Fri Nov 19 18:27:00 2004

> Pair of LM339's, an LM358, an SG3846, an SOC640B, and one labelled

339 = quad comparator. Maybe part of the power-OK circuit. 358 = dual
op-amp, maybe part of the voltage feedback loop or the current sense.
I'll gues the 3846 is a chopper control chip -- there are a lot of SG
devices for that (the SG3542 and SG3842 seem to be quite common).

Watch out for TO92 packaged devices (look like small signal transistors).
It would not suprise me to find a TL043 or equivalent regulator...

> "M8852 6-1003 876". No idea what that last one is/does - any ideas what

How many pins? It looks like a house-coded number, which may meen a
custom device :-(, it may meen a selected device (which is less of a
problem, and won't affect tracing the schematic if you can identify it),
or may just be a normal device.

A trick that sometimes helps is to work out the sort of function it
should have and then match up known power and ground pins, etc.

> it is? I suspect the SOC chip is an opto-isolator (it only has 6 pins
> and is curiously in a white package). Not sure about the SG chip but I

Almost certainly an optoisolator. Probably for the voltage feedback loop
(LED on the secondary side of the supply, connected to some kind of
voltage sense circuit, transistor connected to the chopper control side
of things).

> > This is presumably on the mains side of the PSU. Can you find any dead
> > shorts with an ohmmeter? Is there a dead short across the output of the
> > bridge, for example.
> Nope, nothing appears to be shorted at all - or at least I've checked
> the obvious stuff; bridge, chopper transistors, filter caps, power
> diodes etc. - but the fact that this resistor in the mains feed seems
> to just croak straight away seems odd. I'll see how it goes...

I think what I'd do there is connect a light bulb in place of the
resistor (say a normal 100W mains bulb, which should be OK for testing on
light/no load), then pull the chopper transistors and power up. If the
bulb lights brightly you've probably got a short in the
rectifier/smoothing capacitor stage.

Received on Fri Nov 19 2004 - 18:27:00 GMT

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