tracing out schematics

From: Jules Richardson <>
Date: Sat Nov 20 10:48:46 2004

On Sat, 2004-11-20 at 00:27 +0000, Tony Duell wrote:

> > "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.

It's only an 8 pin device - I'll get to that one eventually; it's
somewhere toward the output side and I've only got as far as the
rectifier / chopper circuitry so far.

Curiously, voltage select between 115V / 220V is done via a relay,
despite there being a manual voltage select switch on the back of the
PSU. Not sure why that would be yet; there's quite a bit of circuitry
between the switch and relay, and I've only got rough schematics for
that bit noted down at the mo.

> 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.

Yep, sounds sensible.

> > 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).

It appears to be controlling the gate of an SCR, the anode of which goes
to the live input via a thermal resistor (which is placed next to that
5W resistor which keeps failing), and the cathode of which goes to the
'hot' ground rail. Some form of overload protection, I assume.

> 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.

OK, with the chopper array disconnected (plus everything downstream of
it) the bulb lights very brightly when power's initially applied, then
gradually extinguishes over a period of about 5 seconds. DC output from
the rectifier is 300V. Possible dried-out smoothing cap? The bridge
checks out OK on the meter although maybe it's failing under heavier
load conditions...



> -tony
"We've had a lot of loonies around this place, but you're the first one
who thought the sunrise was made out of stale beer. Now are you going to
pick up your flute and leave, or shall I part your hair with this
Received on Sat Nov 20 2004 - 10:48:46 GMT

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