tracing out schematics

From: Jules Richardson <julesrichardsonuk_at_yahoo.co.uk>
Date: Fri Nov 19 08:03:09 2004

> On Fri, 2004-11-19 at 00:19 +0000, Tony Duell wrote:

Thanks for all the tracing tips :)

> OK, ready to start. I would start at the mains input. Trace where it goes
> (probably through a filter, then a bridge, then to smoothing capacitors).
> You do the tracing with a _good_ ohmmeter, combined with visual
> inspection (where does the track seem to be going!) and knowledge (the
> emitter of the chopper transsitor probably goes to the 'hot ground' via a
> sense resistor). Find the chopper control IC, find how it's powered, how
> it drives the chopper, and so on. Then find the feedback loop (votlage
> regulation loop). You will _never_ successfully trace a schematic you
> don't understnad.

There's a handful of ICs on the main board (I'm not looking at the
secondary board yet, which I suspect is more to do with battery charging
than anything):

Pair of LM339's, an LM358, an SG3846, an SOC640B, and one labelled
"M8852 6-1003 876". No idea what that last one is/does - any ideas what
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
haven't tried looking it up yet - quite possibly just another amp of
some sort.

This thing has no less than 8 chopper transistors, arranged in something
like an H-bridge configuration as typically found in motor speed
controllers.

> Personally I'd not pick an SMPSU as my first serious schematic to trace,
> but you can't always choose what you have to do :-)

Well it'd be nice to get the machie running again. As I recall there was
no obvious failure, just switched off one day, switched on the next and
it was totally dead. Hopefully whatever failed in the PSU didn't take
out too much other stuff with it! :)

> > Aside: the PSU doesn't blow fuses, but is totally dead and doesn't even
> > attempt to start. I found a dead 5W 10 ohm resistor (open circuit, no
> > discolouration) in the live feed upstream of the bridge, which seemed
> > like an odd component failure. I had a spare in the junk pile so
> > replaced it, and that one immediately went the same way.
>
> 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...

cheers

Jules
Received on Fri Nov 19 2004 - 08:03:09 GMT