tracing out schematics

From: Jules Richardson <julesrichardsonuk_at_yahoo.co.uk>
Date: Tue Nov 23 06:30:55 2004

On Sun, 2004-11-21 at 19:42 +0000, Tony Duell wrote:
> > > Now what happens if you add the choppers?
> >
> > try the whole PSU with the bulb in place, or isolate everything
> > downstream of the choppers and just try those hooked to the DC output of
> > the bridge? (i.e. by removing the main switching transformer and the
> > small transformer that drives the gates of the choppers I should be able
> > to isolate everything else)
>
> I was going to suggest running the entire PSU, but in this case, can you
> find the drive from the chopper control chip to that driver transformer?

Yep. It's a couple of tracks between the SG3846 PWM IC (which I've found
a datasheet for now) and the driver transformer, but I can cut them and
bridge them later.

Interestingly, there's no feedback between the low-voltage side of the
switching transformer and the circuitry surrounding that SOC603B 6-pin
IC, so maybe it isn't an opto-isolator at all. All the circuitry
surrounding it is on the 'hot' side of things, and its sole function
seems to be to control the SCR which sits between the live input to the
PSU and ground on the bridge output.

The main PSU board seems to be responsible for generating +5V only; the
second board (which I haven't even looked at much yet) handles +12V and
the battery backup control.

I think whoever made this thing was involved in some "design the most
complicated PSU possible" competition :)

I need to check for shorts on that second board, though. It gets fed DC
output from the bridge, but when I tried the lightbulb trick the other
day I didn't have it plugged in. Quite possible there's a short on that
board and the 'main' board is fine... it's something I should rule out
sooner rather than later!

cheers

Jules
Received on Tue Nov 23 2004 - 06:30:55 GMT