tracing out schematics

[Tony Duell]

> > > 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.

Odd.. I wonder how the voltage feedback works, then. It's _possible_ that
the chopper control circuit is on the isolated side of the PSU (and
therefore no opto-isolator on the voltage feedback loop),
transformer-coupled to the chopper transistors (DEC were fond of doing
this), but I wouldn't assume that without checking

The circuit you've found sounds like something designed to blow the fuse 
(or at least shut the supply down) if there's a problem. Maybe part of 
the crowbar cirucit (although I wouldn't have thought that shorting the 
input would shut the outputs down fast enough), maybe something to blow 
the fuse if the input votlage selector is set incorrectly (the HP 
Integral PSU contains a triac circuit the sole purpose of which is to 
blow the mains fuse if machine is connected to 230V mains with the 
selector set to 115V).

> 
> > 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

Maybe OK. It could just be the charging current of the smoothing caps 
(they'll cahrage a lot more slowly with the lamp in series). Now what 
happens if you add the choppers?

-tony