tracing out schematics

[Tony Duell]

> 
> 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

That does not suprise me. Since there's a pulse transformer in the 
chopper base circuit, the controller IC is on the isolated side of the 
PSU (the isolation is performed by the pulse transformer), so there's no 
need for any optoisolator in the voltage feedback loop.

One question. Have you found how the controller IC gets its power? Is 
there a separate (maybe linear) PSU for this?

> 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.

It may also pwrovide +5V. It sounds like the second board handles the 
battery-backed lines, presumably for memory.

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

No, that honour goes to the supply in the PDP11/44. The official 
scheamtic is over a dozen A3 pages (!). It's 3 choppers running off the 
same (coke-can size) capacitors, one to provide the rest of the PSU 
control circuitry with power, one for the +5V/+15V/-15V logic supplies 
(the +5V line is rated at 125A IITC), one to provide a 36V line that can 
be battery backed. The last is then regulated down (more switching 
regulators...) to provide +5V/+12V/-12V for the memory. Oh, and there's a 
full-H driver for the fans that runs off the +36V line.... As I said, 
complicated...

> 
> 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

I will refrain from flaming you just yet :-). Seriously, it would be 
useful to know which, if either, board has a dead short!

-tony