tracing out schematics

From: Tony Duell <>
Date: Wed Nov 24 18:49:12 2004

> > One question. Have you found how the controller IC gets its power? Is
> > there a separate (maybe linear) PSU for this?
> Hmm, no linear PSU. I do hope it doesn't rely on the battery backup for
> the memory in order to start! How do typical SMPSU's start up given that
> there's no initial drive to the choppers to kick things off?

All sorts of ways. It's easy when the chopper controller is on the mains
side of the PSU, becasue you can use a resistor from the +ve side of the
smoothing capacitor to get things going, then take a supply from a
separate winding on the chopper transformer. This you clearly can't do if
the controller iC is on the isolated side of the supply.

In that case, you can have an auxilliary supply (which may be
linear or another switcher -- the 11/44 supply does the latter). Or a
suparate startup circuit on the mains side of the PSU to kick the
choppers into operation and get _some_ outputs which will then get the
chopper control IC going (I remember with horror the PSU in th DEC
(Hitachi) VR241 monitor!). I've never heard of one that needs a battery

> > 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...
> Ahhh... I didn't realise there was an H-bridge for the fans; I thought
> they were AC direct from ~ 30VAC on the transformer. The fans decided to
> keel over in one of our 11/44's after one of the feed wires got chewed
> and shorted against the case - everything else is still operational.

A common problem that will kill one (or more) of the transistors in the
H-bridge. Pull the leftmost PCB in the PSU case, the fan driver is
towards the bottom of that board (watch out, one of the choppers -- for
the startup supply -- is on there too, so there's mains around!). I think
there may also be a picofuse on that board that you should check.

Received on Wed Nov 24 2004 - 18:49:12 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:37:18 BST