Replacing failed powersupplies on qbus PDP-11s

[Tony Duell]

> Finally, does anyone know of any good guides on repairing switch modes?
> Whilst not keen, I guess I should bite the bullet (hopefully not
> literally) and learn how to repair them.

I've repaired SMPUs... The first one terrified me, now I am still 
cautious, but I will do them.

Until you get some expernince, don't even think of repairing a PSU 
without a scheamtic. There are several difference circuits, the control 
circuitryt may be on the mains side or the output side of the 'isolation 
barrier' for example, and not realising whcih is is a recipe for giving 
yourself a possibly fatal shock amd more importatnly damaging the 
machine. (Yes I ahve got that the right way round. There are many more 
hackers than classic computer out there...)

There used to be a 'repair guide to SMPSUs' (or some similar title) as 
part of the Usenet sci.electronics.repair FAQ. That is worth trying to 
find and read.

Also, if you're in the UK, try reading 'Television' magazine from around 
1985 onwards (or maybe a bit before that). Many TVs of that period used 
standard-ish SMPSU circuits, and the repair methods are much the same for 
computer supplies.

In general, stanr by looking at the fuse. If it has blown violently 
(shatered or blackened glass), suspect a major short on the mains side of 
the PSU. If it's a rectifier diode, then most likely it was not damaged 
by anything else, and replacing it will get the supply working again. If 
the chopper transistor has shorted (very common, alas), then it may well 
have been damafgd by somthing else. Maybe a shorted rectifier or 
capacitor on the secondary side (but the protection circuit _should_ cope 
with that), maybe a problem with the snubber network, maybe shorted turns 
in the chopper transformer (A replacement is avialalbe 'locally' as the 
manuals say, where 'locally' is either a misprint for luckily, or the 
name of an oscure Welsh village :-)). 

If the fuse is OK, and the supply is 'tweeting' (trying to start up, then 
shutting down, and repeating), then the problem could be a short on the 
scondary side, it may be a fault in the voltage regulation loop (so that 
the crowbar is tripping, shorting out the supply and shutting things 
down), or maybe an open-circuit or high ESR capacitor. The last is very 
common, some people who go in for 'lucky dip servicing' just test all the 
elctrolytics in the SMPSU with an ESR meter and replace any that seem a 
little high.

If the fuse is OK and the supply seeems dead, then probably the startup 
circuit has failed. In smaller supplies, this is often a high-value 
resistor (220K or so) from the +ve side of the mains smoothing capacitor 
into the depths of the chopper circuit. It oftne fails open-circuit. If 
you can find it, test it.

Beware of the last fault. Some SMPSUs don't have bleeder resistors on the 
mains smoothing capacitor, relying on the chopper circuit to discharge 
them. If this isn't running for any reason, the capacitor stays charged 
and will zap you. Now how do you think UI found that out :-)

-tony