Replacing failed powersupplies on qbus PDP-11s

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