On Sun, 5 Oct 1997, Marvin wrote: Are you sure? _None_ of my IBM TechRefs (I have : PC, XT, AT (2 vols), O&A (2 vols), Scientific, EGA, PC-jr) have any PSU schematics at all. Well, the PC-jr PSU card is there, as is the schematic of the professional monitor PSU (the CGA monitor PSU is missing...). Horrowitz and Hill 'The Art of Electronics' has the Tandy 2000 PSU schematic in the 2nd edition (silver cover), along with a circuit description. The best source I've found for SMPS operation/repair is 'Television' magazine. This is a UK magazine for TV service engineers, and has a lot of useful hints. Most SMPS's work the same way, so if you can fix the one in a TV, you can do computer ones as well. When the chopper fails, it's worth looking for other faults that caused that. Things like : Snubber capacitor gone O/C (VT100's are famous for this IMHO) Shorted turns in the transformer. A 'ringing' tester is useful here. Base resistor gone high, or its parallel capacitor gone O/C Collector current similation resistor gone high (TDA4600-based PSUs are famous for this - and they do turn up in some computers, like the Tatung Einstein). Other common faults are : Mains rectifier diodes gone O/C or S/C (in the latter case the fuse will blow) Startup components (resistors/capacitors) failed (PSU won't start, but if you can get it going, it will keep running) Output capacitors gone O/C or with high ESR. This causes spikes (of a few volts) on the output lines, which generally trips the crowbar. I've had this one several times. Boschert 2-stage PSUs have an unpleasant and common failure mode that kills all 3 choppers, the control IC, several small transistors, some passives, and the PCB tracks. I can provide details if anyone's interested. -tony

I think: those XT power supplies in PC and XT series uses single power transistor which is not used after the dual transistors type since the AT power supplies, and it's not changed much since even in clone ones as well. Also, judge when cost is too much to fix when you can retrofit a superior better quality power supplies out of PS/2 style PSU into the orignal power supple shells, the fans IBM used is way out high quality. Usually 10 percent out of dud power supplies I repair are non-startable even after I find all the duds. Dunno why. Having diffculty ordering the Bob's ESR meter because of being a Canadian. Any suggestions? Jason D.