Do you have a cable with a white (nylon?) edge connector to fit onto the front edge of this PCB? If not, you need to make one. Actually, these bricks are relatively friendly in that there are no lethal voltages in them (they runn form a 20-30V AC input, which comes current that will flow in the event of a short circuit is not that high either (10's of amps), although it is enough to make small signal transistors explode (that's from experience!). I bought an RK07 that was full of water (seriously, it poured out of the hollow baseplate...). After drying it out, cleaning the heads, connectors, etc, it worked fine... -tony

IF you're missing this, it's not too hard to make it... Well, you may not need to really understand what you are doing at first. About 95% of problems in these bricks are cured by testing and replacing the power transsitors and the electrolytic capacitors. -tony

OK... That's good. I hate lucky-dip servicing too... There are several good references on SMPSU design (the section in 'The Art of Electronics' 2nd edition is a good staring point), and these DEC PSU bricks are a pretty standard step-down regualtor circuit. The only unconvention feature is that the controller is a 723 chip, not a special SMPSU chip. The 723 is basically a votlag reference and a (separate) op-amp in one package, so it's not hard to understand. o-----\ /----------------+-----UUUUUUUUUUUU-------+-------o \ </ | L | -------- | | Im | ----- ===== Control ^ D ----- C Out /|\ | | | | | o----------------------------+------------------------+-------o Here's the basic circuit of the power-handling part of the step-down regulator. When the control signal is asseted, and the transistor is turned on, D is reverse-biased (so does nothing), and the left terminal of L is +ve. Current starts to flwo through L, but the back emf of L effectively drops the voltage that appears at the load (and, indeed, energy is stored in the magnetic field of L). When the control signal is deasserted, the transistor turns off. The magnetic field of L collapses, so that now the left terminal is -ve. A current (effectively supplied by L) no flows via D (the 'flywheel diode'), and supplies the load. C, of course, acts as an output smoothing capacitor and preveents the load voltage changing too much. Yes, that's an oversimplification (but not as bad as some explanations I've seen in print) but it will at least explain what the main components on the schematic do. -tony

It's not cheap in the UK (about \pounds 40, I think), but it is _good_. I think it's worth the money. Of course I don't know what price you were quoted... Oh, the H7441. For some reason I assumed you had standard H744 5V 25A regulators. Those also use the 723. Probably one 555 is an oscillator, the other is the pulse-width modulator or something. I don't have the H7441 printset to hand. But to be honest, problems with the control circuitry are very uncommon in these bricks (and indeed in SMPSUs in general) -- most of the problems are in the power-handling part. As I said, most SMPSU faults are capacitors or power transistors :-) -tony

The value of that capacitor is not _too_ critical. 22000uF would be near enough, for exampkle. You are lucky to have other local collectors. -tony

The good news is that capacitors have got smaller over the years, so a modern one is likely to be able to fit in the case. You might have to make up some brackets, etc to hold it there... You are lucky. I don;t know of any others in the SW London area, or even in all of London. Pity, it would be nice to meet some from time to time... -tony

Jochen, the latter are 32A regulators, right? - If so, and particularly if part of the box is from a different source, you may want to make sure you have the later revision of the power distribution gear. The older version might let out some magic smoke: it was designed for the (IIRC) H744 brick (25A max.). When the old distribution is used with the later, stronger H7441, and with a stuffed backplane, it can get overloaded. I may be able to look up the critical part numbers if you cannot find them elsewhere. -- Andreas Freiherr Vishay Semiconductor GmbH, Heilbronn, Germany http://www.vishay.com