On Thu, Feb 5, 2015 at 4:54 PM, tony duell <ard at p850ug1.demon.co.uk> wrote: OK, please forgive my ignorance, here. To give you some idea of my electronics expertise level, I'd like you to know that I just googled crowbar :D Now, I'm handy with a soldering iron and understand these sorts of things OK once I have a run through, but I just have no idea where to begin. So again, forgive me, but I'm going to ask some very green questions. I'm sure you guys are going to want to strangle me or laugh me out of the room, but, hey, I'm trying, darn it, so please don't plonk me! From the wikipedia article I read, I guess this crowbar thing tripping, is just its way of trying to protect the rest of the system from overcurrent, right? And the dried up cap is one thing what could make it think there was an overcurrent situation, correct? I'v had a little luck in the past with such things, but I don't know that I'm going to understand how to read the schematic. I'm going to try and find one now. Also going to try to find a manual that describes the power supply. But how do I know which one is the output cap? What is a valid test once I find it? I guess I could charge it up on a car battery and see how long it holds or something like that. I don't have much electronics gear here. Borrowed a Craftsman digital VOM from my electrician friend (although it's telling me I have 140v AC on the mains!) and I have a soldering iron, heated desoldering bulb thing and some desoldering braid, flux and lead solder. BTW, according to this VOM, there's 24.5 AC and 14.2 volts DC being input into the H745 regulator. Is this too low? Maybe this has something to do with the H742 adjustment that I didn't find? Or maybe this VOM is miscalibrated. thx jake

I (hopefully) never have and never will flame somebody who is trying to learn. Sharing repair information (at all levels) is one thing this list is for. [...] Overvoltage, not overcurrent. The idea of the crowbar circuit is to short circuit the output of the power supply if the voltage rises too high, thus protecting the rest of the machine. This, will, of course, cause a high current to flow in the PSU circuit and the overcurrent circuit should then operate [1] but that's a result, not a cause. [1] I once had an H754 (yes) where the overcurrent circuit had failed and the crowbar operated. The result was a number of transistors literally blown off the PCB. I think I had to replace 3 or 4 of them, the 723 chip, the fuse, etc. This is as switching regulator. Basically there is a power transistor that is either turned hard on (saturated) or cut off. When it is on, energy flows from the transformer and is stored in the magnetic field of the inductor in the module. When it is off, tht stored energy supplies the load. The capacitor is another energy store, effectively to smooth out the voltage changes from this switching. If it fails (the electrolyte dries up and it seems to have a large-ish resistor in series with it), then you get high voltage spikes on the output. These then trip the crowbar. I've not seen what the 11/45 manuals cover, but somebody else mentioned the 11/34. The 11/34 PSU, for all it is different, uses the same regulator bricks, so I assume that bit of the manual is applicable here. When I have unpacked all my printsets I can give you a circuit reference. But just look for an electrolytic capacitor between the -15V output and ground (I think it is the second largest can on the board, the really big one is the input smoothing capacitor). No, you are looking for series resistance, not leakage. The only real way to measure it is with an ESR meter (a device for checking such capacitors, the name means 'Effective Series Resistance' and it's a very useful tool when working on PSUs. You need the flux-cored 'electronic' solder (resin cored), the real lead/tin stuff is what was originally used and should be used for repairs. Do NOT use any other fluxes, certainly not the acid flux used for plumbing/metalwork (OK, there are times when you add some more resin flux, particularly when doing SMD work, but not here) I think a desoldering pump is useful too, much better than the bulb. You need your own DMM. A 'scope is very useful too. And when you start working on the CPU itself, a logic analyser is handy Those voltages are fine. -tony

I do not want this to be taken the wrong way. I am not trying to insult you, nor do I want to put you off restoring the 11/45 (which is a very nice machine). FWIW, the 11/45 was the first PDP11 I ever restored and the first I ever used. All I had was the printset (schematics and fortunately microcode flowcharts). No technical or maintenance manual. No instruction set listing. No web pages to look at. No bitsavers. No list like this one. I managed it, and you can too. But the 11/45 is complicated, both in terms of the number of components and way it was designed. There are quite a few non-obvious (at least to me) bits of logic circuitry in there. I think you need to read some more general books on electronics too. I am not sure what to recomend here (and of course suggestions are welcome). I do like 'The Art of Electronics' by Horrowitz and Hill, it does start from a basic level, but it may be rather heavy going at the start. It's still a book to consider (and probably buy), since you won't grow out of it. But there may be other inttoductory books to look at. -tony

I have never done an on-line course, and have generally learnt more from books than lectures, but that is probably just a personal thing -- how I am. Of tbe 2 courses you mention, the first sound a lot more relevant for repairing classic computers, but it may not go far enough. It can't hurt to take it, though. I would look at books _as well_. The more sources of information the better ;-) -tony