I do intend to publish the schematics, but right now I am pretty sure they are: a) full of mistakes b) not drawn logically c) there are a couple of areas I couldn't trace without major surgery on the board. I am not a PSU expert, and I am not sure what you mean by a "bias voltage powering the switch logic". As for decoupling it, again I suspect that is way beyond my knowledge. I do have a variac though, and I believe it is not a good idea to power a SMPS from a variac. I guess the decoupling you mention would avoid the problem of using the variac, but I don't know enough to do that. There are no chips in the PSU (apart from a couple of comparators). Quite a while ago, I did put a scope on the base of the transistor I mentioned (using an isolating transformer), and could not see a signal (or rather a very small signal), whereas on the known working one I did see a signal. It has been my aim to discover why this is, but it is too complicated for me, with no real PSU experience, to understand.

Here is a link to what I have now http://1drv.ms/1KQkTBp There are Eagle sch files, and PNG files for those who do not have Eagle. Regards Rob

I did warn you! :-) :-), but thanks for looking and trying to make sense of them. Yes, I wasn't very disciplined with the labelling. Basically on the big schematic, called Secondary I have drawn the connectors for each of the riser boards. The text near the connector identifies which riser board it is the connector for and the connector schematic only has one connector, but it will always be called X1, I didn't make it correspond to the Secondary schematic. I will fix this. Note also that the small riser on the primary side is included in the Primary Control schematic, I didn't do a separate schematic, which was a mistake I think. Thanks, I will check this. I have found lots of mistakes, I am sure this is another one. Yes, I agree, the Primary Control is the one where the incorrect operation seems to be happening, Q301 does not appear to be switching. I think this schematic is as complete as I can make it, although it could still have mistakes.... Some time ago I got a scope on Q301, and I could see the b-e never went high enough to switch it on. There were some very short and very low pulses every 40us, but that was all. When I compared this to the working one, I could see a signal, still every 40us, but definitely switching the transistor on during the cycle.

2015-11-02 22:05 GMT+01:00 Robert Jarratt <robert.jarratt at ntlworld.com>: Checked the new one. There are still something weird. The base of Q301 it is short circuited to the emitter (which if that is true would of cause your problem). But I think this is a drawing mistake. The base signals goes to the D405 diode which is connected to emitter Q405 and emitter Q406 which is then connected to emitter Q301... Please check this again. Maybe there are a short circuit component that makes you trace it wrong (if you do it by ohm-meter) /Mattis

I have been doing some further checking of the schematic for the primary control section, because that is where the failure was, and I have found some significant errors. I have been over it as carefully as I can and hopefully it is now much more accurate. I have also made the connections to the primary side of the transformer correct, although as I remark on the schematic, I don't know how the transformer is wired in relation to the secondary outputs. Regarding the apparent short between the base and the emitter of Q301 (as measured, not as drawn - I have corrected the drawing), seems to be just the low value of R410, the resistance I measure b-e is 39R, which is exactly the marking on this resistor. So I think this would explain that. One other thing worth noting is that there is evidence of heat around D408, but it tests OK (in circuit). The updated Primary Control schematic is still here: http://1drv.ms/1KQkTBp Regards Rob

Koning transformer, it's ones. and supply. Those 780, I Hmmm.... Until your email I was pretty confident this was a switcher. But your description of a linear PSU seems to fit. I have posted three photos of the main board here: http://1drv.ms/1KQkTBp can you tell from that what kind of PSU it is? The big TO-3 on the left is the one that I found had failed. The replacement does not appear to be switching, but I don't know why. Regards Rob

I agree with Wulfman that it looks like a switcher. There's too much circuitry in there for a linear. Also, that pile of small electrolytics is typical for a switcher. That said, it's possible that there may be an auxiliary linear regulator in there. That's common for additional outputs with modest current needs, especially in older designs where the switching circuitry takes a lot of individual components. A TO-3 can isn't what I would expect to see in a switching-only supply. In other words, that particular transistor might be the pass transistor of a linear regulator. paul

duell BU208 is This one is a D905. bench choppers

If there are large-ish electrolytics (100's of uF) with a voltage rating around 200V and no reason to believe that there is a 200V output rail then it is very likely to be a switcher with those being the rectified mains smoothing capacitors. Some SMPSUs (Switchers -- it stands for Switch Mode Power Supply Unit) need a load, other's are happy without one. Occasionally DEC put the specification of the supply in the printset -- they did for the 11/730, and IIRC that supply is happy with no load on all outputs apart from something odd like the +12V rail. I've yet to find a DEC PSU that fails destructively if not loaded, it is more likely to just trip the overvoltaeg protection crowbar, shut down, then try to restart and repeat the cycle, tweeting as it goes Be careful!. There is a third type of supply, and it's one that DEC used a lot, going back at least to 1970 (that is, it turns up in most older PDP11s, etc). This has a big laminated core mains transformer, rectifiers and smoothing and then _switching_ regualtors (not linear) to produce the right output voltages. As with all switchers, that type might need a load (but my experience is that the DEC ones don't). Certainly the 11/730 is a mains switcher (it rectifies the mains to get about 350V DC, then chops it). In fact it's 4 SMPSUs in that little box. The 11/780 draws a lot of power. I don't think they would have used linear regulators (which by definition are inefficient) in that machine. I would bet quite a lot that all MicroVAX PSUs are mains switchers Also watch out that DEC somethimes had a little linear PSU to power the SMPSU control circuitry, at least before the SMPSU had rattled into life. The control circuitry (oscillator, regualtor, etc) is on the _output side_ of the isolation barrier in such supplies, the drive signal is transformer-coupled to the base of the chopper transistor. Those turn up in all sorts of things, the VT100 being the most common example. I hate to say this, but DEC seemed to like complicated and unusual power supply designs, and I would not want to try to repair one without a good schematic. -tony

Firstly, every Astec PSU I've seen has been a swticher. They probably did make linear PSUs, but not common... Secodnly, it may have been made to a DEC design (rather than just a specification). I believe you replaced the chopper transistor. Astec were, alas, notorious for using 'selected' transistors in some of their supplies. Often marked with a house number which I guess yours wasn't, but it is one to watch for. Good idea. Don't do what is suggested in the Philips P854 manual which is that if an isolating transformer is not available to disconnect the earth lead of the 'scope inside the plug and have the 'scope floating. The idea of a metal cased 'scope being a couple of hundred volts from local earth does worry me! -tony

No, probably not. Actually I may have one somewhere, but have never had to repair it, so have never run it without a load. Thanks for the warning though! How unusual. My experience is that a transistor protected by an expensive fast-acting fuse will fail short-circuit, thus taking out the fuse as well. -tony