> -----Original Message----- > From: cctalk [mailto:[email protected]] On Behalf Of Mattis Lind > Sent: 01 November 2015 12:46 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load > > söndag 1 november 2015 skrev Robert Jarratt <[email protected]>: > > > Some of you may recall I have a faulty H7864 PSU, which failed a while > > ago with a loud pop, but no obvious physical damage. I replaced the > > blown transistor (on the primary side of the large transformer), but > > when I power it on, the transistor does not switch and there is no > > output, so clearly there is still a problem somewhere. > > > > > > > > I have been spending some time drawing schematics for almost the whole > > thing. I am now at the point where I intend to compare it with a > > working one, probing each one side by side, to see up to where it > > appears to be working. It would be awkward to have two dummy loads, > > just for lack of suitable equipment. Does anyone know if it is safe to > > run these PSUs with no load? Would my testing be valid without a load? > > > > > > > Maybe you can publish your schematic somwhere. It is much easier to come up > with ideas to pin point problems if we all view the same schematic. I think it > would be possible to identify the problem without comparing the two PSUs at > least in this case where there is some kind of fundamental problem.
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 would start with trying to decouple the bias voltage powering the switch > logic > so that it could be run from a bench supply while powering the main switch > transistor and power transformer from a protection transformer and variac. If > you run like this you could start without any dummy load at all. > When you get to a higher input voltage from the variac it could be useful to > have some small dummy load. 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. > > What kind of chip is controlling the PSU? With bias power applied is there any > switching activity output from the chip? The RC network that usually make up > the time constant should have some kind of sawtooth signal I would guess. If > not it can obviously be broken or some feedback signal has caused it to shut > down, for example due to over current feedback. 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. > > /Mattis > > > > Thanks > > > > > > > > Rob > > > >
