> -----Original Message----- > From: cctalk [mailto:[email protected]] On Behalf Of Paul Koning > Sent: 01 November 2015 16:52 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load > > > > On Nov 1, 2015, at 7:12 AM, Robert Jarratt <[email protected]> > wrote: > > > > 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? > > If it has a transistor at the primary side, and a high frequency transformer, it's > a switching supply. Those want a load, usually. Especially the older ones. > > If it has a laminated iron transformer, big electrolytics for filtering, and > transistors at the secondary side, it's probably a linear regulator supply. Those > don't need a load. You may want to do full testing under load to observe > correct regulation, but for initial testing you can test them without. > > Linear supplies are older; I think by the time of VAX you're likely to see > switchers (with the possible exception of first generation stuff like the 780, I > don't remember if those big supplies are linear or switchers). > > paul
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
