Thanks Eric, I had a private reply that said pretty much the same thing.
Last night I replaced the shorted diode and another electrolytic capacitor and the PSU started up on the bench, powering a load board and an RD53 hard disk. Ripple was good except on the -9V output, but that output doesn’t seem to have any capacitors after the final transformer, so I think it is OK. I need to replace a few marginal capacitors on the video module now, while I have the machine in bits. Then I will put it all back together to see that it still works. I have a lingering worry that the failed diode was on the +28V supply to the video module and that there might be a fault on the video module, but it doesn’t appear to present a short circuit, so I am hopeful. I will do a blog post once it is all back up and running. Thanks Rob From: Eric Smith <space...@gmail.com> Sent: 16 April 2020 00:05 To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> Subject: Re: VAXmate PSU On Fri, Apr 10, 2020 at 10:14 AM Rob Jarratt via cctalk <cctalk@classiccmp.org <mailto:cctalk@classiccmp.org> > wrote: D12 is an MBR3045PT. It tests correctly as a common cathode diode network. However, the forward voltage seems to be 0.19V. The datasheet (https://pdf1.alldatasheet.com/datasheet-pdf/view/53622/FAIRCHILD/MBR3045PT.html) would suggest it should be 0.76V at room temperature. I can see no physical damage to it though. Even when they are good, the forward drop of a diode can be FAR lower than the typical forward drop (around 0.7 for normal silicon diodes) if you're putting significantly less that the rated current through it. 0.19V forward drop is only slightly lower than typical characteristic at e.g. 10mA current. See figure 3. I've seen more than enough variation of diodes from typical curves for that alone to convince me that the diode is bad (though obviously it may be). The real test is how much it conducts in reverse. All diodes will pass a small amount of reverse current. This one shouldn't pass more than 1 mA in the reverse direction at room temperature, even with near the rated reverse voltage (45V) applied.
