Tony Duell wrote: > On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk > <cctalk@classiccmp.org> wrote: > > > In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow > > wire (an isolated circuit). Pin numbers 14 & 13. Try measuring voltage > > across the suspicious connections as any reference to ground may not be > > indicative. > > > > This voltage is also present with the H7819 PSU. It's not clear to me > > what it is used for. > > It's only a guess (this is rather more modern than the DEC hardware I > have), but it may be a supply for a thinwire ethernet transceiver > circuit. This would be insolated from system ground of course, and > I've seen something similar in an X-terminal. >
Hi Tony, After years of rediscovering the same failures over and over again, I finally managed to get in the habit of making notes while I am trying to diagnose faults. Now I just need to get in the habit of reading my old notes when I start to work on a new fault. I did that just now and I found that I have a MicroVAX 3100 model 76 whose internal thinwire tranceiver failed last summer. Not finding any obvious reason for the failure, I replaced it with an external tranceiver plugged into the AUI port and then forgot all about it. I should check that this floating 9V output from it's H7821 PSU is working correctly. (I wonder why it is described as -9V on the label of the 30-35042-01 PSU in my VAX 4000/100A? If it had been shown as +/-9V for example or even just plain old 9V, I would have been less inclined to assume it was referenced to ground.) Regards, Peter. > > -tony > > > > > > > Maciej
