Apparently some of the other DEC power supplies include a fuse on the AC input for the 5411086 board! I guess they discovered the problem in the field, too. It is a little ironic that the board that generates power fail blew up :) That's why I'd suggested the analog circuit that generates AC LO/DC LO -- both of them had gone away on the backplane. I also find it interesting that DC LO really has nothing to do with the output of the various independent regulator modules. You'd think since DC LO is open collector, each module would be able to pull it down and it'd only be released when all modules were stabilized. That is of course one of the issues with memory write-protect lockout on nonvolatile RAM on S-100: you have to wait til all three supplies stabilize, and then factor in enough time delay for each board to also have stabilized before releasing *PRESET and/or memory write lockout. I guess that's why they built a signal in for just that purpose on IEEE-696! The current draw through the connector should be 2-3 A, the combined output from the 5411086 board's +15V and +8V supplies isn't supposed to exceed 4A and is fused at 5A. It's switchmode (albeit kinda weird early switchmode, using a uA723) so current out should be less than current in, even accounting for efficiency losses. A 1A load on the +15V supply didn't register on the (granted, somewhat insensitive) power meter I was using on the AC transformer primary side. I plan to include a 0-15A AC current meter on the "actually in a box" version of my AC test supply, for finer measurement of module consumption at given loads. I've also seen various old computers and other machines with fuses bypassed, or something that's not a fuse in the fuseholder (cut off pieces of bolt or rod are common). Not a good choice! I did once see a military computer power supply with "battle fuse" switches -- the switches bypassed the power supply fuses, presumably so that you didn't lose computer power in a critical situation from a fuse breaking due to concussion from a shell hit or something. Thanks, Jonathan On Fri, Jun 7, 2019 at 11:29 AM Herb Johnson via vcf-midatlantic < vcf-midatlantic@lists.vcfed.org> wrote:
This caught my eye, so I looked at the schematic. Ironically, the shorted full-wave rectifier is shown on the "power-fail" circuit drawing. But the rectifier supplies all the unregulated DC voltage. Unfortunately the fuse F1 is *after* the rectifier and not before. Thus adding a fuse before the rectifier is wise.
Bypassing the now-destroyed edge-connector pins is also wise. Did you determine the normal AC current on those pins? I suspect a few amps, thus those pins are at risk anyway.
Thanks for calling this repair out. It's a cautionary tale about vintage power supplies in general; look for AC-side fuses, install if not. I"m looking at an IMSAI; the prior owner bypassed the AC fuse holder! ;(
Shocked, Herb
-- Herbert R. Johnson, New Jersey in the USA http://www.retrotechnology.com OR .net