-----Original Message-----
From: MMRCG@groups.io <MMRCG@groups.io> On Behalf Of David Stinson via groups.io
Sent: Tuesday, September 28, 2021 9:41 AM
To: Boatanchors@mailman <boatanchors@mailman.qth.net>; milsurplus@mailman <milsurplus@mailman.qth.net>
Subject: [MMRCG] "Magic" 5Y3?

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Over the summer, I've finally managed to refurbish a Harvey Wells APS-90 power supply.
It was a rusted hulk when I got it, including the inside and outside of the case, but the iron was still good:
https://photos.app.goo.gl/CGsPor2fGqdXpEkB9
https://photos.app.goo.gl/NCQHtMZ1gDPnoNHJ6

Cleaned the chassis with a Dremal tool wire brush (several of them.  USE EYE PROTECTION!  I pulled wire brush bristles out of my forehead :-D ) then coated the cleaned areas with Rustoleum.
Took the case and front cover to a talented and reasonable powder-coat shop, Misfit Powder Coating- nice work and reasonable in Wylie, Texas https://misfitpc.com/ where they sand-blasted them and powder coated and my, they look pretty now.  I asked for a shade darker paint.
Replaced the writing on the front panel with laser-jet printed decals.  I think it looks spiffy.
I intend to use it to power projects, after installing a regulator in the Low-B+ circuit to fix the output at 250V Regulated, at least for now. Easy to change to any other voltage by just swapping a zener or two.

APS-90 data sheet and diagram:
https://tinyurl.com/vpk53b4

Now- here's the mystery.  The Low-B+ buss is speced for 300V at 100 Mills, using a 5Y3 rectifier.
But this buss is "getting froggy" on me.  When I power the supply, the Low-B+ buss, before the
5Y3 has had even a second or two to warm-up, immediately starts outputting about 460V.
Watch this:
https://photos.app.goo.gl/SFZo7t7RM6x2hqoKA

I have not addressed the extra-high voltage yet.
More concerned with how the thing is delivering "instant on" voltage.  Is the 5Y3 gassy, so it acts like an 0Z4?  Several tests show no evidence of gas.
An open bleeder, so the micro-Amps sourced before the filaments heat is "spoofing" the meter?
Nope; the bleeder is good.

So this is interesting.  Anyone got any notions?
I'll obviously be replacing the 450V caps I just installed with 600V, after which the FET regulator will make the Low-B+ buss "nice."

GL OM ES 73 DE Dave AB5S



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