[Boatanchors] "Magic" 5Y3?

[David Stinson]

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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