[Milsurplus] [MMRCG] "Magic" 5Y3?

Tue Sep 28 12:22:58 EDT 2021

DaveTwo additional comments:How fast does the bleeder resistor get up to temperature?  About 17 watts is dissipated at 450 volts across 25k so I would think that it is getting pretty toasty.  It could be possible that as soon as the bleeder goes above room temperature, it is open circuit.  Have a heat gun?  It is too much of a coincidence that peak and measured voltages are the same.  Just saying.
Does the 500 volt supply do the same instant over voltage trick?  You could trade the 5Y3 and 5R4 for the few seconds it would take for a test to see if the 5Y3 is the issue.Jim

Too much agreement kills a chat.  E. Cleaver

-----Original Message-----
From: Jim Whartenby via Milsurplus <milsurplus at mailman.qth.net>
To: milsurplus at mailman.qth.net <milsurplus at mailman.qth.net>
Sent: Tue, Sep 28, 2021 9:58 am
Subject: Re: [Milsurplus] [MMRCG] "Magic" 5Y3?

I don't think that you have to go that far, just load the power supply with a suitable 10k power resistor to effectively lower the DVM input resistance.  
It almost seem to me that the bleeder resistors are open.  That 25k should draw 10s of mA but it seems that they are not in the circuit.  
325vrms X 1.414 = 460v peak which is what you read on the meter.
Jim

Too much agreement kills a chat.  E. Cleaver

-----Original Message-----
From: Ray Fantini <RAFANTINI at salisbury.edu>
To: MMRCG at groups.io <MMRCG at groups.io>; milsurplus at mailman <milsurplus at mailman.qth.net>
Sent: Tue, Sep 28, 2021 9:13 am
Subject: Re: [MMRCG] "Magic" 5Y3?

When working on analog equipment use an analog meter , like a Simpson 260. See what happens then? 

Ray F/KA3EKH

-----Original Message-----
From: MMRCG at groups.io <MMRCG at groups.io> On Behalf Of David Stinson via groups.io
Sent: Tuesday, September 28, 2021 9:41 AM
To: Boatanchors at mailman <boatanchors at mailman.qth.net>; milsurplus at mailman <milsurplus at 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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