[R-390] 1960 S-W 390A Redux #1: The PTO Pt. 1 of 2

[Larry H]

Jordan,  Thanks for sharing your work with us.  Nice job - looking forward
to the rest of the story.

I noticed that you tested the pto heater system.  One thing though, the
thermostats are notorious for sticking in the ON position and damaging the
pto.  If you run with it on, be vewy, vewy careful.

Regards, Larry

On Fri, Aug 19, 2022 at 10:48 AM Jordan Arndt <Outposter30 at shaw.ca> wrote:

>
> Hi again group...
>
> Ok, I think the best way to describe the process I undertook to get the '59
> Stewart-Warner back in service is to go over one module/deck at a time,
> except for the Power Supply module and the AF deck which I'll combine in
> another
> message since there isn't a whole lot to describe.
>
> I had the good fortune of having a working 1960 S-W in which I could
> pre-test everything but the RF/Xtal Osc. deck. I got the '60 S-W back in
> service by testing its modules in a working (for the most part) Depot Mutt
> 390A based on an early single fuse Motorola chassis and a mix of decks from
> Motorola, Teledyne etc...
>
> Both of these S-W 390A receivers were bought for parts and spares well over
> 20 years ago and they had sat for several years before that, so they were
> more than a little on the rough side...
>
> Module 1, The PTO Pt. 1, Electronics
>
> The PTO I installed in this 390A is a Raytheon rebuild dated 11/23/76, and
> is a version of the Collins PTO using a corrector stack.
>
> When I had originally tested it in the '59 S-W I found that the wiring
> around and connected to the base of  Z702/T701 had been played with,
> leaving
> some conductors in positions so close that tapping the PTO tube resulted in
> a lot of crashing noises. In exploring that issue, I found that someone had
> begun to desolder the wiring and the 2.2k B+ resistor R703 connection,
> perhaps with the intent of pulling the transformer assembly, so I resolved
> those issues and the noise problem was gone. I put that PTO aside while I
> tested the other
> modules using the '59 as a test rig.
>
> After installing it in the 1960 S-W, and while trying to find the reason
> for
> what appeared to be a gradual drop in receiver sensitivity as the display
> frequency increased on each MC band so I tried to adjust C706 which tunes
> the primary of the PTO output transformer. I use a HB tool for adjusting
> the
> adjustment collar of that cap after misplacing(somewhere) the old original
> tool.
>
> The original tool was a 1/4" hollow aluminum tube about 4" long with a
> wall
> thick enough to hold shape after having been pinched to where it can
> engage
> with the
> flats in the adjustment collar of the piston capacitor.
>
> I could not get the capacitor to peak the output, but got closer to the
> peak
> with the shaft of that cap retracted as far as possible. In comparison
> with
> the same cap in a different PTO, it appeared that the adjustment shaft
> must
> have broken off at some point so the "piston" was only very partially
> engaged and short of adequate capacitance to peak the transformer.
>
> That may be why someone had poked and prodded in the underside wiring of
> T701 that I found.
>
> After some thought and examination, I added a dipped 5pF 500V SM in
> parallel
> with C706 inside the can and it now peaks properly with ~3-4mm of the
> shaft extending above the adjustment collar, and the PTO output is now very
> flat across the frequency range of the PTO.
>
> Because I was able to adjust and set the 10 turn endpoints without issue,
> I
> did not
> remove the covers of the PTO.
>
> As it is right now, if I adjust it for zero offset error at xx.500, the
> display is off at most by ~150hz with the offset error from .100 to .800
> from zero to ~100 hz without any readjustment, and I'm good with
> that.
>
> I verified that the PTO oven and thermal switch worked properly and let it
> burn in for a day or so to check for thermal instabilities both electronic
> and mechanical, such as warble or frequency jumps or undue instability, as
> well checking the PTO shaft for any change in the ease of rotation of the
> shaft, which actually became easier after the oven burn-in.
>
> That about covers the electronic/electric aspects of this PTO, and in Pt.
> 2
> I'll describe what I did to ensure the PTO is mechanically aligned, which
> actually is a thing..!
>
> 73...Jordan VE6ZT
>
>
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