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

[Jordan Arndt]

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