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

[Jordan Arndt]

Hi Larry...Thanks...

I see I transposed the production years in my first two paragraphs. I'm working on a 1960 S-W and used a working 1959 S-W to test the modules. Sorry for any confusion about that...!

Yes. I wanted to test the PTO oven and thermostat to ensure it worked properly, and it does. I have a Cosmos PTO that the oven did not shut off and actually melted the dessicant envelopes and the resulting mess had to be cleaned up before I worked on that one. I leave the ovens switch off in operation.

Thanks again and more to follow...

73...Jordan VE6ZT
  ----- Original Message ----- 
  From: Larry H 
  To: Jordan Arndt 
  Cc: Collins R-390 HF Receiver List 
  Sent: Sunday, August 21, 2022 7:05 AM
  Subject: Re: [R-390] 1960 S-W 390A Redux #1: The PTO Pt. 1 of 2


  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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