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Hallicrafters SX-32 rebuild

Discussion in '"Boat Anchor" & Classic Equipment' started by WB3JIH, Dec 29, 2018.

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  1. WB3JIH

    WB3JIH Ham Member QRZ Page

    Latest project on the bench -- 1942 SX-32.

    Got it from the original owner's family, and working through the rig on a rebuild. I've come across a number of "mods" to it, and I'm working my way backwards to factory configuration (even have the original manual!).

    I've come across one though that I think might be a keeper -- at some point, a VR tube was installed in the radio, looks to be a 0B2. This is obviously not on the original schematic, and doesn't appear to be hooked up as I would expect.

    The radio uses a high and low B+ supply of 280 vdc and 100 vdc respectively. I am thinking that the VR tube replaced the resistor network below,

    SX-32 power supply.jpg

    consisting of R31, R32, and R38. These were 11K - 1.5 w, 4K - 7 w, and 2.5K - 2 w respectively. The schematic indicates that the junction of R31 and R32 is the "100 vdc line" to other parts of the radio.

    There's not much else I can discern on this mod. My thinking is this - put a 4K, 10 watt resistor from the 280 vdc line to the 0B2 to get 108 vdc for that bus; sort of modeling it after the SX-42's design. It also looks similar to the R-45/ARR7 schematic using an OD3.

    There looks like a big power resistor in the radio downstream of the VT tube, so I'm not sure what that was there for. In any event, it has deteriorated from exposure.

    Would appreciate any wisdom on this. Of course, I can restore it back to stock configuration, but this particular mod just seemed to make sense. It also would fill an otherwise unsightly hole in the chassis.

    Jeff
     
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    OK, that looks like a fun project. I'm not sure what you're advocating though without an "as-built" drawing for the chunk in discussion. Can you draw where the VR is wired now (on a napkin is fine :p )?

    Do you want to regulate the 100v line? I like doing that to early Halli radios as this makes the oscillator/mixer more stable with power line voltage fluctuations (helpful when listening to SSB with the BFO).

    Usually, this would involve replacing the 11k resistor (R31) with the VR in shunt mode (perhaps leaving a 56k or so R across the VR tube as a bleeder). A 4 k resistor may be a bit low in value. It needs to drop 180 volts and would dissipate 8 or 9 watts... a bit much for a 10W resistor. This also may run the VR a bit hot.

    The original voltage divider pulls 16ma total off of the 280v line. I'm thinking a resistor of about 5.6k feeding the regulator would be about right. This would pull 32 ma off of the 280v line into the shunt and mixer. Splitting the load would have the shunt and mixer/RF stages at 16ma each or so.

    Maybe I'm nutz though... it is the Holidaze, and I'm cooking 4 things at once here while typing.
     
    Last edited: Dec 29, 2018
  3. WB3JIH

    WB3JIH Ham Member QRZ Page

    I wish I could give you an "as-built" design. Right now there is a tube socket with the input hanging in the air and the output of the VR wired to a power resistor on an empty terminal strip. It appears to connect to the 100 volt line...

    The info on the resistor was great help. I have some 6K power resistors from redoing some Swan rigs, so just the ticket. The 0B2 has an internal shunt it appears, and also is rated only for 30 mA... I doubt the extra 2 will be a huge problem, but I did question the shunt resistor...

    Thanks!
     
  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Sure! Sounds like they did a mod in progress that stopped short... well, you can finish it up 50 years down the line! 6k sounds fine.

    What will happen is that the branch on the LV line will split the current with the VR tube so the regulator will only have to sink about half the power the “feed” resistor gives it.

    I’m on a cell phone now and unable to look at the bigger picture so I can’t give you closer values other than “winging it”....

    But you get the idea. Some may disagree, but I really do like to put a shunt reg in that position. It makes the circuits on the LV line more stable.

    The other nice mod is to put a 6SB7Y in the mixer’s position. This gets you better gain in the ‘above 15Mhz’ place on the dial.

    What looks like a shunt in the regulator tube is a jumper to remove voltage if one leaves the regulator out of the socket. You can basically ignore this feature and treat the VR as a 2 terminal device.
     
    Last edited: Dec 30, 2018
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Ok... replace R32 and R38 with the 6k resistor and put the regulator where R31 is, cathode to ground.

    That will set this mod up nicely. The first time you run it, poke a voltmeter around to make sure things are working well and poke a calculator to make sure I haven’t lead you astray (i.e. check the math). :p
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

  7. WB3JIH

    WB3JIH Ham Member QRZ Page

    Glen - Thanks -- I have the actual manual from 1942. Brown paper and owner notes included. I've generated a high resolution PDF, photo quality, in case anyone wants it:

    http://www.podengo.com/SX-32/SX32-manual.pdf

    Chris - thanks for the reality check! I figured the VR tube would be a better solution than some big resistors. I've learned enough to know a good mod versus a bad one when I see it. I've got quite a ways to go before the power switch goes on in anger though.

    Here's the rig "as found..." I just got a meter bezel in the mail from a kind gentleman, and some other parts are on the way. Big things I've noticed so far is the tuning capacitor cover plate is missing and the S-Meter isn't OEM. I've also had to replace quite a bit of wiring in the audio section because of mouse infestation. The dial brake isn't original for this model either, nor is the spinner. Don't really mind either of those though, as they served the owner well on his assignment in the Med in the 1950s...


    SX-32-as-found.JPEG

    --Jeff
     
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Super!

    It’s a solemn affair to restore a radio like that. Give it a loving home.

    Those radios are truly fantastic, but I’m basing that on an SX 28A I had a couple decades ago. It required all new caps and resistors and was really “fun” to pull apart to the modules to get to everything.

    When it was all back together and aligned, it was a spectacular performer. I was truly amazed at how it could dig thousands of obscure signals out of the ether like no radio I’ve had before or since. It did have a 150’ wire hooked to it... and I lived out in the woods at that time (for a couple year’s break from the big apple).

    Very nice find! Please tell us how it turns out.
     
  9. WD5GWY

    WD5GWY Ham Member QRZ Page

    Thanks for sharing the manual!
    James
    WD5GWY
     
    KD2ACO likes this.

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