Hallicrafters HT-45 rebuild

Discussion in '"Boat Anchor" & Classic Equipment' started by WB3JIH, Sep 28, 2018.

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  1. WB3JIH

    WB3JIH Ham Member QRZ Page

    Hi all-
    I’m starting to rebuild a HT-45 Mk IIa. It was converted to use a 3-500 and had a number of atrocities committed against it, so it’s going to be a bit of work. I know the 3-500 isn’t a big deal, but unfortunately the tank coil is pretty badly tore up: bent, distorted, solder blobs, etc.

    I thought that I might just rewind it like I did with the one in my SR-2000, but quickly observed that it uses a flat stock, like a slinky, that I can’t reproduce with my basic tools.

    So question to the group — any suggestions on replacing or rewinding a suitable tank coil? Would #12 wire be suitable to replace it? It didn’t seem to me to be a power thing, since the SR-2000 has similar specifications and uses #12 wire, but the HT-45 coil has more turns over a larger diameter. I don’t mind home brewing a new coil, but I don’t know what the inductance of the original would be, or how the difference between round wire and the flat stock would affect things like Q or power handling capacity.

    Or maybe just buy a tank coil from a vendor selling a 3-500 based amp? Or a good coil from a parts donor?

    Thanks for any thoughts.
    (Cross posted to the Hallicrafters listserv)


  2. N2EY

    N2EY Premium Subscriber QRZ Page

    The ideal solution would be to find another HT-45 and make one good amp out of two hangar queens. But that may not be practical.

    The original HT-45 used the 3-400Z, and if a 3-500Z fits (the latter is a bit taller), you're good.

    A homebrew coil will work OK. Might be a tiny bit more lossy but that's not a reason to stop you.

    If it were me, I'd carefully remove the old coil and measure it, both physically and electrically, figure out/measure the inductance, and make a replacement.

    What do you have for a power supply?

    73 de jim, N2EY
  3. AC0OB

    AC0OB Subscriber QRZ Page

    If you decide to homebrew the coil you would need a coil with the following dimensions:

    4 Shorting Taps on a coil with:
    D = 2" or 50 mm
    Length = 3" or 76 mm
    24 Turns of Wire diameter = 2.05 mm or #12 AWG
    L = ~ 14 uH

    Qcoil = 720

    Last edited: Sep 28, 2018
  4. N2EY

    N2EY Premium Subscriber QRZ Page

    Excellent info!

    Let's see......24 turns in three inches is 8 turns per inch....I might have a suitable form.....
    W7UUU likes this.
  5. WB3JIH

    WB3JIH Ham Member QRZ Page

    Wow -- did you open the cover on yours? :) I have the original coil form -- ceramic bar and it's exactly that dimension, although I count 22 turns.

    That's exactly what I needed to know -- thanks!

    And Jim -

    The power supply is a "homebrew-ish" Hallicrafters supply. It's the original cabinet and the oil filled capacitor, but looks like the transformer has been updated to a more modern piece. There's a dymo label on it that says "2750 V, 700 ma CCAS", and the Hg vapor tubes have been replaced with big HV diodes on a piece of perf board. This is sitting just on top of the capacitor on a flimsy piece of sheet metal. If it was every used, the guy who put it together has a way more lucrative life insurance policy than I do.

    Looks like everything else in the power supply is at least there though.
    N2EY likes this.
  6. AC0OB

    AC0OB Subscriber QRZ Page

    You are quite welcome.

    I don't have an HT-45 but I use a set of equations programmed into MatLab and they have shown to be accurate for my builds.

    With 22 Turns the inductance of the main coil L2 would be 12 uH so the coil L1 would most likely have about 1.5 uH of inductance.

    Good luck.:)

    Last edited: Sep 28, 2018
    N2EY likes this.
  7. WB3JIH

    WB3JIH Ham Member QRZ Page

    Go on... :)?
    W7UUU likes this.
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    I have some parts from old BC-191/375 tuning units. Will have to dig them out but there might be something useful, now that I know the size.

    As for the power supply - a string of 1N5408s is probably your best bet.

    73 de Jim, N2EY
  9. KE0ZU

    KE0ZU Ham Member QRZ Page

    Some of THIS might be helpful. Good luck with your project. Some pics at the end of this thread when you're done would be nice to see.
    N2EY likes this.
  10. WB3JIH

    WB3JIH Ham Member QRZ Page

    Thanks for all the good info. I have a new tank coil rolled and epoxied on a polycarbonate form using the calculations from AC0OB.

    Now -- question is -- where to tap??

    I've used the antenna analyzer and carbon resistor from plate cap to ground before on another rig and got it close, but am not sure if I'm figuring the plate impedance on this one correctly.

    For the 3-500, I'm using 2700 vdc/ (350 mA*1.6) = about 4.8K.

    That seems somewhat high...

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