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Monday, July 18, 2016

Building a VFO. A BIG VFO. IF options?

Once again, The Radio Gods have Spoken (TRGHS).  An off-hand comment at the Manassas Hamfest, a bit of encouragement from Pete Juliano, and the next thing you know Armand WA1UQO has sent me this beautiful National HRO dial and reduction drive. This thing is so nice... Well, put it this way: this is the first time I'm building a rig around the dial! 

Further evidence that TRGHS:  I needed something on which to base the HRO dial and a box for the VFO.  Wouldn't you know it:  That Whole Foods "grilling plank" that I bought a few weeks ago was PERFECTLY sized for this task.  Eerie, don't you think?  As for the VFO box, well TRGHS again:  pictured above you see a side view of the box from one of the Heath QF-1 Q multipliers that I cannibalized for the variable caps.    Finally, for the main tuning cap, I took another look at that old brass variable cap that I took out of a 1930's era British regen (pictured above).  It had been hopelessly stuck for a long time.   I twisted it a bit and was amazed to see that it is stuck no more.  TRGHS!  (I just need to find a suitable nut so that I can mount the old cap in the QF-1 box.)

I'm thinking that this VFO will be the heart of a general coverage shortwave superhet receiver.  I want filters for AM and SSB and I'd like it to cover 5 MHz to 10 MHz.  I've been noodling various IF possibilities, but concerns about birdies and spurs keep driving me back to 455 kHz.   I have a crystal-mechanical filter for that freq.  And a big box of 455 kc transformers. What do you guys think of this option?   


  1. Wow, that is going to be one big rig! Sounds like a cool project. 455 shouldn't be a huge problem, even modest receivers (S-38, etc.) did a reasonably decent job rejecting images below 10 MHz.

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  3. I think 455khz would be a good choice, kind of "retro"

  4. Since its on the plank name it the Carp VFO.

  5. Being British 1930's, the nut for the cap is likely to be an imperial size and Whitworth thread.
    I'm a fan of Whitworth and deplore Metric. The Whitworth thread's profile makes it the most robust ever designed - a Whit bolt will snap beyond its mated nut before stripping.

  6. Dex: No Imperial Whitworths available here. Any suggestions for a North American replacement? 73 Bill

  7. Is the cap under discussion the one in the picture? If so, failing availability of correct single shaft-concentric nut I'd be thinking of mounting it with the 'tripod' legs shown, drilled (and tapped?). A thought ...


  8. Whitworth thread sizes[4]
    Whitworth size
    (in) Core diameter
    (in) Threads density
    (per inch) Pitch
    (in) Tapping drill size
    1/16 0.0411 60 0.0167 Number Drill 56 (1.2 mm)
    3/32 0.0672 48 0.0208 Number Drill 49 (1.85 mm)
    1/8 0.0930 40 0.025 Number Drill 39 (2.55 mm)
    5/32 0.1162 32 0.0313 Number Drill 30 (3.2 mm)
    3/16 0.1341 24 0.0417 Number Drill 26 (3.7 mm)
    7/32 0.1654 24 0.0417 Number Drill 16 (4.5 mm)
    1/4 0.1860 20 0.05 Number Drill 9 (5.1 mm)
    5/16 0.2414 18 0.0556 Letter Drill F (6.5 mm)
    3/8 0.2950 16 0.0625 5/16 in (7.94 mm)
    7/16 0.3460 14 0.0714 Letter Drill U (9.3 mm)
    1/2 0.3933 12 0.0833 Letter Drill Z (10.5 mm)
    9/16 0.4558 12 0.0833 12.1 mm (0.4764 in)
    5/8 0.5086 11 0.0909 13.5 mm (0.5315 in)
    11/16 0.5711 11 0.0909 15 mm (0.5906 in)
    3/4 0.6219 10 0.1 16.27 mm (0.6406 in)
    13/16 0.6845 10 0.1 18 mm (0.7087 in)
    7/8 0.7327 9 0.1111 19.25 mm (0.7579 in)
    15/16 0.7953 9 0.1111 20.75 mm (0.8169 in)
    1 0.8399 8 0.125 22 mm (0.8661 in)
    1 1/8 0.9420 7 0.1429
    1 1/4 1.0670 7 0.1429
    1 1/2 1.2866 6 0.1667
    1 3/4 1.4939 5 0.2
    2 1.7154 4.5 0.2222
    2 1/2 2.180 4 0.250

    Nearly all current still cameras accept a 1/4 in UNC thread in their tripod baseplate though the UNC is close enough to Whitworth that it will fit, and many motion picture cameras accept a 3/8 in UNC and, again, the Whitworth is close enough to fit, while a 5/8 in UNC thread is the accepted standard for tripod mounted land surveying equipment and, once again, the Whitworth will fit.

    Let me know what you need we still have supplies in Lancashire England.

    Taps and dies too ....... there will be what you need in my cellar.



  9. that tabulated beautifully !!!


  10. Yep, Google and Wikipedia work wonders! UNC and UNF were other 'standards' I was looking for - microphone booms and fittings. A case for measurement - a caliper and thread gauge ... another skill necessary to the Electronics Tech: Mech-Engineer! :)

  11. ... and with a measure of competence there, a short step to Roboticist! :)

  12. The HRO dial was important because it gave exact numbers and close together. So even if you didn't know what frequency, you could get back to it. Or you could make the effort to create a graph to link the numbers on the with frequency

    The HRO-500 of course used the same dial, but since it used a VFO covering a fixed 500KHz range, they could put the effort into making it linear, so the readout was direct.

    If you have a capacitor with a good reduction drive, like from frequency meter or a Command Set transmitter, you could go digital. Add a quadrature encoder wheel, as seen in mechanical mice or current radio equipment, and use the information to feed an updown counter. So you get a precise logging scale, the current equivalent of the HRO dial, or the mechanical readout on the R390. Whether it's direct readout or not depends on how linear you make the oscillator tuning.

    Why not a frequency counter? You've already answered that, you're using n HRO dial instead of a frequency counter.


  13. 455KHz seems horribly ancient, not really the best solution when it was about the only choice. Even the BC-348 made an improvement in image rejection by going to 910KHz, though it skipped the broadcast band.

    Complicating things often makes other things easier. A higher IF reduces image problems, but you also can make the preselector separate, no need to fuss with tracking it with the oscillator tuning.

    Long ago double conversion generally meant a fixed frequency in between two others, with just enough selectivity to get rid of the second IF images. So there'd be some IF transformers before te conversion to 455KHz (unless you went to the scheme of the Drake 1A, a crystal controlled converter ahead of a receiver that coveted fixed 500KHz range).

    The first IF wasn't seen as a place of selectivity, until later.

    The SP-600 had such a stage that kicked in on the higher bands, two way FM receivers did it too.

    Many a CB set, cordless phone, 49MHz walkie talkie, baby monitors, even scanners went with a cheap FM broadcast band type ceramic filter at 10.7MHz, enough selectivity to get rid of the image, even if the bandwidth was 350KHz or so. But the filters were cheap, and nothing to align.

    Take advantage of that. All those things are readily available cheap at garage and rummage sales, and they offer up the all important crystal to convert to 455 (or 450)KHz. That makes it viable, if you had to buy a crystal thy' e become so expensive to eliminate the use of odd filters.

    You can even extract the FM if strips from one of those mentioned devices (early cellphones too) and add low deviation FM reception to your receiver for the experiments with FM on the HF bands or for VHF with a converter ahead if the receiver.

    You can always later change the 10.7MHz ceramic filter to a 3pin crystal filter for narrow FM, these re fairly available because if to-way equipment.

    Once you have the double conversion, you can turn the 10.245MHz oscillator into a vxo, providing fine tuning. Sony did this in one early portable shortwave receivers. If you keep the ceramic filter at 10.7MHz, you could make the conversion oscillator cover 100KHz, and then make the first oscillator a PLL, with 100KHz steps. It's much easier to make a PLL with 100KHz reference than 1KHz or lower.



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