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I have a Hallicrafters S-120 1960s vintage vacuum tube receiver (an "All American Five" superhet with solid state rectifier replacing one of the "five" tubes). It works well, doesn't have excessive internal noise, and seems very sensitive (with the regenerating IF used for "exalted carrier" on AM, and as a BFO for CW and SSB).

I've considered using this as a secondary receiver, for some version of "split" operation.

The problem with that is that, while the tuning is plenty precise (with the Bandspread control allowing several turns of a secondary tuning knob to cover a few hundred kilohertz), the tuning indicator needle is significantly off the actual tuned frequency (correctable) and doesn't move at all when using the Bandspread. I had the idea to connect a digital frequency display, which would allow me to see exactly where I'm tuned. I have such a display on hand, built from a kit.

The only place I see to get current tuned frequency from a superhet receiver is to tap into the local oscillator. Am I correct in thinking this will give a frequency 455 KHz below the actual tuned frequency?

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    $\begingroup$ More likely that the LO is 455 kHz above the tuned frequency. $\endgroup$
    – WA9ZZZ
    Commented Apr 17, 2021 at 2:13

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Yes, attaching a general-purpose frequency counter to the local oscillator of a superheterodyne radio will result in displaying a frequency with an offset either above or below (depending on the design of the radio) the nominal frequency of the signal being received.

In order to account for this, apply the reverse offset numerically, between the frequency counter and its digit display.

This may or may not be practical with the counter module you already have.

  • A frequency counter intended for the purpose should have a programmable offset.
  • If it is microcontroller-based and has open source firmware then you could reprogram it (this may or may not require special programming hardware and software).
  • If it is based on discrete logic, then the circuit could be modified to include an adder/subtractor and the offset programmed using wiring or switches.
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  • $\begingroup$ Thanks. The one I have is the cheap 4 1/2 or 5 digit one, $15 for kit on eBay. Pretty sure it's microcontroller, but also near certain that programming isn't accessible. Might be worth getting an Arduino and doing some experimenting. $\endgroup$
    – Zeiss Ikon
    Commented Jul 17, 2019 at 19:01
  • $\begingroup$ @ZeissIkon I'm surprised that you can't find one that will work with your radio. The listing doesn't necessarily have to include Hallicrafters, as long as it is for a radio with a similar circuit. Besides eBay, I suggest you check on the qth.com etc. for sale listings. $\endgroup$
    – Mike Waters
    Commented Jul 17, 2019 at 20:27
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    $\begingroup$ eBay has many $10 8-digit counters (search for "Cymometer") that include an LO offset feature, reached through an obscure menu. They're totally unshielded and may interfere with your receiver $\endgroup$
    – tomnexus
    Commented Jul 17, 2019 at 21:18
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    $\begingroup$ @MikeWaters Haven't really looked yet. The idea just occurred to me at work. I'll check for frequency displays made for superhet vacuum tube radios. $\endgroup$
    – Zeiss Ikon
    Commented Jul 18, 2019 at 11:03
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    $\begingroup$ @ZeissIkon ya the microcontroller clock at ~13 MHz, all its harmonics and other noise. A metal box only helps if every wire going in or out is filtered, so the fun will be filtering the power lead to the frequency counter - ferrites and feed-through capacitors. Anything cheap and digital will radiate, but try it first and see how you go. $\endgroup$
    – tomnexus
    Commented Jul 18, 2019 at 14:44

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