If you spent the 1970s obsessively browsing through the Radio Shack catalog, you probably remember the DX-160 shortwave receiver. You might have even had one. The radio looked suspiciously like the less expensive Eico of the same era, but it had that amazing-looking bandspread dial, instead of the Eico’s uncalibrated single turn knob number 1 to 10. Finding an exact frequency was an artful process of using both knobs, but [Frank] decided to refit his with a digital frequency display.
Even if you don’t have a DX-160, the techniques [Frank] uses are pretty applicable to old receivers like this. In this case, the radio is a single conversion superhet with a variable frequency oscillator (VFO), so you need only read that frequency and then add or subtract the IF before display. If you can find a place to tap the VFO without perturbing it too much, you should be able to pull the same stunt.
In this receiver’s heyday, this would have been a formidable project. Today, a cheap digital display will do fine. As it turns out, this radio has some bands that tune to the VFO’s frequency minus 455 kHz and some bands tune to the VFO frequency plus 455 kHz. With a microcontroller you could deal with this easily, but [Frank’s] solution was to simply use two displays. They are cheap, so why not? The displays are configurable, so you could probably work out a way to use one even if you had to manually throw a switch to do it.
The displays draw power from the radio’s lamp sockets. The real trick to the project is finding a place to tap the VFO frequency and then doing so in a way that doesn’t kill the oscillator or introduce instability. [Frank’s] design uses a capacitor to couple the oscillator’s energy into the counters.
If you don’t want to use an off-the-shelf display, it is pretty easy to count frequency with most microcontrollers. Some have dedicated hardware for this purpose. A common trick is to count the number of zero crossings over a period of time and scale to how many you would have in a second. There, are, however, a variety of methods.
We have to admit that while we enjoy old radios, we also enjoy a digital display. Of course, another answer would be to replace the VFO completely, but that would negate the cool old dial.