IC-502 + DigiVFO

 The Icom IC-502 from the mid 1970's is a standalone 3 watt, 6 meter band SSB / CW transceiver.  I've made contacts with this radio during e-skip openings.  Its a tough little radio to tune because the knob requires the steady hand of a safe cracker to operate.  It also drifted with the original analog VFO during a conversation.  The original VFO is a completely analog device, with less accuracy that we're used to the 21st century.

QRP Labs came out with their Digi VFO product late in 2025 based on the popular Prog Rock 2 product.  This tiny board now boasts a display and rotary encoder, as well as CAT control capability for a modest price.

Based upon the suggestions of others in the amateur radio community, it seems rather easy to remove / or simply disconnect the original VFO and use the Digi VFO in its place (after providing a good low pass filter), The attenuated signal came out to be just less than about 1 volt peak to peak.

The original VFO was easily removed with only 4 electrical connections.  The gear reduction drive and VFO unit separated with just two set screws.

The plan was to make a mounting plate for the Digi VFO and mount it back in the original location and use the original knob.  This required a shaft extension, but that's a simple mechanical adapter that I made on the 3D printer out of PLA with carbon fiber for added strength.

I mounted the Digi VFO board directly to the rotary encoder assembly simply because it was convenient to do so.  The low pass filter design ended up being a modification of the QRP Labs LPF kit to turn the square wave output into a smooth fundamental 36Mhz to 38Mhz signal out of CLK0 pin.  The BFO setting is at 13.99600 Mhz and Low setting for a heterodyne radio Local Oscillator injection. In the Digi VFO setup menu and starts displaying at 50.100 Mhz for convenience of the operator to produce at 36Mhz output at a display reading of 50.000 Mhz

(The Digi VFO could use an option for 10Khz per knob rotation to move quickly through the band with the single click of the knob, but I won't knock the product. It is very good.)

Packaging is key.  At least when the new product is displacing a much larger component, it makes it easier.  I managed to solder everything to mount to the rotary encoder with shorter jumpers than in the previous photo.  It fits nicely in place of the original with a carefully manufactured adapter plate.  The output of the low pass filter is clean and sinusoidal.  The spectrum analyzer shows a clean output from the transmitter.

The result was so encouraging, I decided to modify the original radio cover to accommodate the frequency readout.  The display sits directly above the VFO tuning knob.  The tuning knob uses a shaft extension to employ the original tuning knob without the reduction gearbox.

This project was a complete success bringing life back to an excellent product.