The Encoder Fix Is In, But It Wasn't A Cakewalk

Honeywell encoder shaved to fit

The replacement of the standard K2 pulse encoder, used for frequency tuning, was a bit more challenging than I'd anticipated. Basically, the problem was that the encoder I ordered didn't have a mechanical drawing on-line (even on Honeywell's site) and I mis-judged its size. The original encoder sits in a square hole in the Front Panel board, but this replacement was too large for that hole by quite a bit.

Seeing that I was going to have some challenging mods to make to get this to work, I first made a promise to myself that under no circumstances was I to modify the K2 itself. All modifications had to be to the part. That way, if I did get stuck I could always be sure that the original K2 encoder would fit again.

Encoder mounted on rear of FP board

Because the new encoder would have to mount on the rear of the K2's FP board that meant the tuning shaft was too short. I compensated for that how I could by first shaving down the side of the encoder case as much as possible so it could be positioned closer to the front panel. That wasn't much, but it was enough so that the threaded collar just sat in the hole in the front panel, giving more stability.

Unfortunately, not enough threads were showing to get a nut on the collar. Thus, the encoder had to be fastened to the FP board itself, which I accomplished using a small piece of stiff plastic (so as to avoid a short on the board). The plastic came from the middle section of some 450 ohm window line.This method actually stabilized the encoder a great deal.

New encoder mounted to FP board

Up to this point I'd been pretty lucky. There was room to spare between the back of the encoder and the Control Board behind it, so no worries there. There are a couple small spaces in the square FP hole to route the encoder wires to the front pads. Of course, I'd already put in the new FP U3 and temporarily wired the encoder so that I knew this was all going to work electronically.

Extending the tuning shaft

Still, the encoder shaft was too short, by at most 2 mm. This is because the original K2 knob's inner shaft hole is recessed about 6 mm (probably to allow for the original encoder fastening nut). Trying to extend the shaft in a permanent way consumed well more time than I'd put in up to this point.

I first tried simply gluing on a short piece of steel rod. The only metal-to-metal glue I could find in town though was epoxy and it wasn't up to the job because of the tuning knob weight. I considered welding on the extension, but even if I had wrapped the encoder in a rag drenched in ice water I doubt I could have done that that without damaging the electronics.

Extension with plastic shim

I had to have a physical/mechanical connection between the encoder shaft and extension. The technique I used was to carefully drill a small hole in the end of the encoder shaft, just large enough to take a small self-threading screw of the right length. I couldn't very well turn down the clamping screws in the tuning knob to the screw, so I also had to add a shim, which is just a short piece of hard plastic tubing I had on hand. I filled the space between the screw shaft and tubing with epoxy.

I think this is a permanent fix and no mods to the K2 chassis itself were made. It took a little adjustment of the tuning knob to get it to turn without wobble, but that wasn't much to overcome in the end.

The radio now works FB, the only change being that it tunes slightly faster since this encoder part is 128 PPR (Pulses Per Revolution) whereas the original encoder was 100 PPR. I think I can consider that an improvement! It was a fun project and saved me 60 bux over buying a new encoder from Elecraft.

Please share your experiences with homebrewing a radio equipment repair in the comments!

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