You can try lubricating the shaft.
The tuning knob on the 8800 is a friction or press fit on the encoder shaft. You need to get a firm grip on it and pull pretty hard. Once it starts coming off, it should become pretty easy.
Once you remove the knob, you should see where the shaft goes into the encoder mechanism. There will be a shaft nut there. Where the shaft disappears into the encoder assembly is the point you want to put a drop or two of lubricant.
Stand the 8800 on its back and drop one or two drops (no more) of a very light oil onto the shaft so it can work its way down into the shafts bearing which is usually just a sleeve type bearing. Sewing machine oil is usually a fine oil that will work.
Let that oil work into the shaft by gravity and also rotate the shaft with your fingers. Try pulling the shaft in and out a tiny bit while you are turning it as most encoder shafts do have a small amount of free play. This will help work the oil into the shaft.
In really bad cases, the factory grease may have hardened and getting oil in can be hard or impossible. In those cases, one would need to carefully try and take the assembly apart so the old grease can be cleaned out of the bearing and off of the shaft. This is not easy though and trying it can damage the optical encoder beyond repair. This also requires a lot more disassembly to remove the encoder.
When you replace the knob, you may need to push it on all the way and then pull it back out a very teeny tiny amount. Don't back the knob out so far that it has a lot of play or seems sloppy in its movement.
Hopefully lubricating the shaft will be all that's needed as trying to repair an optical encoder is not an easy task plus some are sealed and can't be opened.
Shafts becoming tight on older era radios due to the original lubricant drying up is a fairly common problem. The FRG-8800 is definitely a radio that's old enough where the rotary encoder shaft may need a little TLC!
I've not had my 8800 apart in years so I don't recall the fine details but I do remember pulling the tuning knob off was the hardest part. I had to pull so hard that I was worried I'd break something but it suddenly slid right off once I put a little more muscle into it.
Most tuning knobs have set screws under the rubber tire of the tuning knob but the 8800's tuning knob has no set screws. It's a friction fit or slip fit on the encoder shaft.
Once you get the tuning knob off, freeing it up should be easy unless there's mechanical damage to the encoder or shaft. No disassembly of the radio is needed to do this other than pulling the knob off.
Pushing the tuning knob back onto the shaft will be a lot easier than pulling it off the shaft was!
I'd probably clean any oils that got on the shaft before you push the knob back on. Use an alcohol wipe if you have one to remove any oils from the shaft. If not, do the best you can with a tissue or something.
There are also no flats or anything on the shaft so you don't need to worry about aligning anything when you go to push the knob back on.
