It’s apparently a common failure mode for the rotary encoder which processes the tuning knob input to fail in the Icom R75 (and many other Icom receivers and transceivers). Mind did recently.
A bit of background: most radios these days are completely computer-controlled. There is no variable capacitor connected to the tuner; the radio synthesizes its intermediate frequency under computer control, and if there is a tuning knob, it merely is a device for sending input to the computer. It may appear to be more traditional than a set of up/down buttons or a keypad, but it’s merely a show. A useful “show,” as tuning with a knob is often more convenient than using buttons.
The symptom of a dead rotary encoder is pretty much what logic would tell you it is: the radio still operates, and you can change frequencies by entering them with the keypad, but the knob simply does nothing.
There’s several fixes detailed in the initial link above. Initially, I was debating what to do and procrastinating the job, because none of them are cheap (replacing the chip in the controller requires de-soldering and re-soldering a surface-mount device, since I don’t have any equipment to do that, I’d have to buy it, since I couldn’t locate anyone who had it to lend, and there goes the clear price advantage of that option).
Then my (un)employment situation resolved itself, so I did the easy out and simply ordered a replacement from Icom, which costs just under $90 (not the $130 it is rumored). From the repair manual (which you should definitely download, as it shows how to take the set apart so you can get at the encoder), the part number is 6910012480. That part number may differ for products other than the R75, so please obtain a service manual and check it to be sure in that case.
OK, I suppose the real “easy out” would have been to send the thing in for repair. That would have doubtless cost at least twice as much as fixing it myself did, I was reasonably sure what the culprit was (the symptoms made that pretty obvious), I’ve taken apart reassembled electronic devices many times, I know the precautions to take to avoid damaging things, and this particular repair didn’t even require me to warm up a soldering iron.
So it was pretty much a no-brainer to do it myself. That said, the way the Icom parts person was taken aback at a mere customer troubleshooting his radio then having the audacity to order a part with which to repair it did make me have a little bit of doubt. Which was unfounded: when put back together and powered on, I was rewarded with a fully-working receiver.