Looking for help with Mixet shower valves

This is a rather odd situation. There are apparently a few million or more Mixet shower valves / faucets out there as they’ve been made at least since the 1960s. (The main question, when shopping for parts is before or after 1968.) Not all that long ago, I’ve seen Mixets for sale at hardware stores. Now, they seem to be out of business.

There’s no lack of replacement handles and innards, and even retrofit kits for when the valve’s body wears out, eliminating or at least greatly stalling the need to rip the wall apart and replace the thing. There is a scarcity of information, though, with regards to the care and feeding of them.

My problem is one of my Mixets is really tough to turn the temperature adjuster. (These things have a “set and forget” lever for the temperature and a knob to turn the water on and off.) It turns so hard that the temperature adjuster lever broke.

Anyone know how to loosen the thing up?

That happened to the shower valve in my apartment. The landlord simply replaced the entire valve.

Well, replacing’s not an option for the near future, since we’d have to bust out the wall to do that, and a bathroom remodel’s just not in the budget for this decade.

As a short-term fix, I was able to find replacement temperature adjusters, but that doesn’t resolve the hard-to-turn issue that made them break in the first place.