How to fix "mixing" valve on Temptrol Shower ?

We have a Symmons Temptrol shower control/mixing valve.
It no longer controls the temperature (pressure?) properly and the shower changes temperature significantly when other
water is used in the house. Otherwise it still works fine.
I assume that there's a replaceable cartridge or other item in there ? Can someone give me a couple of sentences on how I'd access the unit and what to replace ?
Thanks, Bob
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