These TC Electronics pedals from the 1980s are tricky to work on and I'm posting this information for the benefit of all brave souls that take on repair of these iconic FX!
The potentiometer knobs are *NOT* a push fit, so the often-seen advice for using string or ribbon to pull them off will certainly not work.
If you gently remove the blue top by lifting/prising your fingernail or similar blunt instrument, this will reveal a screw which releases the knob. Lift the knob off gently as there's a stack of washers beneath and this arrangement must be carefully noted.
Also, in the case of the TC XII Programmable Phaser (and almost certainly other pedals in this series) the jack sockets foul the pots, inhibiting removal of the circuit board, so much careful jiggling is required.
See attached photographs.
I'll post further information from my XII repair if further issues become apparent.