Before you take it apart.Only detector I have from the 80's is the original Eldorado from Tesoro. Currently it won't power up...almost certain the on/off/sensitivity potentiometer has gone bad. I do remember the last few times I used it that potentiometer was real scratchy? I relocated the control box to the rear to give it a better balance and depending on trash would either use 6", 7" or 8" concentric coils. Still a fun detector to use though it doesn't get used near as much as when it was new. Ha! Discriminate mode is mainly used when searching but would also use all-metal mode to size up targets for more possible target information. May not be the deepest detector but Tesoro's were pretty good when it came to hunting trashier sites. I once dug a fairly deep (8-9") early 1800's large copper 1/2 Skilling at an old plantation site using the Troy 9" concentric coil. I need to look in to checking/changing that on/off/ sensitivity potentiometer and take some more walks down memory lane :>)
You could try a good contact cleaner on that pot.
Pevey Funk Out. Made for quality guitars.
Spray Ito the pots. And anything else you want cleaned. Circuit board I'll use a soft tooth brush.
Let it soak awhile. Operate the pots.
Then blow dry.