I have just ordered a heap of these in 5mm
Single Rubber Button Contact Repair 5mm ... I have repaired these before and it's a delicate job to get right ... I read this fixes the button issues and the buttons are responsive to a light touch ... but when you take your control apart treat it like brain surgery and photograph each stage as there are boards stacked with four conductive blocks separating them, these blocks have to go back in the same place the same way up so be careful when you lift the boards as they may stick to the bottom of the graphics board when you lift it and fall off so you wont know which way up they were, and all the small plastic spacers which there are many must go back (none left over) there are also some metal washers that transfer ground to the stacked boards they must go back in the same place, be extremely careful handling the circuit boards (static wise, wear a strap)or you will kill the graphics card and you will get graphics fail on start up I have one control with graphics fail but it still initializes ... there are some 850sx operator stations that have the circuit boards on the side (Sabre for one) but I think all the lathes have it stacked on top of the back of the keyboards also be careful when replacing the power plugs to the board when finished as they can be plugged in backwards or misaligned ... the worst thing you can do is to plug the operator assemblies big brown plug in while the machine with the main switch on as it can be plugged in to a degree backwards and there is 110v on it. if you have a spare panel pull that down first to get an understanding of the assembly before starting on the main event.
There is another fix if you have a spare control just for parts ... get all the buttons out of the spare and go to your machine while it is running (yes running) and gently pull one of the small buttons that is not working out through the front of the grill and one of the wider buttons out also, but only do this on the left hand panel (the emergency stop button side)
Now you have two spots in the running machine (with the machine running) where you can test all the buttons from the spare controller to find all the ones that work and those that don't by pushing the contact part of the button back into the spots on the running machine where you pulled the large and small button out ... if the button is good it should beep consistently with a light pressure ... usually only the most used keys will be no good and mainly on the left hand side ... but they are the same as the qwerty keys so you can easily find a full set from two controls to fill one control.
After you have separated all the buttons from the spare operator station into two piles good and bad and some will have to be cut apart with the scissors as they are joined in groups on the qwerty board but will still work after separated, you can identify all the keys you want to replace on the running machine and go about pulling it apart and replacing them from your good pile of buttons ... but beware if you do pull a key out from the front you will not be able to put it back in properly without disassembling the whole operator station ... just be also aware that many keys on the qwerty and rhs panels are joined together at the flange so you cant stretch and pull them out through the front grill without destroying them.
fyi the CTRL, SHIFT and CAPS LOCK (which i cant see a possible use for as everything is in caps) are not meant to beep when pressed ... you can test the panel after rebuild without reinstalling it back in the bulkhead fully by putting it up on top of the bulkhead plug it in with the """MAINS OFF""" and start the machine after plugging to see if it passes all the tests, boots, and initializes ... I did get a hand wheel "fail" when doing this once but when I reinstalled the operator station back in the bulkhead it must have pulled the face plate down flat and some contact reconnected somewhere and the error went away.
If you attempt this be prepared to either be meticulous or have to redo the job over and over until trial and error hopefully gets you there in the end.
there is a decent amount of locktite on all the small screws so you may have to go back and forth to get them out ... there are some stud posts stacked with the boards between them and when you undo them make sure they are unscrewing in order or you will flex the board like crazy.
back to these blocks again ... they have a raised edge on one end that fits locates into the board to precision locate them ... position these before final tightening of the many bottom circuit boards nuts or they wont locate or make contact with the board at all.
this is one of those jobs "never enough time to do it properly but always enough time to do it over". (ten times over)
I made every mistake above and destroyed lots of sh.t.