Recently, we've had a few problems with our RV4S the worst being many nozzle crashes through the Z axis dropping. A different error was when the machine started up with a 'waitnotbusy' error message prior to the arm startup sequence and warm up routine. More error messages followed and ended with an 'R Axis Move Error' message, during all this the arm did not move at all.
Switched off, dismantled the PC and removed the RV ISA card. Re-seated all the ISA card sub boards contacts by wiggling them a lot and re-inserted the ISA card into the PC a few times to clean the edge connector contact surfaces. Re-assembled and the RV4s ran properly with no startup error messages. It seems this fixed the 'R Axis Move Error' problem.
Went on to place around 3000 components and then the nozzle crashed, (lowered nozzle hitting the front of a feeder hard and snapping off the tip, as usual). So, the above didn't fix the Z axis dropping problem.
Since replacement nozzles seem to be pretty much unavailable now, we have had to resort to making our own using our 3D printer. This is quite time consuming as the nozzle has to be made in two parts and then solvent welded together and finally drilled and matte black coated.
Read spiyda's comments about fitting a switch in the pin 9 connection of the "Z" axis stepper driver CN1B and thought it might be worth a try. After working out which CN1B board it was using George04's wiring diagram (turned out to be 1st board on the left), we fitted an on/off switch on the wire going to pin 9 and set it to the off position. Started up the RV4S and went on to place over 4000 components without a single nozzle crash.
However, we started to see that only some of the components on one particular feeder lane (G0) were being placed - the same lane where previously the nozzle crashes occured. When we checked the status for that lane the machine reported 'Feeder not reachable' and the 'No Entry' style symbol appeared against that lane. Tried to reset the bias but 'Feeder not reachable' kept coming up. Had to set 1st Ref for that feeder followed by selecting 'Yes' to 'Resetting all the biases' to clear the feeder not reachable problem. This was a pain as we had to set the biases for all the other lanes again.
Still, this was much better than crashing nozzles! However, the 'Feeder not reachable' error kept occurring too frequently until it was happening on every board.
At this point we moved the problem lane to another feeder and this seemed to solve the problem. Went on to place over 20,000 components with no nozzle crashes or 'Feeder not reachable' errors.
I think our problem was slightly different from spiyda's in that our z axis drop was being caused by an x/y axis error instead of a z axis worn rack problem, but the same 'switch-off the power' to the z stepper via pin 9 was causing the nozzle crashes. Anyway, thank you spiyda for steering us in the right direction and I hope our experience can be of some use to others.