by RetireeJay » 2017-May-Tue-08-May
The transistor (if truly bad) sits in the path between the Z "endstop" sense pin on the board and the corresponding input pin on the Atmel chip, so substituting a mechanical switch for the probe still won't work.
One alternative for you, if you have a knob on the top of your Z screw (or could put one there) is to use the homing switches for X and Y, but home your Z axis by hand. This works because when you power-on the 'bot it assumes all axes are at zero. The 'bot won't "know" you are doing the manual adjustment, so it still thinks it's at zero after you make the adjustment. People did this with the very first edition of the Simple. You would not use the global "home" command and you definitely would not use the G29 command ever. You've got to make sure your slicer doesn't put a Z home command or a G29 into the G-code. But with this approach you will have no worries about the head crashing into the bed; the firmware never allows the head to move to a position less than zero.
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3, upgraded to MK3S