I'm trying to fix an old but very complicated Calcomp pen plotter (model 1044gt). At the moment the machine is crashing so I have decided to look at identifying the circuitry in more detail.
On a piece of flexible cable next to the voice coil which controls pen pressure I found a completely unmarked round black component with the contacts on metal showing at the two faces. Here is are urls to photos:
Also included is a url to my best-effort at reverse engineering the small circuit board in the pen carriage to which this all connects - linked by JO701 to the main circuit boards in the side of the machine.
The component in question connects to JO704 pin 7 - which as shown in the lower left corner is one of the opamp inputs. The other side of this component connects to JO704 11 and the voice coil. Hopefully this will give some clue to its funtion.
At the moment I measure it to have a resistance of 2 k ohm in both directions. That this is a resistor seems illogical and I wonder if this component has blown. My original suspicion was that it was a diode to prevent bad voltages when the magnetic field in the voice coil collapses from influencing the sensing electronics.
If anything else would be helpful to solving the mystery please ask. Hopefully someone here with more experience than me can tell me what it is or was.
Thanks a lot, Tim
(Oh, and if anyone has any information on calcomp 104x plotters it would be a great help (i'll try positing this somewhere more relevant). Circuit diagrams or the maintenance manual or and explanation of crash codes would be a godsend in particular)