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Help identifying unmarked component - see picture.

Hi,

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:

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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.

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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)

Reply to
tim
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:Hi, : :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: : :

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Looks like a thermistor to me (+ or - tempco isn't obvious so you have to work that out). It is unusual to mount such a large component on a flexible printed connector cable - but Calcomp did some strange things...

Reply to
Ross Herbert

Looks like a Varistor (MOV). A reading of 2k on a multimeter doesn't sound very faulty.

Could be across the coil to absorb stored energy to lift the pen quickly.

Reply to
nospam

It loks like a Siemens (EPCOS) uncoated PTC, of fairly high voltage rating, given the body thickness. There should be black lettering on one side - though this is often faint to illegible.

A 2Kohm impedance at room temperature is abnormally high.(10-50 ohms would be more typical).

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RL

Reply to
legg

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