Weak 9872CT Plotter Platen

Martin Hepperle
 

My 9872 sucks! ... not enough!

After fixing the mechanical problem with the bent chassis I now have a (hopefully) final problem.
 
The electrostatic platen does not work properly.  It provides only a very weak suction force so that the paper starts to slide. Also the force seems to be a bit higher when switched on and then quickly drops to almost zero within a few seconds.
 
Of course I had to take the platen off when I was taking the plotter apart for fixing the chassis but I was very careful.
The white cover sheet already had and still has a 2-3 deeper scratches going down to the copper/epoxy plate. 
The only modification from my side was to glue the lower right corner (about 20x20mm) of the white sheet down with rubber cement because it was starting to lift off the Epoxy board.
(BTW: this was not the best idea because the solvent in the rubber contact cement caused some small wrinkles in this corner of the sheet.)
 
When I measure the voltage with and without the platen connected I obtain about 480 Volts when ON, dropping to 20 Volts and then dropping further when switched OFF. This seems to be normal.
I also tested most of the components on the platen control board so I assume that the power supply part is fine.
 
Across the platen connector I measure infinite resistance (when plugged out).
I am not able to measure the capacity lacking test equipment for such low values.
There is also infinite resistance between the platen connector wires and the metal frame of the platen.
There the comb-like copper traces are connected to the wires - I measure zero resistance between one wire and one comb
All this seems to be reasonable.
 
The platen is built of interleaved +/- combs on the Epoxy PCB board (covered with the white plastic sheet).
 
There are a 2-3 deeper scratches in the plastic cover sheet so that in two or three spots the copper trace is open to the atmosphere.
Could this be a problem, even if a smaller sheet of paper is not reaching these open spots?
 
Unfortunately the theory of operation of the platen is not explained in the handbooks or in the HP-Journal article for the 9872/7221 plotters.
Is there someone who knows how it actually works and whether it is sensitive to cover sheet damage?

Any ideas or experience with defective platens for 9872/7221 plotters?

Thanks,
Martin

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