What the heck is this thing?

[Dave92F1]

I hope this is the right place to post this.

I picked up this Soltec model VP-6414S X-Y chart recorder at a ham fest.



This is what it looks like inside:



It's an analog XY recorder (basically a XY pen plotter) from the late 1980s.

It mostly works and I'm trying to restore it. But there's a mystery.

There's a switch marked "CHART HOLD", which can be set to "HOLD" or "FREE". How the heck is that supposed to work? There's no obvious mechanism to hold or release the paper. And if I flip the switch, as far as I can tell nothing happens at all.

There's compartment on the right which looks like it was designed to hold a roll of paper:



On the top of the printing surface there's two yellow LEDs (they're on all the time the thing is powered), and no other obvious mechanism:



I suspect the two LEDs were meant to shine thru the paper to help you line up the X and Y zero points for calibration. No mystery there.

But if I look on the underside of the printing surface, I see this:



The little PCBs hold the LEDs and series resistors. But what's under the RTV goop in the middle with the 2 wires going into it?

I probed around on the top side with a magnet while flipping the CHART HOLD switch each way - nothing obvious.

Both the LEDs and the mystery RTV thing plug in here, just to the right of the red high-voltage arrow shield:



On that connector, the top 2 pins power the LEDs (about 14 VDC on them).

The bottom two pins go to the mystery RTV thing. In the "FREE" position there's about 4 VDC across them (at extremely low current). But in the "HOLD" position there's about 1400 VDC across them (also tiny current).

What's under the RTV, and how is it supposed to work?

[DavidAlfa]

Maybe a Hall sensor or Reed switch to detect zero cross or calibrate the zero position?

[TimFox]

On that type of unit, for use with thin graph paper ("onionskin" weight), there was often an electrostatic mechanism to hold the paper down after the user located it properly in the frame.
I don't know exactly how that was built.

[Gyro]

Yes, It looks as if it has an inverter for electrostatic hold-down on the bottom left of the PCB image (secondary under the black plastic with the High Voltage symbol). It should be current limited but watch your fingers! The hold-down is usually a set of interleaved PCB fingers underneath an insulating top sheet [EDIT: but might be a single plane and then a grid on a layer above it].

[Gyro]

I knew I had it somewhere. Electrostatic paper holder article attached...

[p.larner]

if it was me that would be getting a 7w laser in place of the pen,teady made laser engraver.