HP 7475A Plotter: Refilling the Pens

Plotters date back to the days before companies started using DRM to protect their monopoly positions, so refilling plotter pens requires little more than prying out the plug and squirting in more ink. Refilling the disposable liquid ink pens and the green ceramic pen suggested this would work.

I shaved down the side of a Genuine HP pen to find out why the plug didn’t pop out. It turns out the plug has a long and aggressively ribbed profile to ensure a gas-tight fit:

HP Plotter Pen - exposed plug
HP Plotter Pen – exposed plug

The easiest way to refill those is to drill an off-center 1/16 inch hole in the plug, then inject ink into the sponge with a syringe and blunt needle (and bulk ink!) from an inkjet cartridge refill kit. Angling the needle through the sponge close to the pen wall, then filling slowly, loads the sponge from the bottom up and expels the air along the way.

Inmac pens have a shallow plug, more of a flat cap, that pries out with zero drama:

Inmac Plotter Pen - removed plug
Inmac Plotter Pen – removed plug

Dripping the ink atop the sponge seems to work well, although that sponge is definitely over-filled.

Inmac caps push back in place with zero drama.

The pens have fiber nibs with vent channels along their sides that allow air into the reservoir, so overfilling the sponge nets you a mess when you take the cap off the nib: those same channels allow excess ink to run from the reservoir around the nib, without (much to my surprise) wetting the fiber tip.

About 0.2 ml of ink fills the reservoir to saturation, 0.1 ml leaves it wet, and 0.05 ml seems to work well. The 1.0 ml syringes I’m using require about 0.05 ml to fill the (blunt!) needle shaft & hub, plus the syringe tip below the 0.0 ml index, so measuring the ink by drops might make practical sense.

The old physician’s trick of expelling that air by inverting the syringe and pressing the plunger until liquid squirts from the needle is so not happening…

I’ve had zero success refilling fossilized pens, probably because the OEM ink slowly evaporating from the nib clogs all the gaps between the fibers with pigment or coagulated solvent. Preemptively refilling good pens when they first show signs of running dry generally works well.

Given the number of New Old Stock pens I have that are still in their original wrappers, this is more of a “Does it work?” exercise than a necessity.

But, y’know, maybe becoming the last plotter pen refiller on the planet will be my ticket to fame & fortune! For sure, we’ve all seen over-hyped Internet startups with worse business plans and (the admittedly few) typewriter repair shops occupy a stable niche.

16 thoughts on “HP 7475A Plotter: Refilling the Pens

  1. typewriter repair shops

    Another niche seems to be sewing machine repair outfits. We have one good one in town, but my sister-in-law in the Reno, NV area hasn’t been able to find a competent repair place for her old, high end machine. Shipping one to a specialist seems spooky…

    1. her old, high end machine

      Aye, the elaborate kind running a Windows 98 UI locked to an ISA-bus dongle. When that old PC stops running, you can always use the sewing machine for straight stitches…

    2. Reno? Call one of the casinos that has a stage show. Speak with the stage manager. Ask who creates or repairs the costumes. Call that person. Ask who repairs the sewing machines.

      Locally here in L.A. there are a number of Hollywood wardrobe costume shops. A few of the largest ones (with a large number of machines) have full time in-house repairmen The rest of the shops depend on several really good (and therefore not cheap) traveling repairmen.

      It’s interesting, but locally, the really good sewing machine shops do not advertise, and have no signs. It’s all word of mouth.

      1. Caught. I’ll pass the thoughts on. I believe it’s pre-computer, back when the mechanics were getting awfully complicated. The last “fix” lasted 6 months, so the S-I-L bought a disposable Brother. (Julie’s done the same thing, keeping her 30 year old Elna for the good projects.)

  2. What distinguishes a “plotter” pen from a pen pen, besides the shape?

    1. I think the tip is harder to withstand being hammered against the paper, the fiber cylinder behind the nib provides faster ink flow, and the fluffy reservoir holds more ink. Other than that, the innards look pretty much the same; cut-down Sakura pens work pretty well at slower speeds.

      I’m really hoping modern inkjet juice won’t go hypergolic with whatever’s inside some of those pens: the old dyes from the red-orange end of the spectrum, in particular, deposited really weird crusts inside the caps!

  3. 0.05mL is a truly astonishingly small quantity to fill a pen that size!

    1. That’s what I thought, too. Turns out if the fluff isn’t entirely dried out, 0.05 ml will saturate it!

      The right amount may be even smaller; I figure nothing exceeds like excess and I can wipe it off if that’s what it takes. [grin]

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