HP 7475A Plotter: Rehabilitation

[Update: Wecome Hackaday! You may want to look at:

Searching for 7475a will bring up many, many other posts]

After mentioning that I wished I still had my HP 7475A plotter, Dithermaster sent me one from his heap. As he explained, a mouse family had used it as a combination hotel-granary-latrine:

HP 7475A - chassis latrine

HP 7475A – chassis latrine

For whatever it’s worth, if you must get a bazillion seeds out of a plotter, ship it halfway across the continent: UPS performs a lengthy three-axis vibration test that shakes all the loose bits through the vents.

You’ll probably want the original HP 7475A documentation from the (unofficial) HP Computer Museum before digging in. Not mentioned anywhere: the two washers at the rear edge of the case are not identical. The one holding the power supply in place is slightly longer than the one at the serial connector. Mine are now color-coded to their locations.

A critter whizzed on U13, the serial adapter chip, just beyond the big black filter capacitor:

HP 7475A - PCB latrine area

HP 7475A – PCB latrine area

I rinsed everything (except, no fool I, the membrane keypad at the front of the PCB) with warm water, flushed the latrine areas with dilute baking soda (alkaline, to neutralize the urea), rinsed with hot water, blew-dry with compressed air, then let the pieces sit for a few days.

After reassembly, the plotter didn’t start up. It’s a third of a century old, what did you expect?

Measuring the electrolytic capacitors showed they were all in surprisingly good condition, with only C27 and C34 (on this Option 001 = RS-232 board) having moderately high ESR. They’re the pale blue axial caps just right of the heatsink, both 22 μF 25 V:

  • C27: Processor Reset timing (U14 – p. 6-27/6-28)
  • C34: +5 V filter cap (U21 – power supply p. 6-31)

The corresponding caps on the Option 002 = HP-IB board are C20 and C25. FWIW, if you have an HP-IB plotter, you should probably just hack an Arduino into the motor control connections and run it with Grbl; you’d get a bare-bones plotter eating G-Code, not HP-GL, but that’s not entirely a Bad Thing. Adapting the tool change code to handle the pen carousel is left as an exercise for the desperate.

I replaced the offending caps with 33 μF 50 V radial caps from the heap:

HP 7475A - re-capped PCB

HP 7475A – re-capped PCB

And then it performed its Demonstration Plot (load paper, hold down P1 + P2 buttons, turn on power) perfectly. The fossilized pens left no trace behind; we all expected that.

The serial port connection on the back required, from bottom to top:

All of which came from the Big Box o’ Serial Adapters and produced this rather unsteady ziggurat:

HP 7475A - serial port adapters - typical

HP 7475A – serial port adapters – typical

Seeing as how I’ve been adapting serial connections since before the HP 74754A was a thing, the Adapter Box has All! The! Adapter! Genders! plus Der Blinkenlights! They don’t come in nearly as handy nowadays, though, which is a Good Thing.

Some optimization pared down the ziggurat and added a short extension cable:

HP 7475A - serial port adapters - hardcore

HP 7475A – serial port adapters – hardcore

Eventually, I’ll build a custom cable, but it’s good enough for now.

The switches select 9600 b/s serial data in 8N1 format. Yes, the plotter tops out at 9600 b/s, but remember we’re dealing with a pen plotter that executes terse ASCII commands. It offers both XON/XOFF and DTR/DSR hardware handshaking to prevent overruning the internal 1 kB buffer, plus a myriad other software-selectable options relevant to long-forgotten datacomm systems.

Lest I forget, dots now mark the switch settings for 9600 8N1, A (letter) paper, US (inch) units, direct serial connection:

HP 7475A - DIP switch settings

HP 7475A – DIP switch settings

And then it Just Worked: type IN;SP1; into minicom and the plotter grabs Pen 1. The rest is a simple matter of software.

Now, to deal with the pen situation…


  1. #1 by smellsofbikes on 2015-04-20 - 11:25

    It’s been several years since I last looked, but when I did, I found a place that supplies computer ribbons for old printers, that also still had 7470 and 7475 plotter pens.

    • #2 by Ed on 2015-04-20 - 11:30

      NOS plotter pens are available on eBay at four or five bucks a pop, but what’s the fun in that?

      I’m getting pretty good results from Sakura Micron pens through the magic of 3D printing… [grin]

  2. #3 by smellsofbikes on 2015-04-20 - 11:26

    Also, it’s probably easier to get a usb-gpib adapter off ebay, for those of us who have gpib plotters. These days you can find ones that appear to work reasonably well for under $70.

    • #4 by madbodger on 2015-04-20 - 11:33

      I have a pair of what were once very fancy expensive serial-GPIB adapters that came with a rack shelf I bought at a hamfest for $5. I also have one of the cheap USB ones. They don’t support all the fancy GPIB signalling, but they’re good enough for me to talk to my remaining test gear with GPIB interfaces, which is all I need. Still haven’t tried my GPIB digital photo printer (electromagnetic deflection of a high-rez CRT with P15 phosphor).

      • #5 by Ed on 2015-04-20 - 11:43

        GPIB digital photo printer

        With wet chemistry, I’m sure!

        Vassar’s art center is running a big daguerreotype exhibit with lectures & show-n-tells. If the process didn’t involve iodine, bromine, and heated mercury vapor in a darkroom, I’d be on it like static cling…

        • #6 by madbodger on 2015-04-21 - 09:12

          I only have the 8×10 film holder for it, which is designed for Polaroid film packs. Technically it is wet chemistry, but at least it’s enclosed in the film pack. I’m too cheap to actually buy 8×10 Polaroid film and the outboard gear to process it, however. However, I’m intrigued by the very high res 5″ flat face CRT and the drive electronics (the deflection yoke is a thing of beauty, with lots of elaborate windings) to accurately place the beam within a few dozen micrometers, and the very pretty violet phosphor.

          • #7 by Ed on 2015-04-21 - 14:49

            8×10 Polaroid film


            I know that Polaroid film remains a Thing, because hipsters, but a quick check says those big sheets run just shy of $20/shot: priced beyond my toy budget!

    • #8 by Ed on 2015-04-20 - 11:36

      There’s still a market for adapters in the fractional-kilobuck range, at least according to Amazon. Must be plenty of old instruments floating around out there…

      • #9 by smellsofbikes on 2015-04-20 - 14:33

        National Instruments $650-per-unit, guaranteed to work with LabView. There are over 1000 instances of this in my company alone, and my friends in biomedical fields say it’s not just us electrical engineers that are using labview for everything.
        I have a cheap prologix converter at home, and it does a great job with all my home instrumentation: scope shots from the lecroy! I haven’t yet tried it with labview, though.

        • #10 by Ed on 2015-04-20 - 15:00

          it’s not just us electrical engineers that are using labview for everything

          Our Larval Engineer reports that LabView is how their project / lab measurements get made, by and large. The days of manual scope probing are over…

          • #11 by smellsofbikes on 2015-04-21 - 13:55

            Ironically, now that digital cameras are ubiquitous, we’re back to taking scope screenshots by camera. There was a brief, wonderful moment in which it was via gpib/network, but that’s too much work compared to pulling out an iphone…

            • #12 by Ed on 2015-04-21 - 14:52

              pulling out an iphone

              Then ignoring the fact that a blurry picture doesn’t get less blurry after you paste it into the report / article / blog post. [mutter]

  3. #13 by madbodger on 2015-04-20 - 11:30

    I added a vinyl knife to mine, but then bought a cheap Chinese vinyl cutter and gave away my plotter too. For RS-232 connections, I came up with an 8P8C standard for serial, and made adapters from that to 9 and 25 pin connectors with various genders and pinouts. So I can whomp up pretty much any serial cable at any length in a few seconds. As you pointed out, this ability is needed less frequency these days, which is indeed a Good Thing.

  4. #14 by Red County Pete on 2015-04-20 - 14:44

    Memories… We did a lot of IC layout at HP (most of the low-to-medium end optoelectronics from HP came through our shop) and the 7475 was one of the go-to plotters, both for CAD as well as general graphing. My personal favorite was the grit-drum-drive E sheet plotter, AKa the Elephant..

    On pens, going outside HP, you had a lot of options. One manufacturer (CalComp?) did a roll-type plotter that used a range of pens, including the india-ink pens in colors, ballpoints, and the usual felt tips. Somebody offered a knife-pen to cut rubylith. Mercifully, cad-cut ruby was a technical innovation I managed to miss. (Even the dinosaurs at Nat-Semi adopted CAD by 1976, though I remember Bob Dobkin griping about it….) I doubt the cutter was for the 7475. Don’t recall it having much downforce.

    • #15 by Ed on 2015-04-20 - 15:04

      I doubt the cutter was for the 7475. Don’t recall it having much downforce.

      I’ve blown a remarkable amount of time demonstrating that’s absolutely true: try not to ridicule me too much as the tale unrolls.

      But the pen adapter works surprisingly well, so it’s not been a complete waste of time, at least as such things are measured around here…

  5. #16 by rkward on 2015-04-20 - 15:50


    Contact me offline regarding plotter pens and which ones you are looking for. I think I have quite a few that are NOS.


  1. HP 7475A Plotter: Full-up Sakura Micron Pen Tests | The Smell of Molten Projects in the Morning
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