PCB Plotter

In another thread, I was looking for brainstorming for desktop PCB fabrication. It came down to pretty much any method would require an X-Y table. When I was going through my storage looking for parts, I stumbled on the old HP 7475A plotter that I had picked up years ago for dirt cheap (it was non-working.) I had intended to repair it and convert the markers to resist ink (I was successful doing that.) But life took a detour right around that time and I never got to fix it, in fact, I had forgotten that I had it.

Well, here it is in front of me and instead of repairing it, I am thinking about replacing its entire controller to make it more specific to what I want to do with it (not absolutely sure yet which route I plan to go - laser seems like the best option ATM) and to make it USB.

The HP 7475A is a flatbed plotter which means it moves the paper for the Y axis, and the pen runs along the X axis. It has a carousel which holds 6 pens for color plotting. It has a 12" wide printing bed.

Two DC motors run the X and Y axis and have quadtrature encoders to track position. There is a solenoid for pen up/down and a stepper for the carousel. I’m not sure how I will incorporate the carousel, but intend to include control for it in any case.

I found the service manual for it, so I have the schematics and pinouts of the various parts and some specs (still trying to locate the specs on the motors and solenoid.)

Anyway, so that is my current project. Is anyone interested in collaborating on it? I can design the hardware and modifications with little trouble, but the code may end up a bit difficult for me. If this doesn’t get any bites, I guess I will return once I have all the hardware done and am looking for just code advice. Just thought I would introduce the project and see if anyone was interested in helping brainstorm it.

7475A-37.jpg

Block Diagram(small).png

After I complete the electronics, one of the first mods I see that I need to do is with the feed rollers. First the rubber is worn off of them and they are only driving the edges. For accurate positioning and for handling PCB stock, I am going to need it to run across the entire board with even pressure. Also, some means of registration would be a good idea.

Found the details for the motors and pinout for the encoders. Apparently these made it into the surplus market (the motors) and other people already dug up the specs.

3.59A stall current for the servo motors (130mA no load). Not too bad. I won't need to roll my own H-Bridge for it. They are rated at 19.1V but are being driven with an unregulated 18V supply currently. Very manageable specs. The stepper had all the specs printed on it. Now to see if I can find specs for the solenoid (though not too difficult to figure that out on my own.)

So instead of paper being squeezed between a roller and capstan or similar, you need to move a tray or similar in out of the print area/band instead?

Yep. Exactly. A PCB would fit in there fine now, but the capstain would only drive the edge. I attached a picture to show what I am talking about. It would probably work as-is in most cases for smaller boards, but I am concerned about it twisting as it is driven. I am thinking that maybe just a guide that can fit up against the other edge might be fine, though.

Hi, i really like your work so far. I own a 7475 too.

I Modified the plotter to use full length permanent ink fitted into old dried pencils. I am still trying to make things work, i have a bad rs232 cable, i am using usb 2 rs232 ... rst cts not working ... i have to figure it out.

Keep your posting on.

Marcel Racine Quebec, Canada

It has been a while but you may use my servo code GitHub - misan/dcservo: Position control of DC motors to control each motor (a single arduino micro for each motor). As for the main controller you can use GRBL and wire the step/direction pins to each one of the motor controllers. This way your plotter can accept GCODE. Similar to what I did for https://www.youmagine.com/designs/4xidraw

An interesting project, I also have an old HP7475A somewhere in my garage :slight_smile:

The HP7475 paper feed is designed for paper, where the rolls impress a pattern into the paper, which allows for accurate positioning. I'd suggest that you fix the PCB on a sheet of paper, if you don't replace the paper feed by some other mechanism.

The plotter also aligns a sheet of paper prior to painting, so that the Y (paper feed) axis aligns with the case. Dunno how that works, it looks quite tricky. Without this feature you have to care yourself of proper paper alignment, or you have to cut off the used PCB area as painted.

Hi Retroplayer , although it may be a little outdated , I was reading your project and is very close with what I been dreaming during my holidays , I wonder if instead of doing a crude surgery we install a laser , from a dvd player (just to try) instead of the pen , or better still what if we install the laser inside a pen , and set it off or on depending on the up or down position of the pen (always off when up). We then use photo sensitized copper clad ? . Does this idea seem feasible to you . Hope you read this measage . Regards Hugo (sorry if something doesn´t fit , but is my first message)