curiousmarc

HP 7475A plotter fails - but 3D printing fixes it

Added 2021-08-05 06:04:34 +0000 UTC

I was having great fun with my HP 7475A and giving it a good workout, trying to have it plot schematics and PCB layouts. I had my technique almost perfected (took a lot of trials!) when I prepared for the final video shot of the plot. And of course that's when it failed. It printed in one single color instead of changing colors mid plot. Eventually it failed outright to pick up the sole pen it was reaching for.

I quickly discovered it had to do with the pen turret not rotating properly. It is driven in discrete positions by a Geneva wheel arrangement. The plastic Geneva wheel had disintegrated in several pieces. I did fix it temporarily by gluing it back together on a metal shim.

Not pretty, but it did work. The nibble that engages with the Geneva slot was made out of a rivet... However this is a) ugly and b) will probably not last.

A quick Google revealed that I am not the only one with the problem. This wheel is made from a different, softer plastic than the rest of the mechanism, presumably to absorb the shocks better. But it simply disintegrates with age. There was even a dimensioned drawing available online.

I considered machining it on the mill out of Delrin, but this part screams 3D printing. Mike Stewart has two 3D printers, a traditional one and a super high resolution optical SLA. He quickly whipped out a replacement wheel with each. The gray one is the SLA.

I tried the SLA first, as its resolution and finish were amazing. And indeed, it fit super well, down to the friction fit with the D-shaft.

You can see how the Geneva wheel engages on the slots of the turret carrier. It works great. We'll see how long it lasts. If it doesn't, I'll have to spend some time machining one out of Delrin, but that would probably take me the better part of a day.

In the meantime, the printer works again, and I am back to plotting! Here is the PCB layout of my ersatz 1 GHz ECL gate that I made in the HP 8082A pulse generator repair episodes.