HP 7090A Measurement Plotting System

[Tony_G]

Hey All,

Just dropping this here in case someone else runs into this.

My unit broke its "Geneva Wheel" (see the awesome CuriousMarc video on this - https://youtu.be/G5_YltaYZsg) so I needed to repair it. Firstly I did the old "super glue the crap out of it" and ended up with it looking like this:



But it kept breaking so I had a friend of mine print a replacement using the STL file that CuriousMarc included in his video description:



That solved problem one but problem two was that it would often not swap pens. I asked about it on the VintHPcom group and was suggested to look at the spring in the pen holder.

My spring worked fine (provided quite a bit of tension) but I discovered that the little rubber caps in the carousel would stick to the pens. I think the controller is using the back EMF of the motor to work out when it has a pen so the additional pressure required to release the cap was tricking the controller into thinking it already had the pen.

I've been cleaning them over the last few days (there was a lot, a lot, a real lot, of old ink in those rubber caps) in a jar with some IPA so I'll update here with the result once I reinstall but for now it is swapping pens flawlessly.



Hope this helps someone in the future.

TonyG

[Tony_G]

For anyone who might be interested - Here is the plotter running its test plot without the rubber caps in the carousel - So far seems to be working flawlessly (fingers crossed that I haven't just jinxed myself):



I extended this test to more complex plots:



Source and HPGL files are located at https://github.com/TGoodhew/HPGLTest

TonyG