Welcoming another Penman #penplotter turtle home. Hope to learn how to repair these and get them working again. Many thanks to @Retro_CAD for this one! #plottertwitter #RetroComputing 
Here’s a video of a Penman plotter in working order.
https://twitter.com/paulrickards/status/1226560693798875137
The new Penman fails to detect the page edges during a home cycle. Without this, it can’t begin a plot correctly. Edges are detected through two small slots on either side of the turtle with an IR emitter and receiver pair. Black paper is recommended to increase edge contrast.
The IR detectors go into an LM358N op amp then to the controller. The IR LEDs are connected to 5v in series through a single resistor. IR detectors respond when I shine an IR remote on them (see scope).
The IR detectors go through an LM358N op amp. I’m guessing the LEDs in these are fading in brightness and sensitivity. Is it better to increase IR emitter brightness or increase IR receiver output via the op amp? And how?
So I remembered I had a sheet of glossy black paper and tried that with the Penman honing sequence. It worked! But does highlight how I think we’re right on the edge of tolerances in the paper detection circuit. It would be great to have some headroom.
Found the Penman product brochure (1/2)
• • •
Missing some Tweet in this thread? You can try to
force a refresh
