HP41C Display corruption

[KC9UDX]

I bought this 41C with the intent of converting it to a 41CL. I'm still not sure I want to do that yet.

But here's what it does: The right hand portion (6 characters) of the display incorrectly display the characters H..N. That is, if the character 'H', 'I', 'J', 'K', 'L', 'M', or 'N' appear in the right hand side of the display, they appear with extra segments. See here: https://www.youtube.com/watch?v=dx0EgmivTl8

When I first got it a year or so ago, the problem was worse, and covered the whole display, and more characters. I wrote a program to display each of the 127 displayable characters in sequence, and ran it continuously for a week. The situation improved to where it is now, and does not improve beyond this.

I could swap the mainboard with a CV and free up an I/O slot, but I'd prefer not to at this point. Also, it seems to me that the problem could be in the display itself.

Does anyone here understand the architecture enough to know what could cause this? It seems to me that RAM would not be the issue, and it could not be as simple as a broken or shorted path from the mainboard to the display.