Weird HP85a failure mode - ASCII mapping error?

[mattc5000]

Hi,

New to the HP vintage community and trying to solve a weird problem with my HP85A. The computer turns on and boots correctly, but every other line on the screen is half filled with the characters "IMMMMMMMMMMMMMMM"

When you type in commands, many of the keys don't map to the right letter, but it is dependent upon what column you are in on the screen. For example, if you start typing RRRRR, it comes out RVVVV, however for some letters, it works right most places on the screen.

I've plugged in a cartridge with the service ROM and the CPU, RAM controller, RAM, and ROM tests all come back OK. Of course when the results are printed on screen, they use the wrong characters, so the CPU OK message comes up as CTU$00 and RAM OK comes up as REM$00.

I replaced all of the RAM chips, as I thought it might be a screen memory problem, but that did not help.

My best guess would be a weird bit flip in the ROM that maps ASCII codes, but I'm not sure which one that is.

Anyone ever see anything like this? Any guesses on which ROM might be bad? Or how to burn a replacement?

Thanks

Matt