[ScottishColin]

This all checks out - moving the resistor changes the characters on the screen.

But I think they're actually the inverse of the codes you have listed. Would it be possible for you to let me know which combination of pins I should use to get something 'sensible' - eg the letter A and I can try that to see if they are inverted or not?

Colin.

Originally Posted by julie_m

With UF9 out, try connecting each of pins 2, 19, 5, 16, 6, 15, 9, 12 to 0V via a 470 ohm resistor. If the character generating circuit is behaving properly, each of these should produce a screen full of a different character; they will be hex codes FE, FD, FB, F7, EF, DF, BF and 7F respectively. The first 7 will all look quite different from the "default" checkerboard; with UF9 pin 12 to 0V, the pattern will be a checkerboard, but with the dark and light squares swapped over.

See this, borrowed from https://www.masswerk.at/nowgobang/2020/petscii:
Attachment 229059