My Obtronix Apple 1 reproduction has a bad Signetics 2504. I built the kit a while ago, but I think I would have noticed the problem then. So I guess it got tired!
The Signetics 2504 is a 1024-bit dynamic shift register, 8-pin dip. It's used here to as part of the display control. The problem I saw was odd numbers (1,3,5,7,9,A,C,E) in every other screen column were displayed as the preceding ASCII character (0,2,4,6,8,@,B,D). Data from the keyboard was stored correctly in main RAM. It was just the video memory was displayed incorrectly.
Anyway, does anyone have a source of a Signetics 2504?
logjam- a long time ago, you said you had a source for the 25xx chips. Are they still around?