V3.1 boards. These have an additional LED, which I have set to show activity, currently on for reading, off for writing, but I may also look at on for ROM access, off for RAM access etc.
early 2001 repairs.
POKE 16384,0Then try poking 255 there and read that back, and you should get 255.
POKE 16384,255If either or these responses are wrong, you can work out which chip is stuck high or low by converting to binary, so for example if the response is 8 and 255, one bit is stuck high. That is 0x08 in hex, 0000 1000 in binary. So it looks like bit 3 is stuck high. You would get 252 rather than 255 if it were stuck low. From the schematics at zimmers.net archive (pages 5 and 6), you will see that bit each bit is provided by a single chip, the 4116 being 16K x 1 bit. In this case, bit 3 is UA12.