Cromemco D+7A troubleshooting question

All -- I?m finally getting around to building the JS-1 joystick replica and using a D+7A board I got a few months ago. I have the JS built but now I?m trying to recalibrate the board per the instructions on page 4 and I?m having trouble getting it to read the right values. The calibration procedure, basically, starts with putting 2.54v on an analog input and using a short program to read the input and display it on the front panel output port of my IMSAM. There are two problems. When reading the A/D and displaying the input, bit7 is stuck ON. The second problem is that I can?t get the input to read the maximum positive of 7Fh (excluding the seemingly stuck bit7). The version of the schematic I downloaded isn?t a very good scan, and doesn?t seem to be the same revision as my board (because the adjustment pots have different reference numbers). There are a few common chips that touch bit7 (like the LS367) which I can swap, but the others, like the AM2502 successive approximation converter, isn?t as common. So before I go swapping chips, does this board have any common failure mode I should be looking for? Thanks! Rich -- Rich Cini Collector of Classic Computers Build Master and lead engineer, Altair32 Emulator http://www.altair32.com http://www.classiccmp.org/cini