HP-97 repair questions (a lot of)

[Accutron]

The 97 uses two different PIK revisions: 1820-1723 and later 1820-1952. The mainboard has two primary revisions. Machines made after the introduction of the 97S have unpopulated vias configured like a DIP socket where the BCD interface pod would attach in a 97S. Pre-97S mainboards can either have tinned or gold traces. The post-97S mainboard always has gold traces, and is electrically identical to the pre-97S revision AFAIK. There are also a number of different ACT, CRC and RAM/ROM chip revisions. First-year machines have an inverse trig bug you can easily test for. Topcats have linear travel keys with coil spring return which close electrical contacts (no optical components, but it's the best keyboard they ever used in a small machine.)