Casio PB-100 / Tandy PC-4 versions and documentation

[Jeff_Birt]

I recently picked up a Tandy PC-4 which is a Casio FP-100 in Tandy clothing. It only needed minor repair (I did a video, to be released December 29, 2021.) After fixing it I was looking at what it would take to build a RAM expansion module for it using the Tandy PC-4 service manual as a guide. Then when editing the video, I realized that I had a later revision of the main PCB.

The original PCB for RS Catalog #26-3650 (PB-100) had a small board bodged on to correct a timing issue which could scramble the expansion RAM. They went on to release 26-3650A and 26-3650B which did away with the mod board and moved that functionality into the RAM module. I found this out thanks to someone who used to work for RS still having the Tech Bulletin about it. I'm not sure what the difference between A and B might be. I think the 'B' model is what Casio called the PB-100F.

I found a cheap PB-100 (original I think, and not functional) on eBay this morning and grabbed it. I'm hoping to document what the differences are and would love to find the Tandy or Casio service manual for the later revisions. I have attached the Tandy Tech Bulletin and the link to the Tandy service manual for original version is below. If anyone else happens to have more documentation I would greatly appreciate it. Thanks -

https://archive.org/details/manualzilla-id-6027413