I can't find A schematics at the moment either, but here's a link to a document that tells you how to convert your A to a B ... I posted this on one of the first pages of this thread, but the original link has gone stale:
https://www.magentacloud.de/lnk/aiyzLQpEAnd this technical manual
http://www.os2site.com/sw/DEC/rainbow/doc2/pc100tm1.pdf also seems to talk about the "A" model.