Hi Colin, here's some pics of the TB1 internals, first with the psu in place....
and the second with it removed....
Now to what concerned me, a standard 486 AT psu, has two 6 pin molex connectors on it, whereas Testbook has an 8 pin molex and a 20 pin ribbon header between the motherboard and the PSU. From memory, I recall many moons ago, there was a web page, by a german chap, who was using "Shore power" on a testbook, however IIRC, this was a testbook 2 unit. now here's the crunch, the 8 pin molex on TB1, is this the output from the psu to the motherboard, with the input voltages being supplied via the ribbon cable ( and various components on the main motherboard), OR, are the inputs via the Molex, and the outputs from the ribbon, supplying the processor, ram etc ?. The pins themselves aren't marked up as such, so really a stab in the dark as to where to connect
Deeper delving, via the HP Museum, of all places, Say that TestBook, was designed as a 386 machine running DOS, as a point of sale type machine, with customisable input/outputs, which would be supplied to the end user, with turnkey operation, but were to be supplied in minimum mutiples of 50!
Amazing what trivia you can turn up on the Web