Solved two major faults on this ordinateur français:
[1] the keyboard membrane had a broken track fixed with some silver conductive varnish (Jaycar part NS3030),
[2] the MK90.090 (v3) cassette recorder or LEP (magnétophone or lecteur enregistreur de programmes) had a blown 8V linear regulator (7808).

An alternate & rather clever method of repairing the keyboard membrane:
http://dcmoto.free.fr/bricolage/clavi...

Once the keyboard membrane was fixed, I was able to also verify that the light pen (crayon optique) also works I had the TO70 connected via SCART to my Sony RDRHX900 HDD/DVD recorder and then via SCART to my Sony KV21LS30U CRT TV.

This is the first TV I ever bought, and the first TV we got as a married couple. Bought it in UK (made in Spain ... different) ... paid extra to get the stereo speaker model rather than mono ... and have lugged it back to, and around, Australia. It wasn't compatible with the Aussie analogue broadcasts (sound carrier at a different offset frequency or something, irrelevant if you're going via a settop box or similar) ... but it has two SCART interfaces so was great for excellent video game console output (like Sega Dreamcast)!

Weirdly/interestingly, the TO70 has a 2.1mm DC output socket that supplies about 13VDC ... centre positive. However the MK90.090 cassette recorder expects a centre negative input. Strange decision. Anyway, I created a cable that swaps the polarity so I can power the LEP from the computer.

A known issue with the MK90 is using the wrong polarity DC power supply which can destroy the small 100mA MC78L08AC linear regulator (in TO92 case):

https://forum.systemcfg.com/viewtopi...

My local Jaycar only has the larger 1A version available (in TO220 case) so I'm using that for now.

Once replaced, the TO70 could now detect the presence of the LEP.

I wasn't able to replay a WAV file via the MK90, but I was able to confirm it is working by loading software from the Amsoftbranded cassette that was included ... something called "ORDIBABY" by CobraSoft. Appears to be some sort of game for kids written in BASIC (possible to LIST the code after loading).