@hamid_1 Hi there Hamid, I was only thinking the other day that this thread would probably be of interest to you! Thanks very much for the pointers and guidance, I'll follow that up tomorrow and report back with my findings.
Sounds like you have a nice selection of sets from this period. Sonatel seemed to pop up out of nowhere around 1979/80. I wonder if they were exclusive to Argos? Aside from the MR-T750 they had a small range of clock radios too, but nothing beyond that as far as I'm aware - no cassette recorders or music centres etc.
I'd forgotten that Fidelity revived the HMV brand around this time too. Their audio separates range and some music centres were available in HMV guise. Your Fidelity/HMV portable is a rare survivor so well done for saving it!
I'm also not sure if the expected voltages are measured with or without a signal applied - this would also make a difference.
Without a signal, Hamid.
Sorry folks, a rather longer delay than I'd have liked getting back to this. Although I'm on furlough the days still whizz by, mostly filled with house renovation tasks.
Further to Hamid's advice, the resistance of the contrast slider seems to vary correctly (although no doubt it would benefit from some switch cleaner which I don't have). 33v is present at R42 and when rotating the tuning knob the voltage at the relevant tuner pin varies from 0 to 24v, so it seems that the tuner is working. Certainly tuning across the output of a vcr causes the hiss to turn to a slightly louder 'mush' so it seems to be at least partially 'seeing' the signal, although absolutely no disturbance to the blank raster on screen.