Please have a look at the service manual, page 18 (Use the link someone posted before) and make sure your model is not a 8020/19 model. A Frech model will not work with that cable too.
In that page you can see the DIN pinout.
Sound is not comming from the PAL encoder circuit, so it is weird that it is failling too.
I would say it is a problem with the RF modulator or (maybe) the pal encoder chip itself.
If you do not have a way to check the VDP signals in connector JN10, then you should consider to disconnect RF modulator and try.
Its definitely the 8020/00 . It has a sticker on the back of the board and the keyboard is QWERTY. The vg8020/19 has an AZERTY keyboard.
I am going to try to remove the RF- modulator and see if that makes a difference.
UPDATE: I disconnected the RF modulator and the results are sadly the same. I am going to contact someone with a scope and see if there is a signal on JN10.
