[Keith]

Thanks for the further replies. I'll check the sync coupling cap, although I've not seen a leaky compression trimmer before. With the extra capacitance of the scope probe after the differentator capacitor, most of the break-up disappears. The remainder at the top is due to the recovery after frame sync which will be the next thing to investigate. The other issue is the loss of the bottom 10% of the frame. I'm guessing that the timing of the vsync is out but I need to use a scope with delayed timebase to have a closer look.

Hugo - I have been offered the loan of a known good test card generator from an amateur radio acquaintance. I shall take this up at the next opportunity.