Ok with the A inten or Mix mode - it is even weirder than I originally thought.
In A mode all the sweep rates seem to work.
In A Inten or Mix mode the A sweep (clear skirt) doesn't work in the 0.2 and 1ms/div settings. In this case the waverform starts at the middle (near delay point) and seems untriggered (jitters a lot). If I fiddle with the hold-off I can make it look correct.
Ditto in A inten or Mix mode the B sweep (grey knob) doesn't work in 50,20,10 or 5 uS/div modes. When it isn't working in Mix mode you see a small pause at the delay point (the signal stretches out for a quarter of a division) and then it reverts to the A sweep rate. In A inten is just intensifies from that point on.
When B is not working then if I watch the signal at Q984 there is no saw-tooth but when I click to a setting where it works then the sawtooth returns.
Totally weird. Can't see what these sweep rates have in common or why this would fail this way.
Tom
