[RogerEvans]

Symon,

In dual + chop mode (pic h) there should be two traces, is one just off the bottom of the screen?

Are (i) and (j) also in 'dual + chop' mode? The two traces look absolutely identical apart from a slight horizontal shift, is it correct that one is 1000Hz and the second is 1000,000Hz? Can you generate both pictures again with both traces on screen so that you can see if the 'chop' between the two channels is interfering with the displayed signals? Generally you would use 'alt' with fast signals so that each channel is drawn on alternate sweeps and 'chop' with slow signals where there is time to switch the spot back and fore many times in a single sweep.

You said in one post that the ZUP signal looked the same in single beam and 'dual+chop' mode, so for now let's assume ZUP is OK when it leaves the timebase board (you could say just where you measured it in case we come back to it later).

If the sweep speeds are correct then pictures (b), (c), (d) and (e) are consistent with the unblanking lasting for about 3usec and fading away over 2usec. It should stay active for the whole duration of the sweep.

Chop mode appears to activate the CHB signal from 'Y intermediate amplifier' board, I think CHB is short for CHop Blanking and on other scopes that I have used, this signal blanks the beam while it is moving from displaying channel 1 to channel 2 and then unblanks it in the new position. Activating this signal is probably what makes the display start working. The manual says that the chop frequency is around 500kHz so a new unblanking pulse is generated every 2usec or so which is faster than the 3-5usec we deduced from (b) to (e) pictures.

ZUP, CHB and the unblanking signal from the digital side of the scope (ZW) get combined in the circuitry around T903, T904, T905 and then drive the opto-isolator IC901. A good place to start would be to set the sweep speed to 2usec and, using a well insulated scredriver, adjust VR901 a little in each direction to see if it has an intermittent contact. BE VERY CAREFUL around VR901, it appears to be on the -2kV high voltage board and the potential for harm to yourself or the scope is not negligible. If in doubt don't touch it! If IC901 is socketed, then WAIT 30 MINUTES after turning off the scope and unplugging it , then remove and reseat IC901 to clean poor contacts– one side of the opto-isolator is at -2kV so you really need to be sure the -2kV is discharged properly.

Regards,

Roger