Tektronix 7844 traces fade with scan speed

[Gianni]

Hi,
I have a 7844 oscilloscope that has two problems:
1) traces fade with scan speed, until they are no longer visible at 50ns/div; if I raise the intensity, I only lose the focus. The problem is exactly the same on both beams.
2) the readout "dances". I have already tried changing the whole board with another that is certainly good, but nothing changed. All voltages seems to be right and clean.
I have a video but I am not able to upload it (which format should I use?)

Unfortunately I have not a high voltage probe, but in 7844 I do not find a test point for it however.

Can anyone please give me some direction where to investigate?

Thanks

[perdrix]

Which timebase are you using?  What frequency is the signal - if you're using a 1kHz signal you should not expect to be able to see it at 50nS/div
 - think about it for a while and you should be able to work out why.

D.