Hi Ondrej,
Videos are very useful.
I was surprised sweep and gate signals are not aligned but I think you are using different probes for each. Using a BNC cable for all of them will eliminate any mismatch. Just for reference here is what I had observed:
/g/TekScopes/photo/262059/0?p=Created,,,20,2,0,0
Setup: Inputs were set to 50-ohm (used BNC-T plus termination for CH3). No signal input, free running.
Raymond's question reminded me my earlier message:
Are you sure B sweep shaft is not rotated out of alignment? You can look at J1 (relay signal) and check it switches at .1us/.2us boundary.
I read your reply about timebases being locked. You may already know that but you can pull the time/div knob and rotate clockwise to speed up B timebase compared to A timebase. In that position B is not locked to A, and you can check if B can be rotated past the 1ns/div setting. There is a lock mechanism that won't allow B slower than A, so don't force in that direction.
I will look at the video again an reply later, I wanted to catch you while it is still earlier in the evening.
Ozan
