CS-4125A Problem

[1843]

Hi all,
Does anyone have a schematic for CS-4125A scope?
My scope has a problem which when I set the time/div to 200nS or less,the CRT brightness drops and whenever I set it less than 200nS further CRT becomes darker until it entirely is dark and I can't see anything...
Thanks for any help.

[1843]

oops!
200uS is right, I have said 200nS.
Also when I use its x10 , the problem occurs again.