Tek 465: a trace is blurry in some conditions

[gabriele]

Hello all,
I recently brought an analog oscilloscope, a Tetronix 465, the problem is that the trace in the channel one is blurry when the timebase is set from 2ms/div to 0.2ms/div. I suspect some contact issues because sometimes pressing on the timebase knob cause that the trace to became normal.
I never dissasembled an oscilloscope, despite my suspect, i need help to know what could be the problem and how could i can solve it. I attach some pictures of the issue, in the first two photos the trace on top is CH1.
Sorry for my english if it is not good, it is not my native language.
Thanks in advance.

EDIT
Pulling the switch to limit the bandwidth to 20MHz hide the problem completely.

[JohnPen]

First thought is to buy a switch cleaner aerosol spray.  With the scope off spray the switch contacts while cycling the switch over it's full range.  Similarly do the rest of the scope's switches while you have the case open.  Let the cleaner evaporate and then try the scope to see if the problem is fixed.

[tggzzz]

First thought is to buy a switch cleaner aerosol spray.  With the scope off spray the switch contacts while cycling the switch over it's full range.  Similarly do the rest of the scope's switches while you have the case open.  Let the cleaner evaporate and then try the scope to see if the problem is fixed.

The switches are strange and fragile. RTFM to find the approved way to clean the contacts. In most cases it involves IPA, shiny paper, and delicacy