Tek 2235 blob on screen - Normal behavior?

[RetroGameModz]

Hello all electronic enthusiasts.

This is not really much of a repair question, since I do suspect that this is a common behavior on some analog oscilloscopes rather than a fault condition. I'm not going to do much about it anyway since it doesn't bother me that much.
However, what I'm wondering here is what might be causing this kind of behavior. I'm asking this purely out of curiosity, and I'm pretty sure that there must be someone on this forum who has a pretty good idea of what's going on.

It's regarding my analog Tektronix 2235 oscilloscope. There is a big blob on screen following the movement of the actual electron beam. The electron beam drawing the trace and the blob are moving in proportion to each other.
I made a short 2 minute video about it to easier demonstrate how this looks like.



Maybe you could test this for me on your analog scopes and tell me if they are doing the same thing? That would be very much appreciated.

Thanks in advance.

[c4757p]

Normal. "Reflections" of the electron beam inside the tube. Turn your intensity down, it's probably too high.

[AF6LJ]

I have seen that and have always believed it was reflectd light off the glass on to the phosphor. There would be a reflection caused by the transition from the glass to the air, that would generate a reflection that could be seen on the inner surface of the CRT front glass which would illuminate the the phosphor.

My Buck-Fifty....  

[Gertjan]

I checked on my Tek 2235, and on my Hameg HM1004-2. Both have the same behaviour.
So I guess it is normal....

But I only see the blob when the intensity is set very high. Under those circumstances (like in your video!) I start to worry about the Phosphor layer in the CRT....   
So take care with your experiments!

Regards, Gertjan.