Dim oscilloscope screen

[yugu]

(I posted this post in the test gear forum https://www.eevblog.com/forum/testgear/dim-oscilloscope-screen/ before I realize this may be a better place.)

Hi guys. I bought a $50 analog oscilloscope from ebay (I'm a fan of Dave  ). It's Instek GOS-635G. It works fine except that the screen is quite dim. When I turn the intensity knob up from the min position, the screen doesn't show anything before 70% position. From 70% to 90% position, the brightness keeps going up. And then it starts to drop if I continue to turn it up. So the maximum brightness happens when the knob is at 90%. But it's still very dim. I can barely see the trace.

Does anybody have any clue about what's wrong and how to fix it? I am wondering maybe there's something wrong with the potentiometer (it feels smooth though). But would like to hear what you guys have to say before I open it up 

Thanks!

[lowimpedance]

Follow the replies  in the other thread, and as you suspect check the intensity pot. Perhaps post a photo of it so we could see if its a special or just a generic style pot. (ie sealed , open , high voltage rated etc)

[oldway]

Follow the replies in the other thread too, and it seems to be a faulty crt. (low cathode emission)
If HV stabilisation is faulty, pattern expand with higher brightness ajust.   
Low cathode emission was a common fault in crt TV sets and HP digital scopes with crt, but I have never seen any analog scope with this fault.