Someone experienced repairing scopes???

I`m repairing an old Hung Chang 5502 scope, it had some problems, arcs, just the half of the screen was displayed, focus and intesity don`t work, etc. I have repaired everything but I can`t make work the focus and intensity. I don`t understand how this part work, the intensity voltage comes from the left side of the schematic but this is coupled with caps so how the DC can pass through the caps to change the grid voltage??? ???

how many volts should vary the grid 4 and 5??
There is something that commonly fails in these circuits?
what test can I do to narrow the problem?


here is the full image
http://www.twin-x.com/groupdiy/albums/userpics/File0019.JPG

any Help is welcome, I have had this oscilloscope for months on my desk :-\

Rafael

Do the 2 neon bulbs in series light up?

no, the neon bulbs don`t light up, they used to be on but I have already repaired that problem, a resistor open. 

It somehow seems like one of those voltage multipliers has a bad diode or cap.
More of a guess, but hopefully a good one.

I `ve re checked the diodes and they are ok, I`m not sure what is the best way to test the caps. 

I´m very confused  voltage multipliers work with AC, there is ac from the HV coil tap, also from the shop blank signal but the focus and intensity controls are DC. I can´t see how it provide the offset voltage through the caps.  ??? ???

all what I need is make more negative the first grid :-[