I assumed that the voltage multiplier is broken, so I decided to look what's inside the black box. Getting to the pcb was not very easy, after I removed the outer plastic layer with a simple wood saw, a very bad smelling block of nasty synthetic material appeared.
I got it off the pcb by using a screwdriver and pliers. I accidantally damaged some caps by doing it so I wasn't able to determine what components were causing the voltage multiplier not to work correctly.
Then I desoldered all parts and cleaned the PCB in the sink with warm water. After that I had to decide what components I should use to substitute the old parts. The diodes were not labeled at all and the caps had an uncommon labeling.
It was "W470 4kV". I think the "4kV" stand for 4 kilo Volt maximum voltage. But I was not sure if the
470 stands for
470pF or
47*10^
0pF = 47pF. I decided to use my new Fluke 17B DMM but I was not able to measure the capacitance. Every time I added another of these caps in parallel to my test leads, the difference between the capacitance was not constant but dropping.
However, I bought 470pF 4kV caps and GP0240 diodes, which are rated for 4kV and 250mA:
By the way, here is the schematic I figured out for the multiplier:
Then I soldered all components on the board. It wasnt so easy for me because I had to keep the old solder in the holes liquid so the legs of the new components could slide through. I kept the little ceramic rings at the legs from the old caps and used them again. Maybe they will prevent arcing. I reused the old resistor because it wasn't damaged.
After I finished the soldering I hooked it up to my scope and secured it with a peg so the board was floating in the air and could not create any shorts.
Much to my astonishment a trace appeared, which made me very happy.
Next I wanted to put the pcb back into a secure casing. I decided to use some old silicone rubber I found in our basement. I made a little cardboard box with the backside of the old case attached to it which had the clips on it so it can be connected to the body of the scope.
Unfortunately I used to much binder when I mixed the silicone rubber, which caused the material to get stiff very fast. I had just enough time to fill the box and stuff the PCB in it. The result was not perfect so I used tape to seal everything up.
Then I read the first chapter of the calibration in the service manual and adjusted the pots so the traces were equally bright and focused. I did not continue with the next chapter because I am not sure if I can adjust it without any other equipment than a second scope and a multimeter. Is it nevertheless possible for me to calibrate the scope? Could it be done with the calibration voltage of my second scope?