Hameg HM103 unblanking, semis OK, 68 pF caps?

[jonpaul]

Bonjour a tous:

For the Hameg HM103   scope  with dead unblanking circuit continued saga from September post asking for parts....

I have replaced the optocoupler,  NPN, PNP BF199  BF440, and checked all resistors and trimpot, all are fine. 

Blanking logic input pulse is fine, so quite a puzzle.

Last possibility: leaky coupling caps 68pF/2kV, I have seen many posts referring to their failure.

I have no way to test  caps here. I have only a variac, VOM and soldering iron!

I ask the Hameg veterans to please  inform me:

Could a failed cap just kill blanking, but scope works fine and traces is good otherwise?

How to diagnose if a cap has failed?

Will a failed 68 pF/2kV kill the semiconductors?

where else to look for the problem?

Your assistance is greatly appreciated!

Jon