Morris, Hi. The 200Meg and the oil filled HV caps are the usual failure points. The case does have to be connected to the - side as you found out! (same on the -2500V supply) The HV can be up to 4000VDC without harming the tube. To check and see how accurate the divided sample pin is you need to measure the output with a HV divider probe at the same time. The 3500V supply is not so important the -2500 you would like to know more precisely. Cheers, Corby