[Amps] measuring cap value

[Jim Hoge]

Hi gang,

In the course of trying to diagnose my Henry 2K floor model amplifier issues, I have tried to measure the doorknob caps with an MFJ 269. I'm sure this is far from ideal but it's the only device I have that can measure the caps. The two parallel plate blocking caps are high on the list of suspected failure. According to the MFJ 269, the caps are good but when sweeping the frequency upward, the reactance goes out of range. I would think the reactance would remain within the metering range of the analyzer for the frequency range of the amp. The cap value is relatively good up to about 12 mHz before it is out of range. BTW, each cap was removed and tested with the analyzer by leads of less than 2 inches. The two plate blocking caps as well as the 5 switched caps that follow them measured nowhere within the +/- 10% of the rated values. None indicated a short with the analyzer or an ohmmeter.

Some background on the amp: In bypass mode, things are fine. In operate mode, you can see a small dip in plate voltage and you can hear the amp go under load. With swr/ power meters both between the transmitter and the amp and between the amp and antenna, swr is normal but power output is about a sixth of power input .The tubes have tested good in another amp. The relays have been shown to be good. That is what brings me to the plate blocking caps. The Henry employs a pair of HEC doorknob caps at 7.5kv and 1000 pf wired in parallel.

In summary, will the MFJ suffice for ballpark measurements or will I miss issues associated with the caps? Based on what I have measured, are the 2 plate blocking caps still suspect?

tnx es 73,
Jim W5QM