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This morning I began going thru and testing the paper caps. I thought I should test their behavior under DC voltage.
I first examined the 0.01ufd 400v coupling capacitor between the first audio and output stage. I used my old IM18 Heathkit VTVM to initially measure the dielectric resistance. It started slow and climbed to about 40Mohm. A new polypropylene cap would typically climb from near zero to infinity almost instantaneously.
I next applied a 350v DC and monitored current. Interestingly, the initial steady state current was about 1.5mA and slowly began climbing. The current climb began to accelerate and within one minute reached 15mA at which point I cut the voltage. Bear in mind that the 40uF caps reformed and descended to a current of 0.5mA for 24 hours under test.
A similar result with the 0.05uF 400v first to second stage coupling cap. The dielectric resistance was close to 80Mohm yet with 350 volts applied, the leakage current rose from 0.5mA to 4mA within one minute and was continuing to climb when I cut the voltage. I then applied only 20 volts to the third 0.05uF 400v coupling capacitor between the second detector and first video amplifier stage. It generally runs with only about 10volts DC across it. The leakage was under 100uA and did not change. So perhaps not under stress, it can remain.
The paper dielectric capacitors appear to be a serious threat if run with a high potential difference across them. It surprised me that a 0.01uF had considerably more leakage than 40uF electrolytic. Anyhow I shall proceed to change a slew of compromised paper capacitors today.
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