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And update on the 721TS running its original capacitors. It has now reached 350 hours. I am writing today because I pulled the chassis to make a minor mode e.g. reduce the value slightly of output transformer primary parallel capacitor at the plate of the 6K6. This is provide the audio with a bit of a treble boost. I find they liked muddy audio in the '40's.
I looked at all the original electrolytics and they are remaining cool. I disconnected two to measure leakage and they are well below 1mA.
I haven't spent much time on the two electrolytics I pulled from a junked 630TS chassis. I had been running these capacitors rated a 450v on a 450 volt supply. I left them 48 hours and the leakage remained about 0.2mA. I had put them aside the last few months and will continue my test on them. Ultimately I planned to tear them apart to look inside but they have held up well.
I think the key is the patient reforming. Never run even a variac to a set 70 years old until reforming the capacitors. While reforming, monitor and limit the current to no more that 10mA at the start and monitor regularly as the dielectric reforms. At the start of the reforming process on the 721TS, the voltage remained at about 10 volts at 10mA for about 10 minutes until it started to climb. In about 6 hours the current at 400 volts was about 1.5 mA and leaving it overnight brought it down to less than 0.5 mA. Testing today the couple I tested the leakage remains the same.
The key is patience. My background is electronics engineering and I have been repairing vintage electronics since the late '60s. In the last 50 years, I have done my share of electrolytic replacements. I am only pointing out that I do not immediately condemn the capacitor until I test it out. I recently restored a 1954 HP High Voltage 0-600 volt Regulated DC power supply and found four of what I would have expected to be quality capacitors bad and had to restuff them. So I insist this doesn't always work. But if good quality Sprague or Mallory electrolytics are used, it is worth giving them a chance.
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