I mentioned earlier that I was reforming the smoothing caps in this set, and though I still have some work to do on the set, so won't know how the caps will perform in use, using my home-brew reformer they've reformed nicely after about 4 hours each. I started them off at 25% their rated working voltage for an hour or so, check if the leakage has drop to below 1mA, and if all's well, go to 50%, 75% than the full working voltage, in this case, 300 Volts. As can be seen from the attached pic, they're down to below 500uA at 300 Volts, which is comparable to a new cap.
I've so far removed seven of the nine-tar encased paper caps, and am waiting the last two to be delivered. I thought that out of curiousity, I'd test the old ones at their rated value to check their leakage. They can't be reformed of course, but my reformer has a series neon which glows brighter when the leakage is higher, and shows the current passed at the test voltage. I've attached a pic of an old cap under test, which was rated at 800V and was the isolation cap for the aerial and earth on this AC/DC live chassis set, so was a safety component. My reformer/tester only goes up to 500V, but in any event, as soon as it got to 150 V the old cap started passing several mA, and the neon was well alight, so I didn't think it safe to subject it to any higher voltage. Had I jacked it up higher, it would have just passed more and more current, and hence, got quite hot.
I've also attached a pic of a new polyester cap under test at its rated voltage - 300V, and as can be seen, the leakage was minimal - about 400uA. The point I'm trying to make, perhaps in a long-winded way, is that you can't simply apply a multimeter on a high Ohms range to a paper cap, and assume that is a cap has such a high resisitance that it's off the scale, that it therefore has a low leakage and can be left in situ. Sticking a 1.5V battery in a multimeter across a paper can is not at all the same as subjecting it to its rated voltage that it would have to tolarate for hours on end in a working set. If paper caps are leaky, they cause all sorts of problems down the line, from sending vales to an early grave, and output transformers with them. Some, such as isolation caps on the aerial and earth, and filter caps across the mains, as on this set have a safety role, and problems that can arise with 'that cap' are well known and often stated.
Hence, if you see a paper cap, yank it out and replace it!
David
I've so far removed seven of the nine-tar encased paper caps, and am waiting the last two to be delivered. I thought that out of curiousity, I'd test the old ones at their rated value to check their leakage. They can't be reformed of course, but my reformer has a series neon which glows brighter when the leakage is higher, and shows the current passed at the test voltage. I've attached a pic of an old cap under test, which was rated at 800V and was the isolation cap for the aerial and earth on this AC/DC live chassis set, so was a safety component. My reformer/tester only goes up to 500V, but in any event, as soon as it got to 150 V the old cap started passing several mA, and the neon was well alight, so I didn't think it safe to subject it to any higher voltage. Had I jacked it up higher, it would have just passed more and more current, and hence, got quite hot.
I've also attached a pic of a new polyester cap under test at its rated voltage - 300V, and as can be seen, the leakage was minimal - about 400uA. The point I'm trying to make, perhaps in a long-winded way, is that you can't simply apply a multimeter on a high Ohms range to a paper cap, and assume that is a cap has such a high resisitance that it's off the scale, that it therefore has a low leakage and can be left in situ. Sticking a 1.5V battery in a multimeter across a paper can is not at all the same as subjecting it to its rated voltage that it would have to tolarate for hours on end in a working set. If paper caps are leaky, they cause all sorts of problems down the line, from sending vales to an early grave, and output transformers with them. Some, such as isolation caps on the aerial and earth, and filter caps across the mains, as on this set have a safety role, and problems that can arise with 'that cap' are well known and often stated.
Hence, if you see a paper cap, yank it out and replace it!
David
