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<div><div><div>Hi Chris,</div><div><br></div><div>The best way to check an
electrolytic capacitor is to use a VOM, not a DMM. The VOM will measure
the leakage of the cap and the one with the lowest ohm reading is what
you want. <br></div><div><br></div><div>First you need to find out which
VOM lead is negative. With the old VOM I use (NRI), the Red lead is
actually the negative lead when measuring resistance. This is from the
VOM's internal battery. I have heard this is not unusual. Use your DMM
to check the polarity.</div><div><br></div><div>Next, set the VOM on
it's highest resistance setting. Connect the VOM to the cap - negative
to negative. The VOM will jump up to a low resistance as the cap charges and then
slowly increase to a few megohms if the cap is good. Finding an
electrolytic with a dead short is pretty rare. <br></div><div><br></div>Instead
of putting a wire across a cap to simulate a short, you might want to
use a low value resistor. If the radio is plugged into a plain power
strip, you can use the power strips switch to turn the radio off quickly
if needed. I wouldn't use a line suppression power strip.</div><div><br></div><div>In the 60 years or so that I have been working on old radios, I've never used a light bulb as a line draw tester. For several years now, I have used a variac with an ammeter. If there is a vacuum tube rectifier, a solid state substitute needs to be used at low line voltages. I will be at the Museum this Saturday.</div><div><br></div><div>73,</div><div>Dave, KA1EDP</div><div><br></div><div><br></div></div><div><br></div>
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On Tuesday, June 4, 2019, 7:24:31 PM EDT, Chris Prata via Thursday <thursday@newsm.org> wrote:
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<div>Hi all </div>
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<div>In the box of old removed capacitors that my friend was kind enough to mail me, there is none which is a dead short and I checked them all (with a dmm).</div>
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<div>One of the main things that I would like to demonstrate is excessive draw from shorted filter capacitors, which can destroy a hard-to-replace power supply Transformer and who knows what else if the unit is plugged in and powered up in that condition.</div>
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<div>The museum seems to possess a countless array of 1930s through 1950s vacuum tube receivers, and I'm wondering if it's possible that someone could work with me to run the test we are going to demonstrate on enough of them to possibly turn up
one which has excessive draw and can be used during the engaging evening demonstration.</div>
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<div>I wouldn't describe this as a critical need as the point can be made with they receiver that I am bringing which will pass the test, and maybe it's just not worth it but then again maybe once we start to check a few it could be kind of fun and
fun and serve the purpose.</div>
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<div>So I'm just throwing this out there in case somebody is into trying to make this happen but I think it would be pretty okay if we don't as well. (and we might even be able to kind of fake it by bridging a component with a jumper inside the radio
but the purist in me would really hate to do it that way if we can do it the real way)</div>
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<div>Regards,</div>
<div> Chris </div>
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