[Thursday] Engaging evening task

[Chris Prata]

A little update, even though I was looking for an example dead short capacitor for the EE #2 night, I did not know that leakage could be seen with a VOM, nor really familiar with a VOM. So, I looked that up and determined one should be on my bench.

As luck would have it, someone was selling a lot of three Simpson 260's online for $10. And he was willing to mail them. They arrived today and the one good one with a calibration tag was actually a high accuracy 270-5! After replacing the batteries, its readings match the DMM for volts and ohms very closely of the few values I checked.

So thanks Dave for the reply below because it moved me (and my bench) forward.  🙂

For the curious:  https://simpsonelectric.com/products/test-equipment/vom-multimeters/270-5-270-5rt

________________________________

From: David Crowell <ka1edp at yahoo.com>
Sent: Wednesday, June 5, 2019 8:44 PM
To: Thursday NEWSM; Chris Prata
Subject: Re: [Thursday] Engaging evening task

Hi Chris,

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.

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.

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.

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.

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.

73,
Dave, KA1EDP


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