[Thursday] Engaging evening task

[Chris Prata]

Thanks Dave, very interesting, helpful, but also a little complicated for me and maybe someone new to a radio we hoping they wont destroy.  My lesser experience level may be a benefit to our visitors for this event, because I have to keep this simple. How to turn on an unknown tube receiver the first time, safely.

I do know a "dim bulb" is cheap and easy to construct, works, and also acts as a current limiter to protect the unit from damage (using the correct wattage light bulb).

As to a capacitor failing to shorted, are you saying a DMM would not show that condition?  (Thats why I mentioned what instrument I was using).  You've got a VOM - Volts Ohm Milliammeter - onto my list to add!

Anyway this event I'm hoping will introduce some considerations for the new person to old radios who might be interested to enter the hobby because they came into an old radio, for example. Thats how I got into it, 20 years ago, bought a Hallicrafters SX71 and the case was electrified from a leaky filter capacitor.

See you saturday... (and tomorrow)

Oh, do we have a VOM at the Museum? Or can you bring one? I can bring these old capacitors to test that way if so...

Chris



________________________________
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



On Tuesday, June 4, 2019, 7:24:31 PM EDT, Chris Prata via Thursday <thursday at newsm.org> wrote:


Hi all

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).

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.

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.

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.

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)

Regards,
 Chris








-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://host194.hostmonster.com/pipermail/thursday_newsm.org/attachments/20190606/b856ef79/attachment-0001.html>