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Some electrical safety for our newcomers

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Some electrical safety for our newcomers Empty Some electrical safety for our newcomers

Post by tpaairman on Fri Jan 02, 2009 12:41 am

I know that there are often questions from new hobbyists regarding safety, so I thought I put up a post with a couple of the basics. Also I know this post is a bit lengthy, but if you are new to this, please take a moment to review what I have posted. You might be quite glad you did later.

The first basic rule is to simply pay attention and know what you are dealing with. Always pay attention. Seems basic enough, but even I didn't follow that rule a couple months back. I had went to unsolder a part connected directly to the power cord of a piece of test equipment. I had just plugged it in to check it out, knew I needed to change this part, but I got distracted for a moment and had to step away from the bench. I came back and proceeded to put the soldering iron on the connection, and got a big bang and sparks. I had forgot to unplug it first.

When I say keep in mind what you are dealing with, that is it is very common for these old sets to have well over the 120 volts from the line running through them. You can expect to have over 300.

Pay attention to electrolytic capacitors. Any capacitor can be dangerous when charged. They can shock you. But the other part with electrolytic is they are polarized, that means that positive MUST go to positive, and negative to negative. If you reverse it, refer back to the big bang. (But no sparks)

The last thing I will point out (for now) is Variacs and isolation transformers. A Variac is simply an adjustable transformer that changes the voltage coming out. You can start on 0 volts and adjust up to line voltage (or more on some models). You plug it in the wall, and plug the radio into the the outlet on the side of the Variac. Whenever you have a radio that you have never run before, ALWAYS inspect if first for any broken wires or insulation, then if that all looks OK, plug it into the Variac, set it on zero volts, turn it on, and slowly bring up the voltage. If your set starts to show signs of smoke, or loud improper noises, cut of the voltage and proceed to repairs.

That leads us to the isolation transformer. A transformer is simply two wire coils back to back. Two electromagnets. Normally they increase or reduce the voltage, but in this case the voltage coming out is the same as what goes in. However because they are two separate coils, there is no physical electrical connection between the line outlet and what goes into the set. So how is this a safety item? Well, think of this. Let's say you plug the set straight into the wall outlet. As you are making adjustments to the set, you accidentally touch a hot wire inside. If your other hand happens to be touching a ground, you will now have a complete circuit across your heart. This includes if you are touching a piece of test equipment of the like. It's a lot easier to do than you might think. With an isolation transformer, wall ground to radio guts are not a complete circuit.

If anyone else wants to add some safety notes, this might be a great place.
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Post by mikehall on Tue Jan 01, 2013 3:03 pm

Question regarding the use of the isolation transformer?---when using test equipment like a VTVM that uses an AC power input--should the VTVM (for example) or other item that will come into contact with the radio be connected through the isolation transformer as well as the radio?

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Post by Resistance is Futile on Wed Jan 02, 2013 3:23 am

No just the radio.

Tektronix advises against using an Isolation transformer on Oscopes at least. If the other test equipment has an internal transformer then that test equipment is probably safe. There are exceptions though from my reading, to cut corners some transformers have no secondary and the primary is tapped for the output. A very dangerous situation.

You have to remember that some probes are chassis grounded, and if you clip the ground to the hot side of the radio you will suffer sparks and worse.

Also some transformers have a safety earth ground (3 wires) and if it has an output 3 wires also you had better measure from neutral to ground on both ends to make sure they are not connected together. Make sure that the isolation transformer isn't plugged into the wall socket when making this resistance check.
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Post by Guest on Wed Jan 02, 2013 11:15 pm

I am a strong believer in the one hand rule. During the portion of the work when it is possible that the chassis is powered up, it is a safe practice to keep one hand at least off the equipment if not off the bench. I also put the knobs back on the controls to perform tests or alignments. And I also use at least insulated if not nonmetallic alignment tools. That is not to discount the value of an isolation transformer or other safety equipment.

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Post by mikehall on Wed Jan 02, 2013 11:41 pm

19&41--that brings up another question I have been wanting to ask--sorry if I am causing thread drift. Could someone do a post or thread on what they use for alignment tools, and where to obtain them? do you roll your own? or buy them? have seen a lot of things about using alignment tools and googled them, but have wondered if the new ones for sale are appropriate sizes etc. for these vintage/antique radios

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Post by Guest on Thu Jan 03, 2013 12:15 am

Here is one source that carries a decent variety...

http://www.stanleysupplyservices.com/search.aspx?q=alignment&go=

And if they break, I keep them and form the remainder into another tool that might suit my needs later. You will see what holds up the best for your needs.

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Post by mikehall on Thu Jan 03, 2013 8:14 pm

Thanks I'll check it out.

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Post by samd on Thu Jan 03, 2013 10:02 pm

19&41 wrote:… I also put the knobs back on the controls to perform tests or alignments..

I'll second this…

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Post by Steve Johnson on Fri Jan 04, 2013 10:58 am

To protect the variac and isolation transformer from shorts in the equipment plugged into them I also added a 100 watt clear glass incandescent bulb in series with the AC output of the variac. If there's a short, the bulb lights at full brightness and protects the isolation transformer and variac. A 100 watt incandescent bulb suddenly going to fully lit means you will also notice the change.
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Post by JasonAMFM on Wed Aug 20, 2014 1:30 am

As far as using a variac or dim bulb tester, are they as necessary when powering up a transistor for the first time as they are when turning on an old tube radio. In other words, can a short kill a transistor as bad as it can a tube radio?
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Post by Bill Cahill on Wed Aug 20, 2014 10:45 am

That depends, and, some transistor sets don't like low voltage, and, can damage circuits.
The biggest worry is to replace especially higher voltage electrolytics first. That would be those in the power supply.

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Post by JasonAMFM on Wed Aug 20, 2014 9:01 pm

Ah, so would a rule (meant to be broken, of course) be: Old tubes: always use a variac/dim bulb the first time. Transistors: cross your fingers and turn the knob? 'laughs'
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Some electrical safety for our newcomers Empty Isolation transformer question

Post by Greybeard on Wed Oct 28, 2020 9:56 pm

Can someone show me a simple means to make my isolation transformer. I unloaded all my bench gear several years ago due to a stroke. I have recently been given three tube radios, and have an urge to tinker a bit. Though according to my doctors, I have completely recovered, I occasionally am kind of foggy, I seem to have lost some degree of memory. I am currently looking over schematics, but haven't seen anything suggesting the safe, proper way of performing this simple task. Any help will be appreciated.

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Some electrical safety for our newcomers Empty Follow up to my question

Post by Greybeard on Wed Oct 28, 2020 9:58 pm

Converting an isolation transformer to a variac.

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