2015 June 15    Electronics  How-To


Repairing vintage radios requires a level of skill to ensure that you don't get fried.  If you don't have any electrical knowledge whatsoever, it's better to buy a pre-serviced radio than to buy a "project".   (When shopping the 'bay for an old radio, buy from someone who knows what they're doing.)

The old transformerless AC-DC radios can have a live chassis if you plug the cord in one way, but not if you plug it in the other way.  

Many people think transformer-based radios don't have this problem. 

I used to think that, too.   

Even if you're just a consumer of vintage electronics and you don't try to repair them, you should read this article.    It could save your life.

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Some old radios and TV's can have a "hot chassis".  Some may even have hot chassis with no insulating case. 

When you have it plugged in, a "hot chassis" is at some AC voltage with respect to ground.   More than about 30 volts AC may be lethal.  Some of them are still "hot" even when the power switch is OFF.

Here's why that's bad. 

Touching that metal chassis, while your other hand is touching anything grounded, could be fatal.   Instantly.

Often that voltage will be at the full 120 volts AC, which decreases the chance of survival.

Even in the early Sixties, they knew it wasn't really a great idea to have full line-voltage on the chassis. 

Safety Capacitors

A real "safety" capacitor does not actually make you safe from getting electrocuted when working on a radio.  All it really does is to shunt AC voltage spikes so they don't get into the electronics of a radio.  These voltage spikes can happen even on a clear day.  Because they act like higher-frequency AC, they will pass "through" a capacitor that would block 60 Hz AC.  This is why they can often be found across the line-voltage wires of electronic devices. 

Basically, they're open-circuit to 60 Hz AC, but they're short-circuit to some types of power spikes.

Now, in many old electronic devices there are capacitors that are supposed to block DC and 60 Hz AC, sort of like safety capacitors do.  These capacitors may have deteriorated, and now there could be mains voltage where it shouldn't be.  It's normal to read 120 volts or so from line-to-ground;  again, it's a problem when you read that sort of voltage from "chassis ground" to earth ground.

Any old piece of electronics could have bad capacitors, and even disc caps go bad sometimes (especially if they took a power spike that was hot enough to bubble the caps on the outside.)

A bad capacitor can definitely put voltages where they shouldn't be.  However, many devices in the 60's and 70's were designed to have a "hot chassis", possibly because it was designed to be inside an insulated outer casing. 

But what happens if that's made of particle board or something... and it falls apart?

1960's and 70's Radios

AC-DC radios were definitely being made in the early Sixties.  I believe some of them had conductive outer casings, too!

Into the Seventies, there were still quite a few hot-chassis radios being made.  Some of you guys may not accept this idea.  (These radios do not get as much glamor as the tube radios, so I think this aspect of early electronics is largely ignored.)

Most electronics people know about "transformerless" radio designs.   The AC line voltage will go right into a rectifier, and from there it directly powers the radio.  It's easy to see how polarity could matter a lot here.

On a hot-chassis radio, the hot wire should be interruptible by the radio's power switch.  If it goes the other way around (power switch interrupts the neutral wire), the chassis could be "hot" even when the power switch is "Off".   That's really bad.

Transformer radios are supposed to be different.  Transformer primaries don't care about polarity.  Whichever way the hot and neutral are oriented, the secondary will still produce the voltage it's designed to make.  Only when you have two or more transformers would it make a difference, because of phase shifting.

So, the common thinking is that if you have a transformer in there, the radio will have a cold chassis.

Is that always true?


Let's see what we've got here.

Safety Check

First thing with any old radio is to make sure there are no electrical shorts that could cause a problem.  After that, one thing I always do with an old radio is to check the voltages everywhere that would likely present a danger.  

If I'm certain there are no dead-short conditions, there's always a stage when I cautiously power it up-- no isolation-- and test chassis-to-neutral and chassis-to-mains-ground

Here's why.

I had this transistor stereo with a nice transformer, an internal fuse, and everything else to make me think it would be OK.   This should not have been a hot-chassis radio, I reckoned.  But something told me to check anyway.  It was just something about the looks of the design;  I couldn't quite place it.

Let's be clear:  I did not open the case to "chase after a hot chassis". 

No, let's be even more clear:  you don't magically make a hot chassis condition morph into existence by "chasing after it" (unless you do something dumb and actually cause it to happen!).  If there is voltage where it shouldn't be, then you're better off seeing that on a multimeter so you can fix it.

So anyway, I had to open the case to replace some bad caps. 

There's a two-prong plug connector in the back.  As usual, that gives no way to ensure polarity.    As I said, though, we're dealing with a transformer radio, so the polarity shouldn't even be a concern.

But what if it was?

With the plug in the one way, everything was good.

With the plug in the other way... whoah, wait a minute here.  The chassis had a couple volts AC with respect to neutral... but 120 volts with respect to ground.    (Which just goes to show... don't assume "neutral" and "ground" are the same... especially in an old radio or something.)

If you touch that metal chassis while touching anything else that has any connection whatsoever to ground, you could die. 

What about your rubber-soled shoes on that carpeted floor?   That insulation won't matter at all, because that's not where the circuit gets completed!

Someone is going to mention (correctly) that I didn't use an isolation transformer.  We'll get to that.  Meantime, realize that you wouldn't have detected this chassis voltage if you'd been using a radio isolation transformer

On this particular radio, I found that the upper prong in the photo should always be the neutral.
Hot wire should go to the bottom prong, as shown in photo.
(When the radio is assembled, what you see here will be upside-down.)

Disc caps do not often go bad, so I opted not to replace these (for the time being).
Now if they were bubbled, that would be a different story.

Get An Isolation Transformer Anyway

If this had been a Hallicrafters S-120 or something like that, I'd have expected to use isolation.  Pretty much everyone who fixes old radios knows that's a hot-chassis radio. 

Generally, don't work on radios or TV's without an isolation transformer.  (And please read this.)

Either that, or correctly install a polarized cord before doing anything else.  You might also need to change the switching and fusing so that everything is on the "hot" wire, instead of the "neutral".  Nothing should interrupt the neutral connection.

So, I had a transistor radio that maybe I would not expect to be "hot-chassis" design.  It had a transformer, and it looked like a modern design.

Earlier I said that an isolation transformer would have prevented me from finding that it was in fact a hot chassis.  That's because the hot and neutral would have no ground reference when isolated.  On a proper "radio repair" isolation transformer, the hot and neutral reference each other.  There is no reference to earth ground or mains ground, so there's no potential difference to ground.   (Just be aware that you can still get shocked when different areas of the radio are at different voltages.)
That lack of ground reference is a good thing here, because it can save your life in certain situations when working on a hot-chassis radio.  It's just that you wouldn't be able to detect a hot chassis in the first place, unless you powered up the radio without the isolation transformer.

So, why get an isolation transformer?  Read that article.

I would still test every radio without the isolation transformer (keeping one hand in a pocket), just as I did here.  If there's line voltage on the chassis with respect to ground, that radio is going to get re-wired with a polarized plug as soon as possible.  And I'm going to make sure it's installed correctly so that the chassis will not be hot.

After the initial testing phase, you should use an isolation transformer.  Just get one.   If you're into vintage electronics, you need one of these right now.  

And actually, if you don't repair electronics, you can still benefit from one.  It filters out a lot of the noise that pollutes AC power lines.  So, it will enhance your enjoyment of a radio.

The Ground Wire

Most isolation transformers you'd buy new are designed to filter out RFI noise.  That's useful, but it won't save your life if you complete a circuit to some grounded object in the room.  (Which you should be careful not to do anyway.)

They have the secondary neutral tied to the ground... which is tied to the primary ground.  Which means that if you touch anything else that's grounded while touching a hot wire, current will flow through you.


There's a very easy way to check.  Again see this article.

For electronics repair, get an isolation transformer that has the output ground isolated from the input ground.  This one indicates the ground is not isolated.  There's also this one from Tenma or this one from B&K, but again it appears the grounds are not isolated (modding this to correct that "feature" would be at your own risk, obviously.)   However, if you want to be certain you're getting the right type of isolation transformer, just get a vintage one that was made for radio and TV repair.  Any of the new ones could have design changes, and they might not even list the critical specs.

Couple things here.  One, it would be nice if every stereo had a 12-volt DC power input.  It works great.   No line noise with a battery!

Two, notice the AC power draw is pretty low.  2 to 10 watts at 120V AC. 



Just because a radio or TV has a power transformer, that doesn't mean it's a cold chassis design.  Be careful.  

If you can read 120V between chassis and GND, and that voltage is zero when you reverse the polarity.... and the radio plays fine, either way.... then it probably is this simple.  It's a hot chassis radio. 

I will say-- one more time-- be careful with capacitors that appear to be "safety capacitors".  They won't keep you from getting fried if you do something dumb.  Capacitors like these are used to separate line current from RF noise.  Even when they work properly, there's still 120V AC, at 60Hz, on those lines.  If you short that (by absent-mindedly brushing against something), you could get electrocuted. 

Most live-chassis transistor radios have the metal chassis inside a non-conductive case, so you're (probably) not going to have a problem just by using it.  It's just that I think it's a lot safer not to have the chassis hot in the first place.  Let's say the outer case got broken, chipped, or peeled away.  Or glue joints dried out, or something.  I think this might be why they eventually went to radio designs where the chassis was bonded to ground.

If you're buying an unrestored radio, at least know what you're buying.   (Please shop for your radios through this link and it helps me keep this site going. Thanks!)

If someone serviced the radio, make sure they knew enough to install a polarized plug the correct way.  

If you have an old radio but you're not experienced enough to repair it yourself, take it to someone who is.    The worker is worth the wage.

That concludes this article.  The knowledge may save your life someday.   If you found this page useful, informative, or entertaining, please help me out by purchasing your stuff through these links.   Your help is greatly appreciated;  it allows me to keep this website on-line and adding more articles to it.

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