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.   The only thing keeping the chassis from being at line voltage, usually, is a fifty- or sixty-year-old capacitor (which can go bad).  This is very, very dangerous if not tended to.

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.

A Quick Note

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In This Article

Easy Stuff vs. Tech Stuff

Hot Chassis Blues

1960's and 70's Radios

Safety Check

Get An Isolation Transformer Anyway

The Ground Wire


Easy Stuff vs. Tech Stuff

Don't get the wrong idea;  there were millions of hot-chassis radios made, and people still enjoy them every day.

Under normal operating conditions, most of these radios have the chassis inside an insulating case.  So, no worries there.

Fixing the radios, though, requires special precautions.

There is a lot of stuff on the Internet where a person could teach himself electricity and electronics;  problem is, it's easy to have dangerous gaps in one's knowledge.   A lot of the foundation for this can be learned effectively only in a physics or electronics course.  Then, study and read after you have that foundation.  A non-science person can learn this stuff, but be sure to get the foundations:  volts, amperes, ohms, capacitance, etc.

I'm not even saying you have to study university-level physics.  Even a good trade school will teach the basics. 

Everything else-- all the fancy stuff, waveforms, signal traces, and all that-- is no good without a basic understanding of why current flows, where it flows, and how.

Radio repair is quite safe, but ONLY if you know your game and stay on top of the safety.  Done right, it's actually more safe than driving a car, because when working with high voltages, you are constantly being very careful.  Driving a car, people think they're safe when they're not. 

That said, most of this would qualify as "don't try this at home" if you're someone without the experience.

Some readers are electronics engineers and long-time radio hobbyists;  for everyone else, consider purchasing electronics that are already 100% safe and tested. 

(Soon I'll be making an article about "buying vintage electronics".)

Hot Chassis Blues

So you've got this radio or old TV.  Before we proceed, a bit of caution is in order.

Even if you don't ever open the chassis, be careful.  You might already know that some old radios and TV's can have a "hot chassis".  I seem to recall seeing or reading about ones with hot chassis and no insulating case. 

Basically, when you have it plugged in, a "hot chassis" is at some AC voltage with respect to ground.   (Some of them are still "hot" even when the power switch is OFF.  Read that again.)

Here's why that's bad. 

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

Sometimes that voltage will be at full line potential (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.  Now, I want to clarify something.  In this article I had used the term "safety capacitors".  The old capacitors were not really "safety capacitors", and you shouldn't think they will keep you from getting electrocuted.  But that's true of modern safety capacitors, too.  (The modern designation has to do with their rating.)  Their purpose is to shunt high-frequency noise to the chassis, which (you expect) has continuity with ground.  Thus the noise wouldn't affect the radio.  Meanwhile, 60Hz current would remain in the circuit where you want it, because it can't pass through the shunt capacitor appreciably.

Now, a source of confusion:  many old radios have capacitors which, when they go bad, can allow 60Hz line current to go where it should not.  And it is sort of a habit for some people to call these "safety capacitors", even though they're really not. 

I think if we look at the design of this radio, we can see it was supposed to have the metal chassis insulated all around by the wooden case.  They knew this was a hot chassis radio.  Many of the tube radio guys do not care for stuff from this era.  The transistor radios, to them, are not even worth working on.  I think tube radio repair info has been selectively preserved.  A lot of early transistor radio knowledge just sort of evaporated.  You can more easily get schematics for a lot of old tube radios than for some of these transistor units.

Here's a key point.  Many radio designs have what look like safety capacitors.  And you will see that they have a transformer.  You will think it's "modern" and therefore the chassis must be at zero volts.  You might be wrong.  These radios may have been designed to have the chassis at full line-voltage.  You would need a schematic to figure out the capacitors' actual purpose.  Even radio repair guys do not always have a schematic for every radio they work on.   Some schematics are very difficult to get, if they even exist anymore. 

The point is:  never rely on something that looks like a "safety capacitor".  Because it might not even be one. And even if it is, it will not keep you from getting fried if you do something dumb.

Just another note here.  You might not agree that this was a hot chassis radio.  Well, here's my view on this.  If the chassis is at 120V relative to ground, that's a problem.  You could call that a floating-ground radio, but that's another way of saying the chassis is at some voltage relative to GND (i.e., it's hot).  Don't get confused between "hot to chassis", "hot to ground", and "chassis to ground" measurements.  The first two should be about 120 volts, normally.  If the third one is about 120 volts, then either you've got a dead short condition, or the radio was designed to have the chassis at line voltage.

If not, then why design it so the metal chassis is behind a wooden casing?

1960's and 70's Radios

AC-DC radios were definitely being made in the early Sixties.  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 that these were "transformerless" designs.   The AC line voltage will go right into a rectifier, and from there, it powers the radio.  It's easy to see how polarity could matter a lot here.

The big thing with known "hot chassis" radios is that the hot wire should go to the power switch.  If it goes the other way around (neutral to the power switch), 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 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.   As in, pretty much everywhere on and in the radio. 

There is 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.  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 is voltage where it shouldn't be, then it's probably better to know so that it might be fixed. 

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

So, 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 that shouldn't be a big deal.

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.  And... you may 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 I would not have initially guessed to be a "hot-chassis" design.  It had a transformer, and it looked like a modern design. (Actually, many radios from this era are indeed hot-chassis.)

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.

If your multimeter can detect a substantial voltage with respect to ground... that means danger, if you touch something grounded.

I would still test every radio without the isolation transformer (keeping one hand in a pocket), just as I did here.  The reason is that if I can detect a hot chassis, that radio is going to get a polarized plug.  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.  I've moved the description to this article.

So anyway, get an isolation transformer that has the output ground isolated from the input ground.  This one might be OK.  You might consider this one from Tenma or this one from B&K.   However, if you want to be certain you're getting the right type of isolation transformer, get 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... this would be plenty for isolation.



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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