Important safety warning regarding the EC-1,
 the S-38 and S-38b Hallicrafters Radios
an essay by

John Fuhring

This warning includes any "hot chassis"
 radio of similar design (including many
 of the famous "All American Five" radios).
This warning does not apply to some late model AA5 radios that
 were designed with an "isolated return bus" (-B bus) so be sure
your radio actually has a "hot chassis" before trying to modify it.

You can tell if your radio has a "hot chassis" or not by
running some simple tests detailed in the text below.

     As they are wired from the factory, these radios are deadly dangerous and you shouldn't even plug them in until they have been made safe.  These radios have what is called a "hot chassis," and that makes them very dangerous to work on or even to plug in.  Do not attempt to test or repair them until you or a skilled technician have rewired them as I will describe below.  I can not emphasize too strongly how important it is that you follow my advice because over the years, many people have been killed by "hot chassis" radios and many others have received painful shocks.  The fact is, the modifications I shall describe are not only very effective, but are quite easy to do and will cost you less than $10, so there is no excuse for not doing the work.  

     Why are these radios so dangerous?  Depending on which way their original unpolarized plug is plugged into the wall socket, the chassis of these sets have a 50/50 chance of being "hot" if turned on and a 50/50 chance of being "hot" if simply turned off.  On or off, the chassis will be hot at some point and you will get a bad shock if you simply touch the insides of the radio.  To make matters really deadly, if the grommets that insulate the chassis from the outside case are in bad shape, you will get electrocuted just touching the radio.  

     How did Hallicrafters and other manufacturers of  "hot chassis" radios get UL approval to manufacture and sell such dangerous radios?  Back when these radios were new, there were no polarized wall sockets and power plugs so radio manufacturers made them safe by insulating the case from the chassis with rubber grommets or by putting the chassis in wooden/plastic cases.  All these radios had Masonite rear covers that prevented probing fingers from touching the chassis and they all had bold warning labels telling you NOT TO TOUCH THE CHASSIS OR CHANGE THE TUBES WITH THE BACK OFF AND THE SET PLUGGED IN. The warning labels said that the set was to be worked on only by a Trained Radio Technician (who knew what a "hot chassis" was).  

     We can't trust the old safety devices anymore because the insulating grommets have deteriorated with age and the Masonite backs of most of these old radios are now missing.  Even if they were still there, the warnings that were printed on the backs of these radios are equally useless because The Trained Radio Technician profession has been extinct for decades (with only a few of us old dinosaurs left).  Today we have to be our own "Trained Radio Technician" and the truth is, most of us aren't "trained" and most of us don't even know what a "hot chassis" is.  The whole idea of this essay is to give you the training you need to make your radio safe before you try to fix it or use it.

     As mentioned, some All American Five (AA5) radios are safe to use without using a polarized plug and without rewiring the on/off switch, but only if they were designed with an isolated bus.  Just looking at the schematic, it is hard to tell if such a bus exists and just looking at the chassis won't tell you anything unless you really know what to look for.
The following test assumes you have a ohmmeter and know how to use it
     The best way to tell if your radio has a hot chassis is to take an ohmmeter and set its scale to read 100K ohms or greater.  Unplug your radio, but turn the on/off switch to the 'ON' position.  Measure the resistance from a blade of the power cord to the chassis.  After you get a reading, read from the other blade to the chassis.  If BOTH readings are greater than 100K Ohms, then your radio has one of these isolated buses, it is intrinsically safe and you don't need to install a polarized plug or rewire it.  BUT  If you get a low reading on either blade to chassis, your radio is dangerous and you should follow the instructions below.  

     If your radio failed the test above, but already has a polarized plug installed, put the on/off switch in the 'off' position change your ohmmeter to the lowest scale (usually the 200 ohm scale) and then measure the resistance from the wide blade to the chassis.  If the reading is high (more than a few ohms), your radio is dangerous and you should follow the instructions below.  On the other hand, if the resistance from the wide blade to the chassis is only a few ohms with the radio on and only a few ohms with it off, somebody has already wired it up for safety.

     As a double check, test the resistance between the narrow plug blade and the chassis with the radio both on and off.  If the resistance is low (a few ohms) with either setting, you have a hot chassis.

     If your radio passes all tests and has a late model isolated return bus, you can still get a slight "tingle" from it. If you have a Hallicrafters S-38c (or later), a S-120 or a late model AA5 radio that you have tested and confirmed that it has an isolated chassis, it is not necessary to rewire it for safety. BUT--- Please be aware that you still might get a painful "tingle" from touching the chassis of these radios.  If that "tingle" bothers you, you can get rid of it by using a polarized plug and rewiring the on/off switch, but that will be the subject for another article.  (I'll provide a link to that article when I've posted it)  In the meantime, below is a schematic of a late model AA5 radio that has had its AC section modified to "tingle proof" it.

Making later AA5 radios "tingle proof"

OK, assuming your radio has failed and needs to be made safe, here's what you do:

    As mentioned, the modifications I'm suggesting are quite easy to do and very cheap.  I urge you to make your radio safe before working on it or, if it is already working, stop using it until after you have made it safe.  All you will need is a soldering iron and a replacement power cord with a polarized plug.  An inexpensive extension cord you can buy for less than $10 new will work fine once you cut the socket end off.

Below are shown schematics detailing what must be done to make these radios safe.  The power cord plug must be replaced with a polarized plug AND the ON/OFF switch must be rewired.  Simply replacing the power cord with a polarized plug without rewiring the switch is worse than doing nothing.  These tasks are to be performed only by technicians who understand AC wiring and who can read schematic diagrams.  If you are untrained and have no experience working with high voltage electrical circuits, don't even try the following or you may be killed.  Wiring up these radios incorrectly is as bad or worse than having them in their original configuration, so be absolutely sure of what you are doing and have somebody else check your work, if at all possible.
(1)  The first thing you must do is remove the old AC power cord and install a new power cord with a polarized plug.  I would suggest you cut the old cord off as near to the chassis as you can and just leave short stubs of old AC cord attached to their original termination points for the time being.  

(2)  Next, unsolder the old power cord stub from the termination point that that is wired to the ON/OFF switch.  It is usually an unused pin on the 35Z5 rectifier tube with a wire going up to the ON/OFF switch.  Solder in its place the new power cord wire that goes to AC Line (AC "hot").  The AC Line is the narrow blade of the AC plug.   After you have done this, take an ohmmeter and double check that the ON/OFF switch is now connected to the narrow blade of the AC plug.

(2b) As an alternative, you can discard the wire going from the termination point mentioned above and solder the power cord's  AC Line wire directly to the ON/OFF switch after you've removed the wire.

(3)  On the other side of the ON/OFF switch, trace the wire to where it is soldered to a lug that is connected to the chassis.  Unsolder that wire at the chassis so that it is no longer connected to the chassis.  You will move that wire as described in step 4 below, but remember where this lug is because you will solder to it in step 5.

(4)  Unsolder the other old AC line cord stub from the where it is terminated on the rectifier tube socket.  This is the termination point that has the 330 ohm resistor on it, a wire from the lamp and is jumpered over to pin 2 of the 35Z5.  Solder on the wire from the ON/OFF switch from step 3 above in its place.

It is extremely important to rewire the ON/OFF switch as described and shown in the second diagram below.  You must not leave the ON/OFF switch in its original configuration with one side connected to the chassis.

(5)  The AC Neutral wire from the polarized plug (wide blade) must now be soldered to the chassis lug that formerly had the ON/OFF wire on it (step 3) so that AC Neutral is hard-wired directly to the chassis as shown in the second diagram.  With a ohmmeter, confirm that the wide blade of the plug (the AC Neutral) is connected to the chassis regardless if the ON/OFF switch is on or off.

(6) Many radios have a jumper between the left (ground) side of the volume control potentiometer to the on/off switch.  If your radio has this connection, you must remove the connection -- by no means leave the potentiometer jumpered to the on/off switch.  The ground side of the potentiometer must now be grounded to the chassis using its own jumper wire.  Pick a nearby chassis ground lug (perhaps the same one one you just soldered to) to ground the volume control potentiometer lug.

     With a polarized plug and the AC Neutral side connected directly to the chassis, you are guaranteed a "cold" chassis that is safe to touch even with the set plugged in.  Be sure to double check and make absolutely sure that AC Neutral is connected to the chassis as shown in the lower diagram and not AC Line voltage by mistake.  If you make this mistake and AC Line voltage is connected to your radio's chassis, your chassis will be always "hot" and your radio will be even deadlier than before.

(7)  For EC-1 radios, you should check the chassis-to-case insulating grommets for deterioration. These are standard large size grommets that are available at most hardware stores.  Make absolutely sure there is no direct (DC) connection between the chassis and the metal outer case of  your radio.  As a final safety check, you should connect an Ohmmeter between the chassis and the case.  With the Ohmmeter set to its highest range, you must measure 500 K Ohms or more.

That's all there is to it.  If you did everything right and you didn't skip any steps, you now have a very safe radio and you may go ahead and plug it in with confidence.

ORIGINAL (deadly dangerous) FACTORY WIRING
Original wiring showing how AC line voltage may be switched
 on to the chassis if plugged in one way and how

AC line voltage will be wired to the chassis through the
 filament string if the set is turned off.  

This is an extremely
dangerous situation and you must rewire your
 set as shown below before you attempt
to work on the radio, open
 the back to change a tube or even play the radio.

Notice that a polarized plug was installed and the on/off
 switch has been moved.  With the chassis always at AC
Neutral and the on/off switch connected to the AC Line
 voltage, the chassis will always be safe.  

Be sure your AC socket is wired correctly and never try to
defeat the polarization of the plug
or use
 an unpolarized extension cord.  

If these drawings simply look like chicken scratching
 to you,
it's pretty obvious that you don't know enough
about schematics or
radios to attempt these modifications
 on your own and you need to call an expert.

     It is not really necessary, but it would be even safer if a three wire plug was used with the ground wire connected to the outside case.  I did this for my EC-1b, but just for "feel good" reasons.   If you use a three prong plug, there will be fewer sockets you can plug into and you may have to use an adapter which, if not plugged in right, will make your radio dangerous.

     If you don't understand how to do this and you don't know about AC wiring, please take your radio to somebody who understands these things and who can rewire the radio for you.  Do not operate the radio until you have made these modifications and don't operate your radio until you  have had all modifications checked and rechecked to make sure the wiring was done right and the radio is indeed safe.  

     I am pretty sure that all houses built after the mid 1950s have polarized wall sockets.  If a polarized plug won't go into a socket no matter which way you try it, you have an old fashioned socket and you must have a qualified electrician replace it with a modern polarized socket.    NEVER, NEVER, NEVER DEFEAT THE PLUG'S POLARIZATION BY USING AN ADAPTER OR EXTENSION CORD THAT ISN'T ITSELF POLARIZED.
Just imagine how deadly dangerous that would be.

This is so important, so let me repeat:
Please be warned, if the wiring is done incorrectly or
 your AC socket is miswired or if you are using an
 unpolarized extension cord, the radio will remain deadly, so
Use only a modern polarized plug.                  
   (2) Make sure your wall socket is wired correctly
Make sure this modification is done right
by carefully testing each plug blade
to the chassis for high resistance
with the radio both on AND off .

One more thing.  
You might want to print out this diagram and paste it to the
 bottom or to the back of your radio.  
It will be a reminder
 to you or anybody who might own it or work on
 the radio in the future that it has been made safe.

additional suggestions and warnings
Please note that the large opening in the AC socket is the Neutral and the small opening is Line.  The large blade on the plug is the Neutral and the small blade is Line.  If you connect a AC volt meter between the large opening and ground, you should measure very little voltage, but if you connect between the small opening and ground, you should see 115-120 Volts AC.  Be sure your meter is set to measure AC volts.   For just a few dollars, you can buy a nifty little device that plugs into your wall sockets.  This thing has lights on it that will tell you if your sockets are not wired correctly.  If you have installed the sockets in your house yourself or if you don't know if your sockets were installed by a professional electrician, I urge you to test them with a AC voltmeter or one of these nifty little testers.  

     One excellent way of checking your radio after modification is to connect an AC volt meter between a ground and the chassis.  The voltage should be zero (or just a few volts) regardless if the set is turned on or off.  Turn off the radio and check the chassis to ground.  If you are showing 110 AC or more, you have a serious miswire and you need to unplug your set and recheck your work.  Turn on the radio and check the chassis to ground.  If you are showing 110 AC or more, you have a serious miswire and you need to unplug your set and recheck your work.

     Another warning regarding these radios:  The indicator lamp is at AC  Line voltage "hot" so be sure the set is turned off or, better yet, unplugged before you remove and replace the No. 47 bulb in its socket.  By the way, after all these years, the rubber insulation on these wires gets rotten and the wires get dangerously exposed.  Bare wires with cracked and peeling insulation should not be taped or glued, but must be replaced with modern insulated wire as part of your safety modifications.  Just think how bad it would be if you reached in your radio to change the bulb and touched a bare lamp wire carrying 120 VAC.  Ouch!!  Bare wires can also short against the chassis and blow out a fuse or start to smoke.

    Finally, if your rectifier tube goes bad and its filament is burnt out, the No. 47 lamp will explode like a small firecracker and you must replace the rectifier tube before replacing the lamp or you'll just burn out the new one.   As a general rule, if your lamp suddenly goes out but all the tubes remain glowing, you simply need to replace the lamp, but if all the tubes go cold, you should suspect a bad filament in one of your tubes and you should check them before changing the bulb.

     There is another way to make your radio safe (but not shockproof) and that is to use a Ground Fault Circuit Interrupter (GFCI)  that is built into a power cord.  Old hair dryers with a GFCI cord can sometimes be found at thrift stores and after removing the cord from the dryer, you could then use the cord as a replacement for your radio.  Having a GFCI on your power cord assures you that your radio is safe even if the wall socket is wired incorrectly.  Note that you can still receive a painful shock, but the shock shouldn't be dangerous.

     Speaking of GFCIs, you might also consider rewiring your wall socket for a ground fault interrupter.  They are very cheap now and it is a good idea to install them in your house wherever there is any chance that somebody might get electrocuted.  Again, you may still get a very painful (but not necessarily dangerous) shock if you touch a "hot" circuit and you could injure yourself while getting away from the shock, so I think that it is still necessary to rewire your radio for safety.  

Using transformers with an unmodified radio

Connecting an unmodified radio to an Isolation Transformer
This will render even an unmodified radio safe -- if there are no ground connections.

     If precautions are made to insure that nothing on the secondary side of the isolation transformer is grounded, this can be a safe way to go, but be absolutely sure of all your connections.  Personally I DO NOT recommend relying on a isolation transformer and I urge you to rewire your radio for safety anyway.

If one side of the secondary becomes grounded, your unmodified
radio remains deadly dangerous.

     When using a transformer for "safety," not only must you insure that the secondary remains isolated, but you must be absolutely sure what kind of transformer you are using.  Many people in countries with 220 VAC (almost the entire world outside the US) use an "Auto Transformer" to get the 120 VAC these radios require.  Using an auto transformer will give the right voltage, but it provides absolutely no isolation and an unmodified radio remains dangerous.

An Auto Transformer has no secondary and does not isolate the radio.

     Again, using a transformer should not lull you into a false sense of security.  The wrong kind of transformer or a grounded secondary will result in an unmodified radio that is a shock hazard.  There really is no reason to not make the simple and cheap modification that will render your radio just as safe as using an isolation transformer.  Rewiring your power connections will make your radio pretty much foolproof and that is good because, after all, you may have no control over who uses your radio once it passes on to a new owner and they may not know about using an isolation transformer.  Of course, if you want to be ultimately safe, you could use a GFCI, an isolation transformer AND modify the power cord and on/off switch wiring.

The folly of using a full-wave rectifier
     Finally, there are some people who think that one of these radios may be made safe by using a full-wave bridge rectifier between the radio and the AC mains.  Take my word for it, this absolutely WILL NOT make your radio safe even if you filter the DC with electrolytic capacitors, you will still get a dangerous pulsating DC shock if you touch a hot chassis fed this way.   Now, isn't it true that DC is ten times safer than AC?  Yes, DC is safer, but not pulsating DC.  Pulsating DC still has the ability to cause your heart to go into fibrillation and that is what kills you.  I could draw out diagrams illustrating why a full-wave bridge rectifier won't protect you, but perhaps the easiest and most convincing way to prove my point is to have you connect the chassis to ground.  Depending on which way your AC plug is wired or if unpolarized, how it is plugged in, you will either immediately blow a fuse when you connect to the wall socket or you will not be able to turn off your radio all the way and it may start to make some very strange buzzing noises.  Consider this too, if you use a full wave bridge and then you filter the output with electrolytic caps, the resulting voltage will be around 160 VDC and not the 120 AVERAGE voltage of AC.  Voltages this high will blow the filaments out of one or more tubes as they will glow white hot just before melting.  If you are lucky, you will only burn out one tube before the whole string goes, but all the other ones will be severely over heated and will most likely be damaged and you can still get a terrible shock, so don't do it.

     The only sure way to make your radio shock-proof is to rewire as I have suggested or be sure you and all others that will use your radio in the future use a transformer that truly isolates the radio from the AC mains -- and can you really be sure the transformer you want to use does that?

The limits of my responsibility and a sincere wish:  
     I have done my very best to warn you about the hazards associated with these radios and I have done my best to suggest safe and effective modifications for making your radio safe.  It is entirely possible that I have omitted something very important, so please consider all aspects of electrical safety while using these radios.  You must realize that I CAN NOT and I DO NOT ACCEPT ANY RESPONSIBILITY for loss of life, chronic health effects (like heart or brain damage resulting from electrical shock), any loss of income or any medical expenses you may incur due to any omissions or mistakes regarding what I have presented here or due to your own mistakes or due to your inability to understand and follow the instructions.  If you aren't an experienced electrical worker who understands AC wiring, don't attempt to work on your radio yourself, but hire somebody who is an expert to do the work.  Don't let your grieving survivors bitterly regret you ever messed with these old radios.  If you have ANY question regarding the safety of your radio, please drop me a line at and I will try to help you.

     I hope and trust that I have scared the bejezzus out of you and that you will take these warnings seriously.  I urge you to make these modifications right away.  Once these modifications are made, these radios are very safe, run very cool, may be left on all day and night unattended and may be used anywhere in the house (except maybe the bathroom) with perfect safety and you will enjoy yours very much.  There isn't anything in these radios that can get hot and cause a fire, or even smoke badly, so use your radio with confidence -- but only after it has been modified.  At 25 watts, they will not run up your electrical bill either.

     If you have any questions or concerns, please send me an E-mail and tell me what kind of radio you have and what your level of competence is.

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