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Hot chassis and Shock Hazards of old AC/DC Radio Tube Gear

CharlieBee

CharlieBee

Super Member
I recently got gifted of a old Hallicrafters S-77A tube shortwave receiver from 1955. An internet search pointed out the shock hazards of the two wire plug and the hot chassis being possible 50/50 chance. This is possible because the switch in only on one side of power cord.

I checked for a hot chassis by grounding one lead of a ac voltmeter to the third ground plug testing with the other lead to the chassis. With the plug in one way the chassis was hot with the power switch Off and with the plug in the other way the chassis was hot with the power switch ON ! A lose lose deal.

I found two helpful sites on how to correct this hazardous situation.

http://www.geojohn.org/Radios/MyRadios/Safety.html

http://boatanchorpix.x10host.com/GFCI_.htm

I wound up rewiring the power switch and using a plug that is polarized to insure the hot wire goes to the switch first and only.

I then got an old hair dryer at the Salvation Army for $3 that had a ALCI (Appliance Leakage Current Interrupter) power plug, that is also polarized. This is the type of ground fault Interrupter that should be used for 2 wire plugged devices.

The standard GFCI Ground Fault Interrupter, also known as a "Residual Current Device" that is used in modern houses in bathrooms, garages, kitchens etc. either as wall sockets or in the circuit breaker. It only works with modern three wire plugs and depends on the third ground wire.

I know that old wire ac/dc 5 tube radios are noted for these shock hazards. I do not know about old tube gear such as amps, receivers and tuners but feel it worth checking them out.

Charlie

As a side note, I initially powered up the Hallicrafters with a dim bulb tester and the tubes lit up and sound was heard a strong station or two for a few days. Then I found the hot chassis, rewired and installed the ALCI plug and now it is dead ! Just another challenge to look forward too !
 
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The really right way to deal with this is an isolation transformer. That way it doesn't make any difference which direction the thing is plugged in. It also doesn't require that the house's outlets are correct, which is handy because a lot of them get hooked up wrong.
 
I'm not clear on what happened after you replaced the cord with a polarized one. Did you harvest the thingamajig from the hair dryer and add it to the radio or what?

Is the radio getting power when it's plugged in? i.e. is there voltage at the power switch?
 
Thanks for the replies on the shock hazard of old AC/DC radios. Here are some answers really late.

Toxcrusadr
Yes I took the wire and polarized ALCI (Appliance Leakage Current Interrupter) power plug from the hair dryer and installed it on the S-77A receiver. With the original two wire non polarized plug and a single switch in only one wire, the receiver case was hot with the plug both ways it was plugged in. The chassis was hot with the plug one way with the receiver switch on and other way with the plug reversed and the receiver switched off. The only way the receiver was safe is with the plug not plugged in!

gaget73
Yes another way to make the receiver shock safe is to use an isolation transformer. The safest way to use an isolation transformer is to install it inside the radio if there is enough room.
Another easier common way to utilize the isolation transformer it to have external and plug the radio into it. This method is not as safe as the isolation transformer can get lost/misplaced or another user or owner does not remember or care to use the isolation transformer. The AC/DC radio still dangerous if not used with an isolation transformer.

Charlie
 
I wouldn't worry about a hot chassis as long as it's correctly installed in the cabinet with no exposed hot screws, or control shafts. It's only hazardous if you go poking around inside while it's plugged in. This should only be done while using an isolation transformer. A GFCI isn't a substitute for proper design (anything from the 50's or later should be properly designed). There were some 1930's radios which had exposed hot screws, and other problems which does make them unsafe. I don't recommend using it in the bath, even with a GFCI...

They made TVs with a hot chassis well into the 90's/2000's, so it's not like a hot chassis is contrary to reasonably modern safety standards. Yes, they did have a polarized plug, but they used a full wave bridge rectifier with the negative grounded to the chassis, so the chassis was always hot.
 
+1 to what andy said.

Inspect the unit carefully to verify that it is built correctly, and that the unit is completely insulated. (antenna connectors, phono jacks, etc.) Make sure the knobs are not brittle or cracked. You don't want them disintegrating as you touch them. Obviously, a brittle case, trim, grills, etc. can pose the same risk.

If the unit checks out, then an isolation transformer is not necessary for normal use. As mentioned, an isolation transformer -- in addition to the other precautions when working on tube equipment is necessary for service, repair or testing.

There were thousands of hot chassis TV's in use well into the 90's, and somehow the human population survived.

A polarized plug will not make an otherwise unsafe device safe. Contact with even the neutral side of the line should not (I'll say never for emphasis) be considered OK or safe. As gadget points out, sockets can easily be wired wrong, and even when wired correctly, the neutral can rise to significant voltages depending on house wiring, other appliances on that line, etc.

Here's an example: Bad white wire connection (loose terminal, or corrosion) at the fuse box -- you are in contact with the neutral because someone thought an aluminum knob looked cool on that vintage table radio, or someone replaced a nylon screw with a metal one -- Then the window AC on the same circuit starts up, or someone turns on a hair dryer or toaster oven plugged into the next socket closer to the fuse panel. The neutral can suddenly rise to the full hot side voltage.

Yes, while you are laying on the ground, the emergency responders may notice that the GFI tripped -- Great.
 
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Andy
The face panel of the receiver is metal and hot, only a fraction of an inch from the knobs and was hot. I felt safer and other would be safer, after wiring a hair dryer polarized ALCI (Appliance Leakage Current Interrupter) power plug to the receiver.
Charlie
 
Many years ago, I learned the hard way the dangers of hot chassis tube radios. I was futzing around with my Zenith TransOceanic H500 battery radio, which was not working at all, while standing on a concrete floor in stocking feet. With the chassis out of the cabinet, I touched the shaft of the volume control, and received a mild shock. Figuring the plug was in backwards (no polarization!), I reversed it, touched the volume control shaft again, and received a much STRONGER shock! :eek: Once my hand stopped tingling, I moved my feet from the concrete floor to a nearby piece of carpet, touched the control shaft again, and didn't get shocked. Since then, I have developed a healthy respect for those hot chassis sets. :oops:
-Adam
 
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