[ODPILOT]

Just gone done restoring my B1750 radio (1954).  I noticed that when it is plugged in , I can feel a current running through the chassis and the metal potentiometer rods used to adjust the volume and the radio stations.   When I unplug the radio from the wall and turn the plug around the other way, the current is no longer apparent (radio works fine either way).     Obviously, the plug from the radio is non polarized while all the house sockets are polarized - as I'm sure they are with your houses as well.  I noticed the same kind of thing happens with a 1960's drink mixer that I've had since I was a kid (it's called a Vornado Drink Mixer). I   Can feel a current running through it, but when I reverse the plug in the wall socket it goes away.   I guess I should figure out which way prevents the problem and then cut off the old plug and replace with a polarized plug - but what's  up with that??  Thanks

[Mondial]

Is this a transformerless AC-DC radio? If so, then there is a bypass cap connected from the chassis to circuit ground which is also one side of the AC line. When the AC cord is plugged in so that the cap is connected to the neutral of the outlet, then there is no AC voltage on the chassis. When the plug is reversed, then the cap is connected to the hot side of the line and a small AC voltage appears on the chassis which is what you are feeling.

 Provided you replaced the cap with a safety rated cap there should be no problem, although it is always better to have the neutral side of the line bypassed to the chassis.

[morzh]

I would polarize the plug, making sure the wide one always goes to Neutral.
The chassis may be floating or straight hot in which case you expose yourself to full 120VAC, capacitor notwithstanding.

However, as long as you yourself are not grounded, you should not feel anything at all, no matter how it is plugged.

[ODPILOT]

MONDIAL - Yes, this is a transformerless radio.   And MORZH yes, I will replace with a polarized plug.    Thanks guys.

[Arran]

 Philco did not seem to go for the idea of building any true "Hot Chassis" sets, they almost invariably used a capacitor like a .22 uf in parallel with a 220,000 ohm resistor tied between the B- line and the chassis, the older ones will have a crude RF choke in series with the .22 uf as well. Here is something interesting to try, when you say that you don't feel a current when the set is plugged in a certain way, and the radio is on, try touching the chassis when the radio is turned off. I would be willing to bet that you will feel a little bit of current whilst the set is turned off if there is no current whilst the set is turned off. So basically you can take you pick, do you prefer a buzz from the chassis whilst the radio is on or whilst it is off? Actually since the chassis is inside a wood or plastic cabinet, with plastic knobs nobody will be normally making contact with the chassis.  So this is why I don't totally buy into the polarized plug idea, when it comes to AC/DC radios there is really no way to make them 100% safe short of running them through an isolation transformer, only ways to make them less dangerous. Another more economical suggestion would be to operate them through a GFI outlet, like the use in bathrooms, that way if you end up with a ground fault situation, the breaker will trip instantly.
Regards
Arran

[morzh]

0.22uF at 60Hz presents 12kOhm impedance which at 120V is 10mA current.
10mA AC is exactly the "let go" threshold.

Some people sometimes think of capacitors as an isolation; it is not. Capacitor same as resistor is a galvanically coupling element.


PS. A human will feel at least 1mA of AC current.

[ODPILOT]

Arran - interesting question about whether or not I feel the current with the radio on or off. When I got home from work - and checked it out - here's something that I didn't expect: With the radio ON and the cord plugged in the "wrong" way - I can feel the current. With the radio OFF and the cord in the "wrong" position, there is no current I can feel. However, with the radio OFF, if then I then reverse the plug, I can feel the current with the radio off. Is that weird, or does that somehow make sense?

[ODPILOT]

When replacing the old power cord, does anyone every use a 3 prong plug in order to ground the chassis, or would that blow something up??  [Image: http://philcoradio.com/phorum/images/smi...prised.gif]

[morzh]

Do not use 3-wire plug.

As for On/Off touch-feel and plug position, Arran explained that to you before, read it one more time, it is not weird, it is quite logical. I do not want to repeat what he has already said.

The very fact that you feel the voltage means you are completing some sort of circuit with the ground. Weak though it is. What kind of flooring do you have and at what floor of the house?

[mikethedruid]

One can feel the AC from body capacitance providing a virtual ground. This is probably what you feel when the "hot" side of the house wiring (black, or sometimes red or blue, the shorter slot) is hooked to the chassis side of radio, but when the neutral, grounded side (white, the longer slot) of the house wiring is hooked to the chassis side, since it is then at earth potential, you do not feel it. If you replace the plug with a polarized one, make sure the wider prong is connected to the chassis side. Also, with SPT cord, the smooth side is used for the "hot" conductor, and the ribbed conductor is used for the neutral. cord sets with molded on plugs should already be polarized this way as it is the national electrical code standard, and the U. L. standard.

[PAradiogeek]

Odd,

Here is an explanation that I always found easy to understand. It says the same things Arran told you, but in a slightly different way. Sometimes hearing essentially the same thing with different words, helps us understand.

http://www.antiqueradio.org/safety.htm

[morzh]

Mike

Human capacitance varies with surroundings but typically is estimated to be within few tens to a couple of hundreds of pF which is very, very low, the human ESDA body model being 100pF+1.5kOhm.

Obviously when one stands close to a large metal grounded object like a fridge or something, or a slab foundation (or basement floor) close to Earth surface the capacitance will somewhat increase, but at 60Hz you have to have about 20nF capacitance to start feeling the current (1mA AC or so).

Which is why I asked where the radio is situated. Because while operating a truly Hot Chassis models (the two I did for Kirk) without knobs and plugged directly into Mains in my house on the second floor, I did not feel squat while tuning or turning on the radio, not before or after it was turned on.

[ODPILOT]

PAradiogeek-   thanks for the link.  It does exactly explain everything we are talking about.    It also mentions that there is the potential to be killed with the amount of current hanging around in the chassis of these radios.  At the moment, the radio is sitting on a piece of Styrofoam, which is resting on my wooden work table (table top is wood, legs and frame are metal), in my garage.   When I've been working it on,  I must say that I'm usually barefoot and standing on a carpet remant on the floor of the garage.  Perhaps this carpet remnant has saved me from getting a shock.      Great info everyone -  atleast  I know I didn't imagine this problem !!

[morzh]

This answers it...you are in the garage. Barefoot.  



In any case: do read up upon electrical safety. It is plain dangerous to work on anything powered from MAINs without fully understanding what you subject yourself to.

[ODPILOT]

This answers it...you are in the garage. Barefoot.  



In any case: do read up upon electrical safety. It is plain dangerous to work on anything powered from MAINs without fully understanding what you subject yourself to.

Morzh -   Once again, you are EXACTLY correct.   When I touch the chassis or metal volume control rod barefoot, I feel the current( probably should stop doing that), if I put sneakers on - I feel nothing, no matter how the plug goes.  I'm finished with this radio anyway and it works well, so now it goes back into the wood cabinet.  If you all notice the lights flickering later, it 's just me.