Shocked by my Predicta!

[Kamakiri]

I was watching my Predicta Holiday the other night, and everything's working fine, so I reach for something sitting just behind the set, my forearm touched the metal swingarm on the right hand side, and I felt a shock! It seems as if the outside of the metal support/swingarm is charged with AC voltage, by the strength of the shock (like a jerk I tried it a couple times to make sure it wasn't just a static charge) I'd guesstimate it at around 30V (about double a Lionel train transformer).

Any ideas as to what could be causing it or how to fix? The TV's operation is about as perfect as a Predicta can get.

[Sandy G]

Kam- You ain't the only one !! I've been "bit" several times by old tube stuff-or at least got a nice "tingle" to let me know its alive, present & accounted for. I think lots of that old stuff was grounded to the case/chassis by rubber grommets, & 40-50 years later they've lost their insulating properties. I don't know much about the finer points of this stuff, but wonder if putting a modern 3-prong plug grounded to the chassis would help ? Any of you electrical wizards out there have any clues ? -Sandy G.

Hey bud - were you by any chance barefooted and standing on a concrete or ceramic tile or terrazzo floor? I know that on older washing machines with bad grounds, you stood a good chance of getting knocked on your *** under these circumstances.

Quite likely, the television chassis is seeking the closest route to ground and it looks like you might have provided a shortcut. I know my old Zenith 6V27 Tombstone radio will light up my life if I touch the chassis barefooted because I have terazzo (stone) floors in my living room.

There's nothing inherently wrong with it. It's just the nature of the beast.

Hope this helps - Fredro

[Kamakiri]

I had thought of that actually, but I was standing on a piece of carpet and wearing socks.

I should just write a bitch and moan letter to Underwriters Laboratories

Many old sets have a cap from one side of the AC line to chassis, and this is enough to cause a "tingle". Try reversing the AC plug in the outlet, but use a multimeter to check results instead of your body! Measure from the part of the set you touched to a real ground, like a cold water pipe.

Originally posted by Sandy G
I don't know much about the finer points of this stuff, but wonder if putting a modern 3-prong plug grounded to the chassis would help ? Any of you electrical wizards out there have any clues ? -Sandy G.

Yes, it will help. The hot lead (black wire) from the plug should go to the switch of the TV or radio, the neutral (white wire) to the other side of the AC input, and the safety ground (green wire) to the chassis. TRANSFORMER SETS ONLY - DON'T DO THIS WITH AC/DC (SERIES FILAMENT) SETS!

Assuming the house wiring to the AC outlet is good, this mod should eliminate all shocks and tingles. You can purchase an inexpensive plug-in outlet tester at hardware stores that uses 3 neon bulbs to test for correct wiring and functioning of your AC outlets. It will indicate open grounds and reversed polarity.

[Chad Hauris]

Couldn't you also use the grounded power cord on a "hot" chassis instrument with metal cabinet (where the common negative is isolated from the cabinet by a resistor and capacitor) by grounding the ground wire to the metal cabinet (not the chassis)? That way, if the chassis shorts out to the cabinet, the power line circuit breaker or fuse is tripped, and there is no way the cabinet could ever become "hot".

Improving the safety of hot chassis devices is more complicated than you might suspect. Check the following link for a thorough treatment of the subject:

http://antiqueradio.org/safety.htm