Hot chassis: check those antenna connectors

[andy]

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[Jeffhs]

The antenna connectors aren't the only things charged with line voltage in hot chassis TVs, especially today's solid-state sets. I just posted to another thread regarding the dangers of connecting external speakers to RCA/Thomson TV sets. These TVs are also of hot-chassis design, meaning, among other things, that if the speaker leads short to each other or to ground, extensive and permanent damage to the chassis will result.

BTW, Andy, you were very lucky you were not injured or killed when your coax cable sparked against the 300-ohm terminals of that RCA set you mentioned. As well, it was fortunate that the spark did not cause damage to the input circuits of the tuner, or to the tuner itself.

AC plugs, polarized or not, must never, I repeat never be modified in any way, and whatever you do, don't simply plug the two wires of a line cord into an AC outlet if, for example, you do not have a 2- or 3-prong plug to replace the original, if the latter is broken, damaged or missing. Whomever filed down the wider prong of the plug on that TV, however, was being very foolish, as is anyone who removes the third (ground) pin of a three-wire AC plug to avoid having to replace the outlet or to use an adapter. Also, jury-rigging the antenna connectors on that set so that the 300-ohm terminals were wired directly in shunt across the 75-ohm coax connector (or vice-versa), bypassing the isolation network, as the set's former owner apparently did, is inexcusable because, as you mentioned, it is a fatal accident waiting to happen. The set will now be 1000 percent safer to use, now that you have moved the isolation network to its proper location in the circuit.

[Kiwick]

Interesting... i used to think that American plugs weren't polarized apart from those with the third grounding prong

here in Italy and in most continental European countries we don't have any kind of polarized plug, even grounded plugs can always be inserted both ways.

Virtually all European TVs made before 1980 or so were tabletop sets with hot chassis to save the cost, bulk and weight of a proper isolation transformer... and in many color sets the presence of a full rectifier bridge meant that the chassis was ALWAYS live at half the line potential (110v with our 220v power) regardless of plug polarity but they were perfectly safe as long as their safety devices (covers, isolated sockets...) were in place and undamaged.

[marty59]

On hot chassis sets that can be plugged in either way at least take a sharpie and mark that hot blade to ensure you always plug it in the same!!

[Don Lindsly]

The older RCA portables had the line isolation network between the tuner antenna terminals and the antenna connection on the back of the TV. Always use an isolation transformer or a floating antenna connection when connecting directly to the tuner on a hot chassis.

[andy]

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[mbates14]

off-topic: You know, i never understood why they put the holes in the prongs. its always presented me an opportunity to make a redneck extension cord, which i have in the past, although highly dangerous.

[Findm-Keepm]

Originally Posted by mbates14

off-topic: You know, i never understood why they put the holes in the prongs. its always presented me an opportunity to make a redneck extension cord, which i have in the past, although highly dangerous.

IIRC, weren't the holes used for lockout protection?

http://www.bradyid.com/images/BradyID_Large/65673.jpg

The locking mechanism engages the holes in the prongs. Used extensively in industry and military to prevent use of equipment or unsafe equipment.

Cheers,

[matt_s78mn]

On a few occasions, I've seen a nail inserted through the holes and then bent at a 90 degree angle as a sort of redneck way of keeping someone from using that specific piece of equipment.

Originally Posted by mbates14

off-topic: You know, i never understood why they put the holes in the prongs. its always presented me an opportunity to make a redneck extension cord, which i have in the past, although highly dangerous.

[peverett]

This could have been factory. I have seen several mistakes/shortcuts that were obviously factory on radios/TVs. One was a series string tube radio with the line cap wired prior to the switch-the cap was shorted. The other was broken tube socket that had been kludged at the factory to make it work. I replaced it.

I always use an isolation transformer when working on TVs/Radios, but once you have it re-assembled, you are not expecting this kind of thing.

[Telecolor 3007]

The guys from "Luxor" had a great ideea in the past. They dind't equypt tvs with a transformer, but instead they've put a device that didn't allowed the tv to be turn on if the chassie was hot! I thinkl they did this with some live chassie radios.

[Kiwick]

The worst death trap of all times must have been the infamous Geloso G255 hot chassis tape recorder, the microphone jack's ground was hot too and you had to use exclusively the intact, well insulated microphone supplied along with the recorder if you wanted to stay alive

Trouble is, some people used uninsulated metal bodied microphones or home made aux cords to record from radios or other sources with this recorder and ended up shocked or electrocuted.

The later G256 variant was fitted with an isolation transformer, likely due to users filing complaints/lawsuits