WARNING: Make sure your antenna connector isn't hot! - Videokarma.org TV - Video

radiotvnut · #1 10-17-2009, 01:26 AM

Was playing around with the '98 Zenith that I did the tube swap on with it connected to the Channel Master converter box without problems. For some reason, I decided to connect a DirecTV box to the set just to see if it would power up and display anything. I connected the coax cable from the box to the set and as soon as I plugged in the AC cord to the box, I heard a loud hum, detected a burning smell, and the lights dimmed in the shop. Unplugged everything and tried the box again and no problem this time. I then suspected a problem with the TV; so, I checked between the outside conductor of the coax connector on the TV and earth ground and got the full 120 VAC! I reversed the AC plug and got zero volts. The TV plug is polarized; but, I can plug it in both ways in the cheap outlet strip I was using.

Had I sold this set to someone, one of three things would have happened:
A. They would have blown the house fuse and maybe damaged the TV.
B. They would have got a good jolt.
C. They would have ended up in a pine box.

In the past, I have taken for granted that the antenna connector is always cold. Probably because I've never really had a problem with a hot antenna connector. I've only heard about hot antenna connectors a few times from other repairmen. You can bet that from now on, I will check every antenna connector on all sets I fix to make sure they are safe. The last thing I'd want to happen is for someone to get killed over a set that I worked on.

This Zenith TV uses a switching power supply with the "hot" ground being on the primary side and the "cold" ground being on the secondary side. Obviously, something is shorting the grounds together.

I just thought I'd pass this along in case anyone else has made the same mistake I did by assuming the antenna connector is safe. Check those antenna connectors!

stromberg67 · #2 10-17-2009, 06:28 AM

Great advice! I have been zapped because of failed line bypass caps. Just finished repair/restore of a 1969 Sylvania portable color set, and found the Black Beauty line cap had exploded, shorting the line to chassis somehow. I always initially use an isolation xfmr, even if the set has a transformer pwr supply.
Good Day to All
Kevin

kx250rider · #3 10-17-2009, 10:25 AM

I've seen SO MANY people blow up nice gear that way. Besides worn-out or bad outlet strips and extension cords, there are plenty of reverse-wired houses out there. I've found at least one reversed outlet in every house or apartment I've done any electrical work on, lived in, or checked the outlets in for any reason. I bought a quick-test plug with 3 LEDs on it, which will show if (and if so, how), an outlet is miswired. It can be a lifesaver, and that is a literal reference!

In addition to the potential of miswired outlets, there's also an unusual but even more dangerous situation: The neutral line fails at the power pole. Then, you have both sides of the outlets in your house hot, as with no neutral, the current is flowing from hot to hot, through all appliances and lamps plugged in. You'll get a variety of weird voltages at any outlet, from 0 to 220 live! That happened to me after the earthquake in '94. The power came back on a few days later, but the neutral was yanked off the transformer up on the pole.

Charles

andy · #4 10-17-2009, 12:49 PM

000

drh4683 · #5 10-18-2009, 07:45 AM

I had a fun situation in my old house working on one of those early 70's hot chassis zenith portables. I had the back off with the outdoor antenna clip lead on the terminals. The clip lead fell off and landed on the chassis. The chassis was hot and grounded out through the coax with a loud bang. Blew the fuse in the house and left a nice arc mark on the chassis. Fixed everything and then I found out I had no picture. I thought something got ruined in the TV and checked another set. Same thing, no picture. That short burned out my distribution amplifier! Ever since then, I've always made a habbit to check a hot chassis to make sure its not live when Im working on it.

kx250rider · #6 10-20-2009, 12:16 PM

One of our tenants had a TV plugged into an extension cord, in the kitchen to an outlet they couldn't reach, and they wanted to hook a DVD up to it. So they plugged the DVD player into another outlet. The TV was reversed in the extension cord, and BANG! Kitchen and bathroom both dead, and it didn't trip a breaker. It took me an hour to track it down to a burnt (formerly loose) connection in a daisy-chained outlet, upstream in the circuit, in the bathroom. The tenant lucked out, as the only thing blown was the bathroom outlet.

Charles

radiotvnut · #7 10-20-2009, 01:01 PM

I fixed the hot Zenith. Turned out to be a bad degaussing coil. With the coil plugged into the chassis, I got a 5 ohm reading between the wide blade of the power plug and cold ground. With the plug removed, I got an open circuit. Checking from the degaussing coil itself to the CRT ground strap showed a short. Evidently, there was an area of exposed bare wire on the degaussing coil that was making contact with the CRT ground strap. To be safe, I replaced the degaussing coil with one from a junk TV and it appears to be fine now.

andy · #8 10-20-2009, 03:23 PM

000

Geoff Bourquin · #9 10-20-2009, 08:22 PM

.....electric baseboard heat. About 20 years ago I was working on someones' set, and I had just unscrewed the cable from the set and let it fall to the floor. It bumped the baseboard heater and POW

. It burned a small hole in the top of the heater. Of course, I was curious about this. Turns out the heater was wired incorrectly. The metal frame of the heater was connected to one phase, not to ground. The cable was grounded, so when the two touched, it connected 120V to ground. I'm sure glad it wasn't me that touched the heater! Before I continued working I shut off the breaker for the heaters and insisted (in writing on the TV repair invoice) the customer have it repaired ASAP

ChrisW6ATV · #10 10-21-2009, 02:47 PM

Most service manuals spell out a safety procedure to use after any repairs are done, to find any hot-connector problems. I remember one TV when I was a teenager that gave me a tingle from its antenna terminals when I was touching something else. That same one, if I remember right, gave me a shock from its AC power plug, after it was unplugged. Somehow the power supply cap charge was on the AC cord.

Zenith26kc20 · #11 10-21-2009, 02:52 PM

My Harmon Kardon Citation IIIX tuner had the metal coil cord shield short to the AC switch blade. In the middle of dinner we were greeted by the most unbelievable hum from the stereo. House fuse (1947 house, 60 amp service) blew and shut everything off. Replacing the fuse revealed the Magnavox TV I had been modifying for a couple of years was turned into junk. Virtually every IC was bad, some blown apart! Lots off blue air in the house that night! I now use a 5KV isolation transformer on the stereo so no more TV's get fried in case of problems.
And, the Citation is repaired and still working fine(I didn't kill it). Miss that TV though......
Have been to a few houses that, when the customer rack mounted their gear, pinched the line cord on one piece. It usually blasts the cable box or satellite receiver. It also burns interconnects off.

bgadow · #12 10-21-2009, 10:55 PM

One of my first experiences in radio "repair" (well, tinkering, I don't recall repairing much) came at around age 14. I was in my bedroom with a Philco chassis on my desk, plugged in and running. Loktal chassis AC/DC set. I did okay until I rested my bare feet on the electric baseboard heater. Yow! Did it more than once, which proves how sharp I am...

I was playing around with a little handheld GE vacuum from the 40s or so. I decided to see how it did at picking up the crumbs that were on the electric stovetop. Some metal on the vac hit some metal on the stove and POW! Blew the fuse, and the vac hasn't run since. I don't understand what happened exactly.

kx250rider · #13 10-22-2009, 10:22 AM

Quote:

Originally Posted by bgadow

I did okay until I rested my bare feet on the electric baseboard heater. Yow!

Lucky! Those hand-to-foot discharges are the deadly ones... If that had been DC, or even if it struck just right as AC 110... Not good!

Charles

peverett · #14 10-22-2009, 05:00 PM

Some models of AC/DC radio sets from the 1930s and 1940s have one side of the line connected directly the chassis. The screws on the bottom(or the control shafts if the knobs are removed) can be hot when plugged in.

Be careful with these.

kx250rider · #15 10-23-2009, 10:43 AM

Quote:

Originally Posted by peverett

Some models of AC/DC radio sets from the 1930s and 1940s have one side of the line connected directly the chassis. The screws on the bottom(or the control shafts if the knobs are removed) can be hot when plugged in.

Be careful with these.

That's why they were referred to as the "Suicide Radios", and pulled from the market. A lot of deaths from those sets! Someone I know came home to his USC dorm room, and found his roommate deceased in the bathtub with one of those little metal 4-tube radios playing on a table next to the tub. (that was right after WWII).

Charles