variac question
 

I was told that I should run my television on a variac do to the lower line voltages in use when the tv was made. My television states 117 volts on the back. My line voltage is 125 volts, Is this enough difference to need a variac? I do run this set alot. Thanks, Doug,

I wouldn't usually sweat it, but 125 is kinda high. It wouldn't hurt to use the variac. Some sets have a provision for changing the tap on the power transformer for areas with high line voltage; this should be noted in the Sams if it is an option.

125 is kind of high, and those old sets don't really have any B+ regulation. It could overtax the flyback, I suppose. But you can get a step-down transformer, or a Bryan says, change the tap on your power transformer. A Variac is an expensive way to go, but it will work. I'd like to see my old sets running on about 110. Another old trick is to put a weak 5U4 in the set if it uses one, or whatever the B+ rectifier is if it's a tube ;)

Charles

You can also use a big 6 volt power transformer wired as a bucking transformer to drop the voltage down a bit - see: http://www.allaboutcircuits.com/vol_2/chpt_9/5.html

Just find a transformer capable of handling the current. Connect it according to the diagram (primary to secondary on one side, connected to the line, other end of primary connected to the line and the load, and the other end of the secondary to the load). Check it with your meter before plugging the set into it, if wired wrong you'll add six volts instead of subtracting it!

-Ian

Did you check your voltage with a load ?
If not then double check the voltages with a load and see if it will drop to a lower level .




Barney

I operate every vintage hybrid tv set with a variac. Here in Europe, vintage tv sets do not have a mains transformer. Filaments are heated by serial mains voltage. And when they are cold, they are overheated by the full mains A.C.. This reduces the lifetime of the tubes. So I use a variac and watch the tubes' heating while power on.

- Eckhard

Another way to drop the line voltage is to use a power resistor, inserted in the power feed, that, with the current the TV set draws, drops you about 6V. The set usually has a label on the back telling the power consumption, say it's 175W. Divide that by 117V, and that gets you an approximation of the current draw. Here we get 1.49 amps, call it 1.5 amps. Ohms law then gives us 6v/1.5a = 4 ohms. Now how much power rating for that resistor? Vsquared/R = P, 36/4 = 9 watts, a 10W resistor would do. Or use a 20W resistor so it won't get as warm to the touch. Hide it inside the cabinet.

A resistor will reduce the turn-on inrush currents you get from the set's cold tube heaters and such.

If the voltage sags more than a half volt with a 100W load, there's likely a house wiring defect that needs to be corrected.

My National TV-7W actually has a line voltage switch in back of the chassis.

http://www.antiqueradio.org/art/NationalTV-7W07.jpg

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Variacs can be inexpensive, I got a 15amp unit on ebay used for 40 shipped.

My thought, You should have one anyway if your playing with electronics!

Evan

Another solution instead of a variac is one of the RCA "ISO-TAP" isolation transformers. These have taps on the isolation transformer to either raise or lower the input/output voltage by as much as 20-30 volts, switch selectable in several discrete steps. Very handy for bench use as well, as it has both isolated and non isolated outputs.

I use one just like what John described-a very handy device. Mine is a later version, after RCA sold the line to VIZ.