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 Post subject: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 8:03 pm 
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Location: Aurora, CO 80013
My home line voltage runs almost 125VAC. I'm finishing a Zenith 10S-155 and I was thinking about putting a DALE 30 ohm 50 watt resistor in series mounted under the chassis. I would mount it to the chassis. Is this enough resistance to make a difference? The chassis' rated consumption at 117v is 110 watts.

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 8:10 pm 
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If your radio draws 1 amp a 30 ohm resistor will drop 30 volts. Wattage is voltage times current or 30 X 1 = 30 watts. This will be quite a lot of heat. Like a small soldering iron.

Better to use a filament transformer bucking line voltage. Filament voltage will subtract from the line. 125 volts - 12 volts = 113 volts

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 8:37 pm 
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Keep in mind that the input voltage stated on the schematic is only for doing voltage checks. Many interpet this to be the operating voltage which is incorrect. The actual operating voltage will probably be 10% above or below that voltage. The radio should be just fine at 120 volts. If that makes you uncomfortable use a bucking transformer as Norm suggested.

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 9:40 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
Search the INternet for "bucking transformer." Lots of posts on this forum about building/using bucking transformers to reduce AC line voltage.

http://www.angelfire.com/electronic/fun ... Boost.html

https://www.antiqueradios.com/gallery/m ... mId=107293

Rich


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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 10:00 pm 
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Location: Aurora, CO 80013
I have a 12v transformer from a previous project that never happened...but I found this on the parts shelf. Output is correct when I set switchable input voltage to 125v. Then I don't have to make any changes in the radio.


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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Mon 06, 2020 10:43 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
That much resistance will make a difference, alright! It will heat up the whole chassis and maybe the room with it, while dropping the line voltage below what the radio needs to operate properly.

The 10S-155 dates to 1936 and uses a 5W4 rectifier. These chassis do not seem to suffer from overloaded rectifiers and roasting power transformers as Zeniths made a few years later became notorious for. However, when dealing with an 84-year-old power transformer, it would be better not to push it more than necessary. If your line voltage is 125 VAC more or less consistently, dropping it eight or 12 volts would be a good idea. That would best be accomplished with a bucking transformer, or with an old fashioned TV line voltage "regulator" which were tapped autotransformers. Since only the secondary carries the full load current in a bucking transformer, you could use a small transformer, perhaps one rated for 1.5 to 2 amps at 12 volts, and hide it in the cabinet somewhere. The transformer you pictured above will do fine.

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Fri 10, 2020 6:56 pm 
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AuroraOldRadios wrote:
I have a 12v transformer from a previous project that never happened...but I found this on the parts shelf. Output is correct when I set switchable input voltage to 125v. Then I don't have to make any changes in the radio.


I have a similar isolation transformer but mine has a fixed primary and three taps/outlets on the secondary. very handy but mine could use a new cord....

Hard to read the label but it is a Stancor P-6415, 117V input, 105/115/125V output, 350VA.

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Fri 10, 2020 7:41 pm 
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Another option---for any set that has a separate filament winding for the rectifier:

Wire in 2 S/S diodes to bypass the rectifier, then either remove the tube or disconnect it under the chassis.

Use the rectifier filament winding to buck the input line voltage. A fringe benefit is that the total transformer VA is reduced, thereby reducing the operating temperature.

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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Fri 10, 2020 10:42 pm 
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pixellany wrote:
Wire in 2 S/S diodes to bypass the rectifier, then either remove the tube or disconnect it under the chassis..

Says here this increases the transformer load.
https://www.w8ji.com/power_transformer_stress.htm

DM


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 Post subject: Re: Reducing AC line input via large power resistor
PostPosted: Apr Fri 10, 2020 11:13 pm 
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Interesting...I feel an experiment coming on...

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