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 Replacing Resistance wire

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Stan Ski



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PostSubject: Replacing Resistance wire   Sat Oct 16, 2010 4:24 pm

Replaced a resistance wire on a Detrola 134. Radio did operate with the old resistance cord so i was able to check the voltage before installing a capacitor. Installed the cap and checked my voltage with a variac. Everything was looking fine. Let the radio play and after awhile the cap got very hot (smoke). I have it hooked up with one side of the line cord going to the on/off switch. The other side of the line cord goes to the rectifier tube. The cap is connected to the rectifier tube and to the filament of the rectifier tube. Just as it shows in diagram 2 at Nostalgia Air. The tube line up on this radio is 6A7, 6C6, 25Z5 and 43. I used a 10 MFD 100V cap. Do i need a larger cap ?I thought 10 mfd was large enough or is it the voltage? Or did a screw up something else?
Stan Ski
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Wcirco

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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 4:28 pm

100V is too low. The cap has to be rated higher that the incoming voltage. 150V is fine.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 5:01 pm

Stupid me!!!!!!!!!!!. Kind of make sense when my line voltage is about 120 volts. So where is a good place to buy these non polarized caps. I checked over at Radio daze and Just radios and do not see any listed. Maybe i will a ad over in the classifieds
Stan Ski
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 5:15 pm

Last ones I bought were from DigiKey. They were Panasonic brand but DigiKey's search is a bit tough to navigate. You gotta make sure the rating is for >150 volts AC.. Regular non-polarized caps are often rated with DC voltage and aren't up to the chore of AC service.
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 5:28 pm

Bingo, found them Stan.

See the AC film capacitors at the bottom of this page.

http://dkc3.digikey.com/PDF/US2010/P1816.pdf

Without redoing your math I think you might want 7.5 or 8 uf.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 5:44 pm

Thank you Jack. I did go to there site and like you say it is not easy to find stuff. I think will order some 10 mfd 180V . This radio has a total filament voltage of 62 volts so this should give me a little margin.
Stan Ski
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Wcirco

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PostSubject: Re: Replacing Resistance wire   Sat Oct 16, 2010 8:44 pm

Stan
If you can only order caps that only have a DC voltage rating, then get them over 200V then you should be good.
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Alan Douglas



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PostSubject: Re: Replacing Resistance wire   Sun Oct 17, 2010 10:25 am

DC-rated caps may not handle the current. The "foil" in many modern capacitors is really a thin layer of metal deposited on the mylar film. VERY thin. And even if it could carry the current, the connections to it may not.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Sun Oct 17, 2010 2:25 pm

I was going to ask if DC caps cold be used. I was looking through my stuff and found some 15 Mfd 350 DC. Before i order some i will check with my local electronic shop tomorrow and see is the have any higher voltage AC caps.
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Sun Oct 17, 2010 5:11 pm

Listen to Alan Smile Don't even try an 200 vdc cap in this application.

Local electric jobbers often have motor/fan AC caps. Not always in the values you want and sometimes all they have are the big 'start' capacitors. These Panasonic types (for example) are about the size of a regular small matchbox and are common in fans. If you have a Grainger outlet nearby they sometimes have suitable ones.
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Amrad



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PostSubject: Re: Replacing Resistance wire   Sun Oct 17, 2010 5:44 pm

I have used these CDE DME capacitors from Mouser.

http://www.mouser.com/catalog/catalogusd/639/837.pdf

10mfd
http://www.mouser.com/ProductDetail/Cornell-Dubilier/DME2W10K-F/?qs=sGAEpiMZZMvOcEq4GH1AAqkqrDJidGzTZ65hdDjcgAA%3d

6.8mfd
http://www.mouser.com/ProductDetail/Cornell-Dubilier/DME2W6P8K-F/?qs=sGAEpiMZZMvOcEq4GH1AAkllLs3eWTkmEkpMJifXHbI%3d

Andrew
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 9:40 am

Amrad, I looked at the caps you listed from Mouser and do not see anything about being non polarized and also being AC voltage. Plus the one at Mouser are about twice the cost of Digikey So i ordered some from Digikey. Question, Why are these so difficult to find and why is Amrad able to use what looks to me a standard cap?
Stan ski
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 10:09 am

Amrad's selection seems reasonable. They are rated at 160 VAC which is sufficient. And being a film cap there's no polarity to them.

Although I use a lot of DMEs I have never noticed them for this application.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 10:18 am

Jack, I am getting confused. I thought they had to be electrolytic caps
Stan Ski
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 10:19 am

nope. Electrolytic caps ARE polarized!
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 11:46 am

Jack. I believe you are incorrect. I am looking at a non polarized electrolytic cap. This is what i i am lead to believe i need to replace the resistance line cord. 10 mfd 150 to 200 AC volts
Stan Ski
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smeezekitty

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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 11:50 am

I think using electrolytics of any kind may burn up under stress.
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 11:59 am

I dunno about electrolytics other than they are not essential for this application. I've never used one in the many that I have done so I don't know the ramifications of using one.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 12:25 pm

Sitting here playing with this radio. In my searching i seen something about using a silicon rectifier (1N4004-30) in place of the resistance line cord. Figured i would give it a shot since i have a bag of these. Seems to work fine. In fact tube voltage is a little lower. It has been playing for over 20 minutes . No heat from the rectifier and dial lights are fine. So maybe this is the way to go? Also a lot cheaper.
Stan Ski
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 1:30 pm

You're going to hate me for bringing it up but your voltage is too HIGH with the diode. Allow me to explain. When you use a diode the effective voltage is actually .707 * the line voltage. So if you have 120 volts available then you will have 84 volts after the diode. (not 60).

I believe you said your filament string was 62 volts?

You can't easily measure this 60Hz pulsating, unfiltered DC voltage without a special voltmeter. If you use a typical VOM it may read .707 lower again, more or less, making it APPEAR as if it were ~60 volts.

If you can find a 75 ohm (or 68 or 82) 10 watt resistor and add in series with the diode you'll be close to dead nuts on target.
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radiotechnician



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 2:15 pm

There are electroltic non polarized caps-NP's. Speakers use them as crossover elements, except when they run out and connect two polarized ones back to back.
Fun and games with NPs in the 70's when I was in speaker manufacturing: Some made better firecrackers than others.
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 2:20 pm

Hate you? Why would i even think that when you are trying to help this ole pollock. I have to believe what you are saying about the voltage. I did order those caps from Degikey so i will just wait for them to come in. I do have a couple more questions. Why does the radio seem to operate fine? I had it playing for a half hour without seeing or smelling any thing. I am in the process of recapping it . The E caps are replaced. I did notice when i have full voltage on it a hear a little buzz. As i turn down the variac it goes away. Is this because the voltages are to high with the rectifier?
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jack shirley



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 2:33 pm

The extra 22 volts in the filament string gets divided up proportionately according to the tubes so its not a great extra amount on any single one. But its enough to shorten tube life.

I'm not sure of the answer on your other question. Prompts me to ask, though, you are using the diode dropper ONLY on the filament string, right?
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Stan Ski



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 3:11 pm

Yes, I have the diode connected to one side of the line cord at the 25Z5 and the other end to the filament of the 25Z5. The little band on the diode is pointing towards the line wire. Got it all recapped and it still has a very slight buzz. It is a little strange because if i only drop my line voltage from 120 to 110 the buzz is completely gone. Sure wish i had one of those caps to shove in there. Guess i have to wait
Stan Ski
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radiotechnician



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PostSubject: Re: Replacing Resistance wire   Mon Oct 18, 2010 4:40 pm

For that slight buzz at 120 volts, try connecting a .001 uFd cao across the diode.

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