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 Post subject: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 5:54 am 
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Joined: Sep Tue 22, 2009 12:34 am
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I had posted about this record player before, but now I have new problems I'd like to run by you guys. It's a Newcomb AV-7, tube "classroom" model. The 4.7ohm 2w "fuse" resistor blew today. It normally got very hot when in operation, a little too hot for my liking, but I am told it is supposed to do this.

A couple of my theories: I replaced the original GE1N1693 in series with this resistor with a 1N4007 at some point years ago. Would this affect anything, causing the resistor to overload?

Another theory: since this resistor is running hot by design, very close to tolerance, would there be a possibility that I may have gotten some low-quality 2w resistors off of ebay? Any suggestions are welcome. I have the record player up and running again with another 2w resistor, but I'm concerned it may blow again.


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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:07 am 
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It it fully recapped?
Phil

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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:11 am 
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Location: Livermore, CA
A 1N4007 will be ok in place of 1N1693. A 1N1693 only has a 200 PIV rating. Not high enough for the application.

Did you replace electrolytic filter caps? 20/40/40/60 @ 150 volts

Original resistor was most likely wire wound but even a 2 watt carbon type shouldn't have burned out. There are some film type resistors that could burn out from an initial turn on surge.

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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:13 am 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2128
Location: Santee Calif. 92071
Use metal oxide two watt resistor in power supply. Use original values of filter capacitors MFD wise. It's not really a fuse.More like surge resistor. Does run warm to the touch.


Last edited by thunderbird281 on Jan Sat 07, 2017 6:15 am, edited 1 time in total.

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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:15 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mpls, Minnesota
Did you use a wirewound fuse resistor? Like this
http://www.digikey.ca/product-detail/en ... ND/4291631

Dave


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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:23 am 
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Joined: Sep Tue 22, 2009 12:34 am
Posts: 132
Thanks for all the replies. It is indeed recapped. Looks like I'm using a "metal oxide film resistor" according to the ebay auction where I purchased these. Should I get a wirewound one instead?

This thing doesn't run warm to the touch. It runs too hot to touch.


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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:49 am 
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Location: Tucson, Arizona U.S.A.
Even with 300 mA flowing through it, a 4.7 ohm resistor would only dissipate less than 1/2 watt. If 2 watt resistors are burning out, you have a serious problem. Some possibilities: the resistor you used isn't 4.7 ohms, perhaps 47 ohms? As someone else said (or at least implied), film resistors don't handle overloads very well; a composition or wirewound type would be better. Even though you recapped it, and did everything else you should, you may have a bad or wrong part, a bad 50L6, a solder blob where it shouldn't be, or any of the other troublemakers we encounter..

If the problem happens when the set is first turned on, I would suspect overload handling ability. But since you said it gets very hot, I would certainly check the rest of the circuit to be sure nothing is amiss there.

BTW, resistors make lousy fuses unless they are specifically designed for the purpose. Indeed, many resistors specify how much overload they can withstand. That 4.7 ohm resistor is there to limit the turn-on surge to 38 amps to protect the rectifier.

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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 6:51 am 
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Location: Mpls, Minnesota
Measure the voltage across the resistor, use ohms law to find the wattage.

Dave


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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sat 07, 2017 9:14 pm 
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Location: Tucson, Arizona U.S.A.
easyrider8 wrote:
Measure the voltage across the resistor, use ohms law to find the wattage.

Dave

Note that the voltage you measure will be half wave rectified and filtered DC. If you have an average responding meter (like most), it won't give an accurate reading for the pulse waveform. A better way, if you have a non-contact thermometer, is to measure it's temperature in the record player. Then connect it to a DC source and adjust the voltage to get the same temperature. The DC voltage and current are easily and accurately measured.

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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Sun 08, 2017 1:25 am 
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Posts: 3951
Location: Boston, MA USA
This type of resistor really is a fuse. That's why it runs hot. If you replace it with a conventional 4.7 ohm resistor you should also wire a fuse in series with it. 0.5A would do.

-David


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 Post subject: Re: Newcomb Resistor Part 2
PostPosted: Jan Fri 13, 2017 6:31 am 
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Joined: Sep Tue 22, 2009 12:34 am
Posts: 132
Looks like we may have a possible solution here. I took the advice offered and ordered a wirewound resistor which arrived in the mail today. I just installed it, and it seems to run somewhat warm, but not hot at all. Thanks for the advice... I would never have figured that out on my own.


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