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[ 8 posts ] 

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Pearson

Post subject: Help me with this math problems Posted: Jun Thu 13, 2019 10:25 pm 

Joined: Jan Thu 01, 1970 1:00 am Posts: 169 Location: Greenville, NC 27858

ecap rated 5z@15750 h
ecap rated 1z@60 h
these are in a Moto # vk101m
What do I need? thanks


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pixellany

Post subject: Re: Help me with this math problems Posted: Jun Thu 13, 2019 10:32 pm 

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Joined: Jul Mon 26, 2010 8:30 pm Posts: 26409 Location: Annapolis, MD

can you give us a link to a schematic?
Also, I might not be the only one that does not know what a "Moto" is
_________________ Mark "Measure voltage, but THINK current." anon.


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Electronic Memory

Post subject: Re: Help me with this math problems Posted: Jun Thu 13, 2019 10:41 pm 

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Joined: Nov Thu 11, 2010 6:03 pm Posts: 838 Location: Pewaukee, WI

Motorola... people that own Motorola smartphones tend to shorten it to Moto... some TV guys do the same.
He ought to do a search for resto threads here and on Videokarma of that model odds are someone has already done the math.
Basically he needs to use algebra to reshuffle the equation for impedance of a capacitor at a frequency to calculate capacitance of an impedance at a frequency. Google impedance of a capacitor and you should get a formula to reshuffle or possibly a calculator page that will take your numbers and spit out an answer.


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Mr. Detrola

Post subject: Re: Help me with this math problems Posted: Jun Thu 13, 2019 11:20 pm 

Joined: Jan Thu 01, 1970 1:00 am Posts: 27867 Location: Detroit, MI USA

A few manufacturers chose to use parts that were rated like that in specific circuit positions in the sweep circuits near the yoke. I believe they bypass the centering controls.
If you look at an RCA 630 schematic, just use the cap values they did in the same circuit position and you'll be good to go. In fact I have found quite a few sets where those original electrolytics were still good, they aren't going to cause any catastrophic issues even if they fail.
_________________ Dennis
Experience is what you gain when the results aren't what you were expecting.


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aj2x

Post subject: Re: Help me with this math problems Posted: Jun Thu 13, 2019 11:56 pm 

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Joined: Aug Thu 12, 2010 6:25 pm Posts: 280 Location: Durham, NC

The "Z" stands for impedance, in the usual engineering calculations. Not sure what Motorola means by it. The relevant equation is Z=1/(2*pi*f*C). Without knowing Z you can't calculate C. After a little searching, I found that this 1948 set uses a TS5 chassis. My Wallace's Telaides book shows one cap (C127) across the vertical centering control, labeled 1ohm @ 60 cycles. This suggests that Z=1, so C=2653 uF, which is a mighty big value for 1948. The pot it's bypassing is only 30 ohms, so a 1 ohm impedance bypass isn't out of the question.
It might be that the capacitor is actually a tuned circuit at 60Hz, the way that some radios have a coil of wire wrapped around a largish paper capacitor to form a lower impedance to some interfering low frequency that the cap alone is unable to deal with sufficiently.
I suspect I've just muddied the water more!
_________________ Mark Nelson A collector of TV signal boosters and UHF converters  God help me! tvboxes.com


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Mr. Detrola

Post subject: Re: Help me with this math problems Posted: Jun Fri 14, 2019 12:15 am 

Joined: Jan Thu 01, 1970 1:00 am Posts: 27867 Location: Detroit, MI USA

RCA uses 1000 uf @ 10 volts and 250 uf @10 volts in similar circuit positions. The 630 schematic shows polarity. It's possible that the originals in the Motorola (and DuMont IIRC uses similar) were non polarized however I don't recall ever having seen that in writing.
Those were very large values in their day, but it was possible to construct electrolytics with high values at very low voltage ratings.
Today, 2500 uf is not a really difficult value to make like it used to be.
_________________ Dennis
Experience is what you gain when the results aren't what you were expecting.


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Tom Schulz

Post subject: Re: Help me with this math problems Posted: Jun Fri 14, 2019 1:05 am 

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Joined: Mar Sun 01, 2009 10:27 pm Posts: 5008 Location: Ann Arbor, MI

The math for 5 ohms @15750 Hz comes out to be 2 uf when I do it I do get 2653 uf for 1 ohm @ 60 Hz, so it looks like I am doing the math correctly.
_________________ Tom


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Pearson

Post subject: Re: Help me with this math problems Posted: Jun Fri 14, 2019 4:22 am 

Joined: Jan Thu 01, 1970 1:00 am Posts: 169 Location: Greenville, NC 27858

Thanks for all the interesting replies, especially Tom's. These ecaps are NP, rated @3v, the chassis is TS7, info I had left out. And yes, Motorola. Anyway, I'm good. Thanks.


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