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#1
08-03-2014, 10:13 PM
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Filter cap ID and replacement
Hey guys. I have a filter cap under the chassis of my DuMont RA-103. Trying to ID it and then figure out what modern filter cap to replace it with. The specs on the cap are as follows:
1.5Ω @ 60~ 4 volts. Listed in the SAMS as vert. cent. cont. bypass. No pf or uf specs listed. What do I replace this with?? Photo of the cap attached. Thanks!!!
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-Al 
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#2
08-03-2014, 10:38 PM
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I have no clue whatsoever as to replacement, but holy crap that's insane! It has an end clearly marked positive, but the 60 ~ should be referring to 60Hz AC, so why is it polarized?! I don't think that's an Omega symbol, perhaps its like the multi-section cap indicators and that's the half-moon symbol? (Only one section though.)
How many leads does it have? My brain has gone into "Deer in headlights" mode from seeing that picture. 
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#3
08-03-2014, 10:50 PM
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Got my answer on FB from Bob Anderson. Need to replace with an 1800 uf @ 4 volt polarized filter cap. He sent me this calculator page and explained:
http://www.pronine.ca/capimp.htm "They use that cap to drive the vertical yoke so it's important to have a certain impedance at the 60Hz refresh rate." Thanks Bob!! Learn something new during each restore!!!
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-Al
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#4
08-03-2014, 11:56 PM
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2000 uF or 2200 uF also works well for these. Any low voltage rating is fine (6 V, 15 V, 25 V, 50 V, 63 V, etc.). Higher voltage is OK, but they start to get large and expensive.
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#5
08-08-2014, 12:52 AM
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Interesting to say the least thinking of anything under a DuMont chassis of that period running at 4V, and with an AC identification yet a polarized replacement has been recommended.
I hope we can elaborate on this one. It can't be a filter for the power supply at 4V. I just want to know what it is and what function it performs. You are liable to find anything in a DuMont! That's what makes them interesting I think.
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"Face piles of trials with smiles, for it riles them to believe that you perceive the web they weave, and keep on thinking free"
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#6
08-08-2014, 01:40 AM
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It's not just Dumont - I first encountered one of these in a Motorola: http://www.videokarma.org/showthread...=248133&page=2
It's used to couple the vertical output transformer to the vertical yoke windings.
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#7
08-08-2014, 05:36 AM
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Banderson, It isn't really used to couple the vertical output transformer to the yoke, it is to bypass the AC across the vertical centering control. The AC (vertical pulse) is a very low voltage at that point, since the resistance of the centering control is low.
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#8
08-08-2014, 11:24 PM
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I am fascinated by these "impedance at 60 Hertz"-type capacitor ratings. This is not the first one that has been discussed here or on the Antique Radio Forums. Do any of you have a good explanation of why these were specified, and rated, that way? They are always very-low-voltage capacitors, too, it seems. My only guess is that anything above about 500 microfarads was considered so odd or rare (or even almost unmeasurable directly at that time), that they did not even try to mark them in terms of capacitance but rather used this type of AC measurement as a more repeatable indicator of value.
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Chris Quote from another forum: "(Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did."
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