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Vintage Mica Cap Value ?

Discussion in 'Troubleshooting and Repair' started by HowardE, Feb 6, 2019.

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  1. HowardE


    Feb 6, 2019
    Repairing a vintage Zenith radio that upon inspection found a cracked mica capacitor made
    by Micamold and Would like to find out value.
    Cap measures 1.0 in. long by 5/16 in. wide by 1/8 in. thk.
    Info on cap as follows: back stamped type 800
    front has three circles and from left to right is orange, white, and blank with silver dot in upper
    right hand corner
    any help appreciated
  2. Bluejets


    Oct 5, 2014
  3. Ylli


    Jun 19, 2018
    Where it is used would be a clue. Otherwise, I would guess 39 pF 10%.
  4. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Provide the model of the Zenith . . . then it will be no problem.
  5. HowardE


    Feb 6, 2019
    This is a Zenith model H725 Chassis 7G01Z and I found out this is a .000039 mf
    What I don't understand is the tube diagram on the bottom calls for a 35c5 tube, the schematic calls for
    a 35b5 so is the right tube a rect or a amp ???
  6. Ylli


    Jun 19, 2018
    Looks like there is a selenium rectifier in there, so the tube would be the 35B5.
  7. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Howard E e e e e e e e e e e !

    Ahhhhhh yes . . . ye olde tyme model H725 . . . and I must have upwards of 3-4 working units of the better 832-835 *** series out in my garage. With one being a " listener ". . . if I'm working out there.
    (*** That's being a fancier cabinet version, instead of the somewhatt blasé, molded bakelite cabinet version, with its ever so all troublesome, coming unwound, floppy carrying handle on top ).
    I have the walnuts and even a "stimulated " limed oak finished one. ( In reality . . . it being a faux finish upon hardboard. )

    What I don't understand is the tube diagram on the bottom calls for a 35c5 tube, the schematic calls for a 35b5 so is the right tube a rect or a amp ???

    35B5 and 35C5 are both being AUDIO OUTPUT tubes of the same type but with DIFFERENT pin out connections to their internal elements . . . both use 35VAC on their filaments .
    More in depth detail follows . . .

    And yes, that would be an old sytle "postage stamp" cased 39 mmfd or pf , silver mica cap, which in nowadays modern stock would be using a deep reddish brown slick epoxy dipped and radial leaded configuration .
    Its being used in the quasi critical frequency / stable local oscillator circuitry.

    Those units are GREAT . . and that's being double emphasis HEAV-EEEE on the GREAT ! sounding units, due to their LARGE central mounted speaker and additional electrostatic tweeter, with other branded units being hum-drum, with their mere lilliputian 4 to 5 inch sized speakers .

    As for your 725 unit, note that it is specified as using a 35C5 audio output tube and if any doubt, check the wire from the primary of the audio output transformer, usually color coded BLUE and is going to the tube plate connection pin, with the transformer RED wire used for its B+ supply . . .unless it uses a hum bucking winding.
    See what pin connection of the tube . . . . that output transformer wire is being connected into.
    A 'B5 uses pin 5 as the plate
    A 'C5 uses pin 7 as the plate.
    That way there will be no doubt, otherwise it can ABSOLUTELY be smoke city, if the wrong tube is installed, since ONLY the pins 3 and 4 filament and the pin 6 . . . G2 grids are being the SAME between the two tubes.

    The power spec on the whole radio is for its being used at 117 VAC line voltage and most of our current AC line voltages are near 10VAC upwards from that. So o o o o . . . some smart DAILY users of these sets, are using a 50 volt filament version of the audio output tube to keep things running cooler.

    If you opt to replace the selenium rectifer . . . as per most rightfull, honorable and esteemed Ylli . . . .with a modern 1N4007 silicon diode, you need to measure developed B+ output of the original selenium rectifier and then in pulling and discarding the old SR, you need to have a series voltage dropping resistor at the AC input side of the anode side of the 1N4007 diode to drop the voltage down, to equate and be in line with what the somewhat lossier selenium unit was providing as B+ supply voltage. Ask if more info is needed.

    Le Zee-nuts 835 series . . . . . . photo gravure. . . .


    73's de Edd . . . . .
    . . . . . . . . . . . .

    A day without sunshine . . . is somewhat like . . . like . . . . well . . . ummm . . . errr . . . uhhhh . . . hmmmmmm . . . NIGHT !
    Last edited: Feb 8, 2019
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