Source for 700v 8uf filter caps

Discussion in '"Boat Anchor" & Classic Equipment' started by W7UUU, Jun 11, 2018.

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  1. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

    Google is failing me :(

    Does anyone have a known source for 700v 8uf filter caps? I'm recapping a Johnson Adventurer but cannot find a source for 700v caps anymore. Unless I'm blind, the "usual sources" of Mouser and Digikey don't list anything that high in voltage. Nothing on Hayseed Hamfest or eBay either.

    The highest I can find anywhere are 450v


  2. KB2WVO

    KB2WVO Ham Member QRZ Page

    not that i have a clue.. but ac or dc ? they have dc 700 volts
  3. K1ZJH

    K1ZJH Ham Member QRZ Page

    I am willing to bet if you open one of the shells for the existing OEM 700 volt caps you will find two 350 volt caps wired in series hidden inside.

    And, they did not use equlization resistors.:)

    600 VDC is about the limit for electrolytic caps.

    There are high voltage film caps, but make sure they can withstand the ripple currents in filter apps.
  4. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    There are no electrolytic capacitors above, generally, 500-volt. What is actually in the 8 mfd 700-volt capacitor is 2-each 16 mfd capacitors in series and then inside a single paper sleeve. You can purchase, usually at a pretty high price, "700 volt to 1000 volt" capacitors. However, what they really are is 2-each capacitors in series.

    What I do with units like the Johnson Adventurer is to put capacitors in series. MUCH cheaper!

    Glen, K9STH
    W7UUU likes this.
  5. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

  6. W7UUU

    W7UUU QRZ Lifetime Member #133 Volunteer Moderator Life Member Platinum Subscriber QRZ Page

    Doh! That's gotta be what they did. Easy peasy. My head wasn't screwed on this morning

    Thanks Glen :)

  7. W9GB

    W9GB Ham Member QRZ Page

    Illinois Capacitor, now a division of CDE, has a good assortment of Axial Lead Electrolytic Capacitors. You can use two (2) 22 uFd 450 Volt units to build an 8-10 uFd, 700 VDC equivalent.
    Mouser and Digi-Key are distributors. LEAD
    Common Size: 16mm diameter, 31mm length

    E.F. Johnson Adventurer
    Restoration Notes by Janis, AB2RA

    Woody, W0UI Restoration

    EFJ Adventurer Manual
    Last edited: Jun 11, 2018
    KL7KN likes this.
  8. N2EY

    N2EY Ham Member QRZ Page

    As others have noted, the trick is to put two lower-voltage capacitors in series. But be sure to use equalizing resistors! Otherwise slight variations in value will cause the one with lower value to have more voltage and it can fail. The equalizers also act as bleeders so you don't get zapped.

    I suspect that the two caps in the originals were carefully matched at the factory before being put into the sleeve.

    I'd use more than the original value, to keep the ripple down and improve the dynamic regulation.

    The Adventurer is a good little rig - let me know if you need anything for it. Note how much smaller it is than the DX-20, yet it runs the same power and uses better components.

    73 de jim, N2EY
  9. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    The site is really going crazy this afternoon! Quadruple posted my response, then eliminated two of them when I edited the post getting rid of the original post!

    Original post:

    The Johnson Adventurer cost $54.95 as a kit and the Heath DX-20 cost $35.95 for the kit. This was a 35% difference in cost. The Heath DX-35 (although with a badly stressed power transformer) cost $56.95 and had more features for less than 4% more. The better Heath DX-40 came out in the same time frame for $64.95 or about 18% more than the Adventurer and the DX-40 had a much better meter than the moving vane type in the Johnson transmitter.

    Glen, K9STH
    Last edited: Jun 11, 2018
  10. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    The site really burped and posted the same message 4-times within a few seconds!

    Glen, K9STH
  11. K1ZJH

    K1ZJH Ham Member QRZ Page

    The need for equalization resistors has meet with some debate in the past. The late Alan Douglas showed they were not needed in a series of tests he post on the antique radios group. Two caps in series are a series circuit and will have equal currents, and the voltage across each cap will be the result of them seeking the lowest shared voltages that result in the lowest leakage currents. If a failure occurs, it probably resulted from a capacitor that would have failed regardless.

    I recently restored a Globe Scout 40A that had two discrete filter caps in series, and they did not use equalization resistors. I doubt they "matched" the caps much beyond using caps with similar date codes.

    High voltage filter caps that had two filter caps hidden in a single tube also did not have any means to force equalization voltages.

    That being said, it is probably good practice, simply because the resistors provide a means to discharge the caps (bleeders) when the radio is powered off. I am sure the examples I had given also were motivated by economics as well, but the fact remains the rigs worked as designed.

    Last edited: Jun 11, 2018
    KP4SX, KM1H and KD2ACO like this.

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