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Johnson Viking Adventurer Capacitor Network Help with Balance Resistors

Discussion in '"Boat Anchor" & Classic Equipment' started by W3KW, Apr 6, 2021.

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  1. W3KW

    W3KW Platinum Subscriber Platinum Subscriber QRZ Page

    I recently acquired a nice Adventurer and it needs just a little work. I will be replacing the cord for a three-prong variant and also need to balance a Cap replacement network. The fellow I bought it from did a neat install of new caps to replace C15 and C16. The Schematic and parts list (see below) list the caps as 8mfd 700 volts. As many of us do, the replacement was made up of lower voltage (450v) , higher capacitance (16 uf) electrolytic caps, as the 700v variants are hard to find I guess. My issue with the install, is that there are no balance resistors in the network. I'm a tinkerer and not an EE, but I do know that much. What I need help with is the watt rating, resistance and the arrangement of the resistors. I don't want to risk doing damage.

    I found this calculator:

    upload_2021-4-6_10-51-7.png

    Here is the Cap Network installed in my Adventurer:

    upload_2021-4-6_10-51-52.png

    Based on the formula above, I calculated 14.4M ohm resistors. I used 600 volts for the circuit voltage. A guess. The article that produced the formula I used, suggests that you could safely halve the resistance, so 7M ohms? But, what wattage resistor to use? I found the following photo, albeit incomplete work, of a balanced cap network in an adventurer. The same principal I'm attempting to replicate:

    upload_2021-4-6_10-57-26.png

    These look like 150K ohm resistors and not exactly neat or complete, but it's the only Adventurer example I could find for this post. These look like 2 watt resistors?

    Here is the schematic and parts list if you are so inclined:

    upload_2021-4-6_11-5-16.png

    upload_2021-4-6_11-5-33.png

    So, if you know the answer, or are really bored and feel like elmering me on how to make this network safer, take a look and please let me know. I assume I merely parallel a resistor of the same value across each capacitor? The idea being to make each capacitor do equal work?

    Learning a ton and very grateful for all the help I get on this forum!

    Thanks.
     

    Attached Files:

  2. N2EY

    N2EY XML Subscriber QRZ Page

    It's pretty simple and you don't really need a calculator.

    Electrolytic capacitors tend to not be precision components - their values can be quite a bit different from marked. Usually they have more C than the marking.

    In most applications this makes no difference. But when you put them in series for voltage purposes, this can be a problem, because if the capacitors in series aren't identical in characteristics, the result can be a very different division of the voltage across them, resulting in destruction.

    The equalizing resistors are meant to deal with that problem. For the Adventurer, you don't need a precise value, as long as they are all the same. 100K at 2 watts is probably fine. The resistors get warm so space them away from the capacitors a bit.

    The original 8 mF 700 volt electrolytics were actually two capacitors in series inside a common cardboard sleeve. They were chosen at the factor to be identical so the voltages would divide evenly.

    73 de Jim, N2EY
     
    W9WQA likes this.
  3. W0GSQ

    W0GSQ Ham Member QRZ Page

    And there were NO equalizing resistors used in that sleeve. When I cut mine open several years ago to restuff I was a bit surprised at that.
     
  4. W3KW

    W3KW Platinum Subscriber Platinum Subscriber QRZ Page

    Jim,

    Thanks for the help. It figured it was easier than I was making out to be but was trying to learn in the process. I’m pretty sure I have a stash of 2 watt resistors so I should be good there. So, I just install, with room to breathe, one resistor in parallel with each cap? So four total resistors? Is it really that easy?

    thanks-
     
  5. N2EY

    N2EY XML Subscriber QRZ Page

    They didn't use equalizing resistors because they carefully selected the capacitors used to have near-identical characteristics.

    We could do the same today, but it would require test gear and a large supply of capacitors.

    73 de Jim, N2EY
     
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Yes, it's really that easy. And you did learn something, I hope.

    Note that you can't use just any resistors - the value isn't critical, but needs to be high enough to avoid excessive dissipation (don't expect a 2 watt resistor to dissipate any more than 1 watt) yet low enough to have the desired equalizing effect (check voltage across each capacitor to be sure).

    73 de Jim, N2EY
     
  7. W0GSQ

    W0GSQ Ham Member QRZ Page

    True enough Jim.
     
    N2EY likes this.
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    There were several designs that used discrete electrolytic caps in series without balancing resistors. So long as the caps are new and have the same date codes they probably will do fine. Capacitors voltages will self equalize; and actually at the lowest possible forming current, since it is a series circuit. If a cap fails, it was probably at a point where it was ready to fail regardless. I doubt those low end ham manufacturers attempted to match the individual filter caps. As I recall the Globe 65 Scout was one example.

    The balancing caps are a good idea if only to provide a discharge path for the electrolytic caps. I'd stick with modern metal oxide power resistors instead of the older carbon resistors. When I was maintaining a fleet of elderly GE base stations they ran the B plus supplies continuously, and the carbon bleeder resistors all would go lower in a value and eventually fail.
     
    Last edited: Apr 7, 2021
    W0GSQ likes this.
  9. W0GSQ

    W0GSQ Ham Member QRZ Page

    Thank you Peter. When I restuffed mine had no trouble fitting in resistors into the cardboard tube since modern caps are so much smaller.
     
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    Watch the heat.
     
    W0GSQ likes this.

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