High voltage electrolytics in old gear

[N8ETQ]

   Hey,

      I have often wondered how they came up with these things
back in the day, when they are just now becoming somewhat
common.


   A couple of weeks ago I posted on the QSO
forum about a partially assembled Johnson
Adventurer Kit that I got to complete.

http://amfone.net/Amforum/index.php?topic=35908.0

   Anyway it had 1 missing part and 1 failed
cap. I added a 20K 10 W screen dropping "R"
and clipped out the failed 8 uF 700 V filter
cap. 1 of 2. The one hooked to the load side of
the choke seems ok but the one on the ripple side
got real hot real fast.

   So I added a few appropriate caps to my order
and they came in today. I waited to make sure they
would fit in the tube before I did anything with
it.

   Well this evening I opened the original
cap and was surprised to see 2 caps in series.
No equalizing "R"'s either. I know some caps
had a "R" built in but if these do I can't
read it. Maybe that's what wrong with him.

  Pix 1 is what I pulled out shown above what's
going in.

  Pix 2 is the finished product. + end

  Pix 3 is the - end. The "Shot Shell" type
Roll crimp is a PIA to be sure.


   So it's now a 12 uF 900V cap w/ equalizing
R's. Should last a while.


/Dan

[Opcom]

That's a great tip for keeping things looking stock and beautiful.
I confess I don't take the time to do it.

[K7MCG]

Looks very nice, but I would be concerned that the heat from the equalizing resistors would reduce the life of the new capacitors.
73,
Chuck K7MCG

[N8ETQ]

Looks very nice, but I would be concerned that the heat from the equalizing resistors would reduce the life of the new capacitors.
73,
Chuck K7MCG

    Thanks, I don't normally do it but with those 2 caps being
the largest components under the chassis I felt it was a good
candidate.

    They will warm up a bit but there not bleeders. 750V/2 X 470K
should be just over 1/2 watt or 1/4 w per "R". I seriously doubt
that in another 60 years there will be anyone left to appreciate this rig.
There just in there to divide the "E". 85C temp = 185F. I use candle wax
to hold stuff in place and have never seen on of mine "Dripping".

    FWIW I'm more concerned about the original remaining cap. As
this thing was designed for 115 VAC. 6:1 ratio on the xfmr adds
about 60 V to the Ep. After I get a chance to check it out
further I may move this cap over to replace the other one and
leave it as a Choke input filter. That should help.

73

/Dan

[WQ9E]

Dan,

Nice work and the original dual unit part did not use any sort of internal equalizing resistors, at least on the type I am familiar with.  One of the old radio magazines had an article interviewing a designer (CD I believe) and the individual units were 450V rated and were considered closely matched during manufacturing so no equalizing was needed given the conservative rating.  That of course could change with aging or especially if one half of the container was mounted near something hot.

You should have no heat problem as your calculations have shown and you would probably also be fine with a couple of modern units from the same production run and no equalizing resistors.  I remember reading the production specs for one of the standard vintage caps and it was rated as -40%/+100% of marked value so it is easy to see why equalizing resistors would be needed for run of production caps back then.

If heat was a concern you could make it look almost perfect by by adding an additional access lead out to the junction of the two and then mount the resistors externally.  The added lead could exit from the side of the cap where it is hidden on the bottom side or out of either end.

[W0GSQ]

Looks identical to what I found and did with my Adventurer. That hard to find high voltage cap you see guys posting about wasn't ever that special after all!

[KA0HCP]

disregard..............