[ARC5] ARC5/SCR274N Tuning Cap Bead Replacement

[DaveJ]

All:

For those of us that have missing glass beads in your tuning capacitors, I
have found a replacement glass bead that works well.

Hobby Lobby's sku 355651 7" long, 4.5 mm diameter Blue Round Rivershell Bead
Strand $3.99 (on sale for 50% off, $1.99).

The SCR-24N and early ARC-5 caps have a bead that looks like the planet
Saturn and later models use a plain round bead that measures 0.177 (appx 4.5
mm) across the N & S poles of the bead.  I'm just speculating but the extra
plastic flange could have been there to increase the leakage or just to make
the installation easier.  At any rate, the "ring" isn't there in the later
model ARC-5s.  

I haven't Hi-Pot tested it yet but I'm not expecting problems. The spacing
on the bead is greater than the spacing between capacitor plates.  I doubt
if any of us are going to run into arc-over problems anyway as very few of
us live over 10,000 feet.

It is a bit tricky getting the bead in place in the capacitor but once you
get it in there, it stays put so you can adjust the spacing screws.  Install
it on a flat, cloth covered area to keep the beads from running away- a lot
like SMD components!

I used a #53 or #54 drill bit to gauge the spacing between the plates.
Measure somewhere in the middle of the cap so you don't have to worry about
bent plates. Adjust both side of the cap to make it uniform.

You can also adjust for minimum capacitance but it is very insensitive and
you can be off by quite a bit.

Photo of bead installed in capacitor attached.

sincerely,


Dave WA4BOJ

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