SB 200 Plate Choke?? Needed

[Steve Harrison]

At 06:54 PM 07/13/98 -0500, Mike Souhrada wrote:
>Picked up one at yesterdays hamfest in Peotone.
>Need a plate choke to replace the broken one.
>PN 45-61  50uH RF Choke
>(A burned up one is ok just as long as the ceramic
>is intact.) Previous owner must have omitted the
>fiber washer on installation and cracked most of
>the bottom off.

You can quickly repair one that's not totally destroyed if you still have
the pieces and can invert the ceramic form so the mounting screw goes into
what's now the top. Build a new fiber washer from cardboard, cut with
scissors or your wire cutting diagonals. Punch a hole with an awl or an
Exacto knife or your longnose plier tips, then enlargen the hole for the
screw by cutting at the edges of the hole with your dikes.

Using epoxy, glue the ceramic pieces back together, trying not to get epoxy
inside the screw hole. You can try to secure a tie wrap around the pieces
to hold them in place tightly, or perhaps use string or a piece of
small-size bus wire. Let the epoxy cure for at least a day, and longer if
you're not the impatient type. Strip off the tie wrap or string (with your
knife) or hookup wire. Then, using a bottoming tap (a regular tap with the
tip ground flat so the tap body screws into the hole as far as possible) or
a long screw of the proper size with several slots filed lengthwise to
carry tapped material out of the form, very carefully clean the epoxy out
of the screw hole. Screw in the top screw very gently with the the
parasitic suppressor solder lug; DO NOT TIGHTEN VERY TIGHT! Since the top
of the plate choke gets pretty warm from the tube radiated heat, you might
want to do something such as wrap heavy bus wire around the top of the form
which will support the solder lug in case the epoxy melts.

Then get back on the air.

Meanwhile, of course, keep searching for a replacement (try Harbach's in
Florida; they have a web site but I don't know the URL) and keep your eye
on the repair job while operating! (You will help it stay cooler and
survive longer if you place a muffin fan on top of the amplifier, perhaps
only running when you're transmitting.) You may be able to obtain something
that will work from Fair Radio, where I saw a bin of ceramic standoffs and
forms just last week during a business trip (a biz trip to Fair Radio??
SURE, the Boss said...).

If you have a grid dip oscillator and find another form that's not quite
the same size, you have little to lose by trying to wind a new choke, using
the dip meter to determine whether there are resonances inside the ham
band. If you have the dip meter, you should try dipping the existing choke
anyway just to get an idea of what it's like which will give you an idea of
what you can get away with on a new choke. If you have to, you can use a
form that is shorter than the original just to mount the choke, then use a
screw with the head sawed off and filed flat to join a shorter standoff to
the choke's; then screw the shorter standoff to the chassis as usual. This
would allow you to use forms that don't have holes drilled through them to
secure the coil's wire ends; just install a solder lug between the two
forms. Don't varnish the choke form since the varnish will probably burn
with the heat from the tubes.

Don't forget that you can use teflon rod if you happen to have a source. It
can be easily drilled and tapped by holding the drill in your hand but will
not have very much tensional strength in the screw threads. But teflon will
also deform when it gets hot and you may notice the rod drooping after
awhile; so keep looking for a permanent ceramic form.

73, Steve Ko0U/1

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