SX-28 Mystery Mod ??

Ron Evans cosmos41 at IX.NETCOM.COM
Thu Apr 13 22:41:24 EDT 2000

At the risk of exposing my considerable ignorance,  I'm hoping someone
can help me with a little bit of a mystery here concerning a very nice
SX-28 I purchased recently.

It came in another case (SX-43??) but also came with the original
cabinet, which I had repainted at an auto body shop.  It came out
looking incredible! But when I took the old gal out of the previous case
to put it in its original
one, I noticed a red-striped white wire dangling down from under the
chassis, unattached on one end.

I could never find where it was supposed to be attached and looking at
the schematic just made things weirder and weirder.  The other end of
the dangling wire is soldered to a pin of a tube socket attached UNDER
the chassis.  The socket and tube are held to the chassis by a clamp in
much the same way some electrolytic capacitors are attached to the
underside of a

The whole thing is VERY "hard-wired" in and seems to be related to the
second stage of the audio output circuit since the mystery tube is wired
to pins of V14, one of the two 6V6GT's in the push-pull circuit.  (The
tube and socket are wired in in such a way that I can't even find out
what kind of tube it is without doing a fairly major "gutting" in that

The "mystery" tube has, in addition a couple of resistors, a capacitor,
and an rf choke wired to its various pins...along with the red-striped
wire that is loose on one end.

The whole thing appears to be some kind of mod, since that tube makes a
total of 16, and the SX-28 only shows 15 on the schematic and in the
literature I've read.

I'm not the greatest in the world at reading schematics, but I can't
find anything remotely resembling this "kluge" on the schematic, and I
certainly should be able to find a 16th tube if there WAS one.

The receiver still appears to "play" but I'm very curious as to what
that "mod" (if it IS a mod) is supposed to do.  I'm thoroughly baffled
and just wondered if someone can help.

The red-striped wire seems to have been attached on the other end
somewhere near the underneath back left side of the chassis, since it
was sort of threaded through other areas to get to that general
vicinity.  But, try as I
might, I can't find any evidence of where it might have broken off from.

Any ideas?

Best Wishes,

Ron - K5MVR "Puzzled in Fort Worth"

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