SX-28 Mystery Mod ??

[Stephen C. Finch]

Ron,

I don't know the radio, but look to see where the plate of the tube is
connected to.  It may be a BFO so the radio could receive SSB.  Or maybe
some kind of AGC.  The part of the original circuit where the plate (or
cathode depending upon the circuit) is connected may give you a clue.

Good luck,

Steve Finch
AI0W

Ron Evans wrote:

> 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
> chassis.
>
> 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
> area.)
>
> 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"