[Boatanchors] Johnson Matchbox 275W

[Rob Atkinson]

Hi Larry,

A lot of the time when you get a Matchbox it will have all kinds of things
done to it by previous owners.  What I have done with one of mine is I
removed everything in there that wasn't needed for it to be a simple
balanced tuner straight through from unbalanced input to balanced output.  I
took all the parts, the relay, the rear panel terminal barrier strip and so
on, and put them in a zip lock back and set them along side the tuner so
that they can be put back in if anyone ever wants to, assuming someone else
will have mine some day.   All I wanted mine for was to tuned balanced line
to 50 ohms tx and rx and I figured I could do the T/R switching the usual
way at the rig with something like a dowkey relay or some other arrangement
and keep the tuner simple.  So now I just have the UHF jack on the back of
the Matchbox with a strap going right from it to the link, and the two
balanced line feed throughs going to the capacitor and that's it.

Originally there was the internal T/R relay that switched in a separate link
tap on rx for 300 ohm balanced line to the rx via the terminal barrier strip
(where you also connected the relay coil v.) and that can still be done for
any rx that has higher Z balanced line connections only, but it seems like
your R388 is 75 ohm unbalanced?   so if I were you I'd just remove or
isolate and bypass all the relay associated circuitry and and run the
Matchbox as a basic straight through tuner and do your T/R switching
elsewhere.   Of course this results in another "highly modified" Matchbox
but it is reversible.   The T/R relays Johnson used, are pretty good so you
could of course work out a way to do your switching with it but your T/R
switch would only work for anything connected to the Matchbox.  I mentioned
getting it out of the MB if you have other feedlines and antennas to switch
and you only want one T/R switch.  I expect I have added to the confusion
but I hope not.

73

Rob
K5UJ

On Wed, Jul 27, 2011 at 1:34 PM, <telegrapher at q.com> wrote:

>
> In hooking my 275W matchbox to the Viking II and back to the R-388 i'll be
> using with it i noticed that i could not get any signal thru it in the
> receive direction.  Well one thing is that the receive side is 300 ohm out
> to the receiver.  Hmmmm, a TV 300/75 ohm balun fixed that.  Still no signal.
>  Why does it have the extra coax connector on the back side.  What was it
> put there for?  Better pull it apart and see what is going on.  True to
> Johnson products, there are somehwer close to 10, 000 screws in it.  Well
> not really but if you've taken the bottom off the Viking II, and i suppose
> the Viking 1 is similar you know how to remove lots of screws.   Back to the
> topic at hand.  When i finally got the cover off the problem became pretty
> apparent.  On the screw terminals for the antenna lead to the receiver,
> nothing was attached to the supposed hot side.  The coax connector that was
> added, apparently part of a mod goes to the common leaf on one side of a 3
> pole relay.  Both cont
>  acts whether operated or released runs the center contact of the coax
> connector to ground.  Wonder what was accomplished there?  The RF input
> connector which goes to another side of the relay was totally bypassed so
> the tuner was in the line all the time and could not be switched out.
>  Simple jumper removal and addition here will fix this.
>
> Why or what was the purpose of the mod to add the coax connector and were
> there other changes included?  I've got the documentation off of Edebris or
> the Bama site but there is no mention of any mods.  I think i've heard from
> other sources that the 275W matchbox did have a coax mod done to it and i
> think it is for the receiver side but not sure.  Any clues from other users
> that have one of these?  Otherwise it appears to be a very nice clean unit.
>
> Larry
> W0OGH
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