Current thinking on tuner design is that the balun should be in thi input,
rather than at the output of the tuner. The problem is that the balun is
operated at the mismatch, call that high SWR, and therefore is inefficient and
does heat up. I suspect that this is your problem. At high power, it can occur.
For full info, read the latest ARRL Handbook. The give an excellent
explanation. Also see the Palstar web site. My AT5K uses the new design theory
and the balun behaves nicely
Dave
WA3MKB
-------------- Original message --------------
From: Douglas <sparks06524@yahoo.com>
>
>
> I recently got an older high power Ten-Tec tuner Model
> 229B. It's basically the same as the current
> production L-network 2KW tuner.
>
> While using 300 ohm line and it's internal balun, the
> SWR increases rapidly with key down time when I'm
> using 500 watts or more power. The impedance will seem
> to shift up, if the tuner is retuned it seems to
> rapidly shift down.
>
> The problem is in the tuner since things work fine
> with my old Murch UT-2000B under identical conditions.
>
> I suspect that something is heating up.
> I've tightened grounding connections and checked the
> balun coil for heating (it never seems warm to the
> touch). Also, I've cleaned and checked the rotary
> inductor connections - seems fine. The fixed
> capacitors also don't seem to be overly heating.
>
> Another thing that might be suspect is the phenolic
> plastic standoff insulators for the 300 ohm
> connections on the rear panel. One arched internally
> and I double insulated the connection screw as it went
> thru the panel with a layer of 3000 volt rated
> shrinkable tubing. No more arcing but the variable SWR
> problem continues. I've never seen non-ceramic
> insulators used in such applications.
>
> Has anyone seen similar problems with this series
> tuner under high power conditions?
>
> tnx,
>
> Doug/WA1TUT
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