I'm encountering a strange problem with my Titan. If I follow
standard practice and achieve a 1:1 match with the exciter, then
take the amp out of standby, the amp typically shows mismatch on
the order of 1.5 or 2:1.
If I then re-tune the antenna for minimum reflected power on the
Titan, the VSWR at the tuner, and also at a Bird inserted right
after the Titan, shows a mismatch of 1.5 - 2:1. The mismatch seems
to be greater at higher frequencies.
The bridge in the tuner and the Bird are consistent -- that is,
when the bridge in the tuner shows a match, so does the Bird, 12
inches of RG-8 from the Titan's output connector.
I have three different tuners -- a Ten Tec 229B, a Ten Tec 238A,
and Drake MN2000. It happens with all of them. I've traded cables,
made them as short as possible, changed the switch that switches
between the tuners (from a B&W to an Alpha Delta). The exciter is
going through the Titan to get to the antenna for tuneup. I've
traded Birds. I've eliminated the Bird. All of the cables are RG-8,
all are short.
So you say, go with the bridges. BUT: Grid current in the Titan
tends to be much lower when the Titan bridge indicates zero
reflected power, and rises rapidly when I don't. And grid current
is the big NO-NO in 3CX800A7's, the output tubes in the Titan. I am
careful to keep grid current down to an indicated 30 mA for the
pair (120 mA is absolute max). I can't get close rated power
without excessive grid current unless I tune the antenna for zero
reflected indicated on the Titan.
The SWR indicator in the exciter (TS850) also agrees with the Bird
and the bridge in the tuner -- IF I bypass the amp's RF circuitry
and connect the radio direct to the tuner (or through the
switches). This points to the RF path within the Titan between the
SO-239 output connector and the final output stage. Has anyone
encountered this before? Any fixes?
73,
Jim Brown K9YC
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