According to the downloaded manual, the ONLY thing that changes between
75 and 80 meters is the value of the fixed capacitor at the loading
point in the output Pi-L network. The coil taps and the input circuit
(they call it a low pass filter) don't change. It also appears that the
rotation of the switch wafer for the tuning capacitors is opposite to
that for the two coil tapping and load capacitor switching wafers.
Changing the padder capacitor collection would explain the different
loading capacitor position, but shouldn't really change the input impedance.
So the high SWR on the 75 meter position must come from a switch contact
problem or a bad solder connection from that band position to the PC
board where the band contacts are connected together.
73, Jerry, K0CQ
On 11/25/2010 10:15 AM, Rob Atkinson wrote:
> Hi Gary,
>
> You never said what your load Z was but it together with your feedline
> length could require more inductance in the amp's pi network.
>
> Once you adjust the "tune" and "load" caps for maximum output you
> should increase loading a bit. This can be confusing. Ten Tec labels
> their capacitors 0 to 10 with 10 being maximum capacitance (plates
> meshed). You therefore decrease capacitance just a bit to increase
> the loading. Watch the grid current when you do this.
>
> I'm interested in how Ten Tec does input Z matching on the old Titan.
> It is possible that the bandswitch in addition to switching taps on
> the output pi network coil, also switches different tuned matching
> circuits on the amp's input. so when you switched to 80 m. you
> inserted a different input matching circuit.
>
> 73
>
> Rob
> K5UJ
>
>
> <<<During the past weekend I participated in the phone Sweepstakes and
> observed
> high VSWR on my OMNI-VII while driving my Titan 425 using the published
> settings for 75 meters when operating between 3.8 and 3.9 MHz.
>
> The manual suggests; BAND 75, TUNE 5.5, LOAD 7.5, FREQ 3.9 MHz. but with BAND
> 75 selected the load control was near 10 and the driver VSWR was nearly 10 to
> 1!
>
> By switching the Titan bandswitch to 80, the LOAD setting became 3, TUNE
> stayed
> at 5.5, FREQ stayed at 3.8 – 3.9 and the VSWR dropped to 1.1 to 1.
>
> Over the twenty plus years I’ve had this Titan 425-E as the original owner, I
> had seldom used it on 80 and even less on 75 so I had not observed the high
> VSWR. A new OMNI-VII with its “High Current” notification and shut-off
> protection got my attention to the fact and drove me to the solution.>>>
>
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