Ken -
I have had power vs band output problems on my Paragon II. I suspect the
same
correction to make all bands full output could apply to your OMNI 6. The P2
uses can
shielded slug tuned coils versus capacitors in the OMNI 6.
The input crystal filter on the PBT board required some adjustment to bring
the power
level to 100%. There are two adjustment capacitors on either side if the
crystal. By
tuning these miniature caps you can vary the drive level.
All this came about when I found the RF out on my P2 was half power on 160,
15, 12
and 10M. I tried to adjust the input slug in the first can (in my case)
which refused to
turn back up when I turned it farther into the coil, Had to send the rig to
Ten-Tec who
graciously did the coil replacement for me. The rig came back 100% RF
output all bands.
Fortunately my OMNI 6+ has no problems. Am referring to an OMNI 6+
schematic to
advise you of the PBT input adjustment that could be the main problem.
Hope this is of some use, Ken,
73 - Don, ve1bn@eastlink.ca
---- Original Message -----
From: "Ken Brown" <ken.d.brown@hawaiiantel.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, May 28, 2008 4:46 AM
Subject: [TenTec] Omni VI problems, continued
> Hi all,
>
> My Omni VI is not producing full power on all bands. I have not fixed it
> yet, though I am probably closer to understanding the problem than I was
> when I last posted to the reflector about it. I have barked up several
> wrong trees, and by describing the process I might be able to save
> someone else experiencing similar problems a bit of time. So here goes:
>
> When I first noticed that my TX power output was low on 160 meters, I
> also noticed that the ALC LED was coming on in CW mode on all bands at
> all PWR knob settings, whenever the key is closed. This is something
> that I had never paid attention to before, so when I noticed it, I
> thought that the ALC circuitry was cutting back the drive. I spent many
> hours focussed on the inputs to the ALC circuit, trying to figure out
> which input might be causing the problem. Eventually I got in touch with
> Ten-Tec service on the phone and found out that the ALC LED always comes
> on in CW mode during key down. I have not found any mention of the ALC
> LED in the CW operation portions of the manual. I have learned a lot
> about the ALC circuitry in the Omni VI, though this has not got me any
> closer to fixing the low TX power problem.
>
> Ten-Tec service suggested looking at the LOW PASS FILTER BOARD. One
> suggestion was that there are diodes on that board which can fail and
> cause a reduction of transmit power output. Another possibility is a
> failed relay on that board. By disconnecting the PA output coax from the
> LPF board and connecting it directly to my watt meter and 50 ohm
> termination I proved to my satisfaction that no significant amount of TX
> power was being lost in the LPF board. This method may be flawed, since
> those low pass filters are not exactly symmetric. They don't have the
> same capacitances on their inputs as on their outputs (except the 10/12
> meter one), so they may provide some impedance transformation as well as
> filtering.
>
> I probed around using a 50 MHz scope, to see how the RF voltage levels
> compared at various places, from the ALC circuits on the 9 MHz IF board,
> through the low level driver and on to the input of the PA. Finally I am
> starting to get some interesting data. The 9 MHz IF TX output is the
> same regardless of band selected, so there is not something going on in
> the ALC circuit that cuts the power back on some bands and not on
> others. The TX RF voltage at the input and the output of the low level
> driver board is NOT the same on all bands. This could be in part due to
> a not quite properly compensated 10X probe and oscilloscope input,
> except the voltages correspond so well with the power outputs on each
> band. I have decided the lack of TX power on some bands is due to lack
> of drive to the low level driver.
>
> The low level driver board gets it's TX RF drive from the BPF/FRONT END
> BOARD, and that gets TX RF from two sources, the 9 MHz IF and the XTAL
> OSC LO MIXER BOARD. Now I am suspecting that the LO power is lacking on
> some bands, though BPF switching problems might also be the cause of low
> power on some bands. I've had some frequency instability on some bands
> too, so I'm leaning towards a fault in the XTAL OSC. That board is not
> easy to get to. I finally saw it for the first time tonight. It is a
> really well built, good looking board, and the schematic shows only a
> single electrolytic capacitor on the whole board. That electrolytic
> capacitor is in the part of the circuit that "sets the bias point of the
> oscillator stage to stabilize the oscillator output level", so it seems
> plausible that a failure there could cause the problem I am having.
>
> I'll let you know what happens after I change C19 on the XTAL OSC LO
> MIXER BOARD.
>
> DE N6KB
>
>
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