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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