I've got a problem with my Omni 6+. This is an Omni 6 with the complete
upgrade, which includes the new 9 MHZ MIXER board.
I first noticed that the readings on my Omni meter during transmit all
seemed to be low (Ic, FWD, SWR). The S meter reads normally. I rarely (if
ever) use the meter during transmit, because I rely on my "Autek" meter to
read "power out" and SWR. For this reason, I thought that maybe Ten-Tec
forgot to align the board when the machine was returned for upgrade, but
that I just hadn't noticed it. (After the unit was returned from Ten-Tec
for the upgrade, the wires from the ON/OFF switch to the DC Input Board were
pinched under the Speaker Bracket, causing a short, resulting in a blown
fuse and damage to a transistor in my power supply. I know that the Ten-Tec
technicians ARE excellent, but they do occasionally make a teensy-eensy
mistake.) So, I proceeded to do the alignment for the 9 MHZ MIXER.
During alignment, I was able to get significant meter movement, but still
have problems as indicated below. Note: all readings were taken with a 13.8
V supply, and a Drake 300W dummy load. Output power was measured with a
Diamond SWR/Power meter, which was confirmed to be approximately correct by
using an RF current meter.
Ic:
Before alignment, the maximum Ic as indicated on the Omni meter on any band
was 5 amps. The actual Ic was much higher, as measured by a digital
ammeter, which was inserted into the power supply line (power supply current
measurement reduced by 2 amps to allow for non-final currents). After
alignment, the Ic indicated by the Omni meter matched the actual current as
measured by the digital meter. I still seem to have a problem, however,
because the maximum current attainable on 20 meters is 12 Amps. Ic by band
is as follows:
160 17 amps
80 22 amps (current limited)
40 22 amps (current limited)
30 22 amps (current limited)
20 12 amps
17 15 amps
15 16 amps
10 18 amps
FWD:
After alignment, FWD readings are as follows. The Omni meter shows zero, or
lower than actual on 160 through 30, and higher than actual on 15 through
10:
Omni Actual
160 - 100
80 - 100
40 20 100
30 40 100
20 70 70
17 90 90
15 100 80
10 100 85
SWR: There is no SWR reading on any band (attached mismatched antenna, and
ran low power).
Other symptoms: No ALC light during voice peaks (even with PWR at max),
although the power output meter indicates RF voice peaks are normal. The
ALC light did go on and off occasionally during alignment, so the LED is OK.
On the bands that had adequate Ic, the Ic limiting circuit and the R54 pot
worked OK.
I studied the schematic, but was unable to achieve a complete understanding.
I suspect that the diodes in the "fwd/ref" circuit on the Low Pass board may
be bad, causing the wrong voltages at connector 12 on the 9 MHZ MIXER board.
It looks like the "fwd" voltage from connector 12 also drives the ALC
circuit. Could the ALC circuit cause the low Ic on 20 meters? Does the ALC
circuit result in signal amplification at the "Tx OUT" connector, or does it
only serve to "limit" the signal?
If possible I would like to fix this myself. I'm out of work right now, and
can't afford to ship the Omni back to Ten-Tec. Actually, we could afford
it, but my wife prefers we use the money for groceries (I can't understand
this, but it must be very gracious of me to adapt to my wife's preferences
on this occasion). Even if I can make the repair myself, I may need to get
the parts from Ten-Tec (some of the part numbers don't look too familiar).
That's the story. If any of you can provide some advice or other insights,
it would be very much appreciated. I may end up calling Ten-Tec, but I want
to get as thorough an understanding as possible before I proceed.
Thanks much,
Dan - W4TQ
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