[TenTec] Omni VI Xmit Bias: TT Mis-Instructions?

[Peter Hoon]

Fellows, Tks for many useful posts on thread "Omni VI + Intermittent Power
Amp?".  Will post results on this later after further testing; so far glitch
has not returned and am looking good on this one.

However, this is a new thead and a new problem.

Questions for you at the end below.

The instructions on page 4-28 of the Omni VI Plus manual (First Printing,
01/97) for adjusting the two bias pots in the transmit PA module read as
follows:

"The bias adjustment pots R19 and R15 are accessible through the bottom
shield of the transceiver.  To adjust PA bias, a dc amp meter must be
inserted in series with the large + 13 Vdc lead coming from the DC Input
board.  Unplug connector 79 from the Low Level Driver/NB board to remove
drive from the PA.  Key the transmitter in USB mode.  While watching the amp
meter, rotate first the final bias pot R15 and then the driver bias pot R19
to produce minimum current.  Now rotate the driver bias pot R15 to produce
an additional increase of 500 mA (.5 Amp) on the meter."

I tried this, and ended up with class C operation and completely garbled
SSB.

The instructions do not make any sense, because if you decrease R15,
clockwise, the idle current drops well below .42 amps and the pot is turned
all the way clockwise for minimum current (an ending position that obviously
was not intended by design engineers).

In the same vein, if you decrease R19 by rotation in a counter clockwise
direction, the minimum current position is all the way counter clockwise,
also a position that was obviously not intended by designers).

What to do?  I went to page 3-14 of the Omni VI Plus manual, and adjusted
the pots to the same position shown in figure     3-14 "Model 564 bottom
view".

On an adjacent receiver, the SSB sounds "pretty good" by my ears and those
of my XYL (though have not tested on air).

Adjustment of bias pots by the "picture book" method certainly is not
optimal in the long run.

In CW, with these pot positions, am getting easily 100 W out on 80M, 40, 20,
17, 12 and 10.  A little low, 80 W out on 160M and 15M (but can be cured by
refined ALC readjustment).  The current draw to the power amp for these
powers is consistently 14.66 amps, which I fear is low.

Questions for your comment:

1)  Were there corrections to the above instructions by TT?  Does anyone
know if TT came up with new, corrected instructions for adjusting PA module
xmit bias pots?

2)  Think I really need to apply a two tone test generator into mike input,
and need to adjust the pots for the "best looking" symetrical double
trapezoid on my scope  Do you agree? (Need to build or buy the two tone
generator). Are there better alternatives?

3)  Is my current draw to the PA module, about 14.66 amps too low under CW
conditions (that is a total, for both the pair of drivers, Q1, Q2 and
finals, Q3 and Q4)?  Somewhere, in the back of my head, I recall a number of
around 18 amps as "normal" for 100 watts out CW in this radio.

4)  Is my adjustment problem due to some other problem with the PA module
(visual inspection and reheating bias pot soldering joints completed with no
change)?

Would appreciate ideas or comments you have for adjusing a broad-band solid
state PA to the best class AB linearity, of the kind used in the Omni VI
Plus (and probably other TT radios).

Thanks

Peter
VE1CHS
Nederland, Colorado