My Omni VI plus developed a problem with the frequency tuning. Tuning across a
steady carrier signal exhibited a beat note frequency jump of several hundred
hertz whereas the frequency should change gradually in almost imperceptible 10
Hz increments. This was not a catastrophic failure but it was annoying when
working CW. It was not noticeable to me on SSB.
After making some measurements on the 5-5.5 Mhz PLL circuit, I determined that
the problem was a shift in frequency of the 39.94 Mhz crystal in the secondary
loop of the PLL circuit. This oscillator is not phase locked. The shift is
probably caused by normal crystal and/or component aging. The primary loop
tunes in 400 Hz increments. The secondary PLL loop fills in the in-between 10
Hz frequency increments required for 'smooth' tuning.
At any rate, the fix was to adjust the crystal to frequency with the variable
coil adjustment L6. See the schematic of the 5-5.5 Mhz PLL circuit.
Remove the top cover four screws.
In order to perform this adjustment it is necessary to move the metal plate
onto which is mounted the Low Level Driver board 72870 (although it is
identified as the 81599 board in the manual), and the FM board 81598, out of
the way. The PLL board is underneath.
Remove coax cables 33, 75 and U from the FM board.
Remove coax cables 24, 31, 32 and 79 and white plastic connectors O two-white
wires, 62 one white wire, 89 seven colored wires, from the Low Level Driver
board.
Remove the six screws holding the metal plate with the two boards.
CAUTION: Three screws are phillips head and the 3 remaining are slot head. Not
knowing any better, I removed all six at once and put them aside. When it came
time to re-attach the plate I discovered that the phillips screws only go into
their 3 original holes. In other words, you can't mix them and re-insert them
in any hole. Why this is so shall forever remain a mystery hidden under lock
and key in the secret archives of Ten-Tec. Hi. At any rate, keep track on which
screws go in each hole in order to avoid a hair pulling session just when you
think you're almost done.
Lift out the board and let it hang out the left side of the cabinet.
Adjust the main frequency tuning knob so that the front panel display reads
xx00.01 Khz on any ham band although I chose 3900.01 Khz. You can hook a
frequency counter to the anode of either diode D1 or D2. L6 is located next to
the crystal Y1. It should only be necessary to turn the adjustment screw a
minute amount not over a tenth of a turn , more or less, either CW or CCW, to
39,940,000 Hz on the counter. Use a plastic screwdriver tip tool.
re-assemble everything and away you go!
After sweating the above procedure and thinking it over, I came to the
conclusion that there is an easier quick and dirty way to accomplish the same
result.
Proceed at your own risk: Remove the six screws. Do not unplug any cables. Lift
the upper left hand corner of the plate as much as you can. You can 'sort of'
insert a plastic screwdriver tool into L6 and adjust the tuning core a smidgen
at a time. After each nudge, tune the frequency tuning knob across a steady
carrier of a CW signal and see if the annoying jump is smoothed out. If not,
adjust the other way and check again. If you adjust too far the tone will NOT
CHANGE as you twirl the knob! If you get lost, go grab your frequency counter
and do it the hard way.
I tried both ways and it worked fine for me.
Afterwards, I checked the transmit output frequency and it was within 3 hertz
of the front panel display readout. I thought this was good enough and didn't
fiddle with things any more.
73, AL. w6wqc
P.S. My frequency counter hasn't been calibrated in a long time. Can't be much
off since I jazzed up the time base oscillator however. :)
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