It's time for another episode of the ongoing saga of N6KB's Omni VI
transmit power output problems and troubleshooting.
In the previous episode I had come to the conclusion that the low output
power on some bands was due lack of LO power at the input of U1 on the
BPF / FRONT END BOARD, which also contains the final transmit mixer
where the 9 MHz IF gets heterodyne converted with the LO to the final
transmit frequency. I had scoped the LO signal level here and found a
good correlation between that level and the final TX output power. On
the bands where the TX power was low, the LO signal level into U1 was
also low. This led me to suspect problems in the source of the LO
signal, the XTAL OSC. -L.O. MIXER BOARD. It seemed like adjusting the
slug tuned inductors on the crystals associated with the bands with low
output power, might be necessary to get the LO performing better on
those bands. The problem then was determining what is the criteria for
proper adjustment.
I guessed that monitoring the DC voltage on connector 84, which
eventually controls D26, a varactor diode, and adjusting the slugs for
the same voltage that the full power band had, might be the way to go.
Other ideas were to determine the total DC voltage range at this point
and adjust for somewhere near the center of the range. I tried these
methods and was not very successful, although I did determine that the
adjustment of the slugs was affecting the transmit power output, which
was encouraging.
I had not found my set of alignment tools, and had adjusted some of the
slugs with a small screwdriver. That was a mistake! I cracked the
ferrite (I guess it is ferrite) slugs for two bands. I called Ten-Tec
service to see about getting some more slugs, and to find out what is
the correct procedure for adjusting the XTAL OSC. slugs. I was able to
get a couple of slugs sent in the mail, and get the essential
information for making the adjustment.
Here is how Ten-Tec told me to adjust the slugs:
1) Check and or adjust as necessary the 20 MHz reference oscillator on
the LOGIC BOARD. A frequency counter on pin 8 of U12 worked for me. It
is divided down to 10 MHz at this point. (pin 9 is the one the feeds the
PLL, but pin 8 is just the inverted version of the same thing, and the
corner pin on the 14 pin IC is much easier to get a probe onto.)
2) Monitor pin 11 of U13 on the LOGIC BOARD with a oscilloscope using a
10X probe. Scope at 0.2 V/DIV (2 V/DIV effective with the 10X probe) and
0.1 uS/DIV.
3) Adjust the appropriate coil slug, for the band you are on, for the
narrowest pulse.
This works! My Omni VI is now putting out full power on all bands. I
have not yet put it all back together, because the 30 Meter Band XTAL
OSC. slug does not want to unscrew out of the coil form. These slugs are
very soft material, and the threads on them wear out easily. Once they
get pushed in to far, if you cannot get them to unscrew, you have to
remove the coil to get the slug back out the other end. The PC board
does not have holes beneath each coil like some equipment I have
serviced, so you can't just remove the board and screw them from the
opposite side. You have to unsolder the coil and the coil can to remove
it from the board, before you can push the slug out. That will be the
next thing I do, and when I get the slug out, I'll probably use the
trick my uncle taught me, which is to put a bit of dental floss inside
the coil form to make the slug a bit tighter.
As soon as I get the 30 meter slug working again, I'll be able to put
the rig back together. The transmit power is already back up to normal
on all bands, so I'm almost ready to get back on the air.
I hope this is helpful to some of you who may have similar problems with
your Omni VI.
Until next time,
73 ES ALOHA DE N6KB
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