[TenTec] Omni VI problems, continued

[Ken Brown]

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