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[TenTec] Omni VI Logic board layout error

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: [TenTec] Omni VI Logic board layout error
From: Gary Hendricks <k4mt1950@charter.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Tue, 29 May 2018 10:32:23 -0400
List-post: <mailto:tentec@contesting.com>
I have been watching the PLL circuit operate on my omni VI and in particular the control voltage fed to the osc board to pull band crystal frequencies.  I noticed after the radio was warmed up the control voltage cycled up and down by as much as .25 volts.  I also noticed I was having trouble decoding FT8 signals as they seemed to have a tilt first left then right. I also tuned WWV carrier and watched it on a waterfall and instead of a straight line it looked like a sine wav as freq shifted up and down.

I looked at ht charge pump consisting on Q3 through Q6 and noticed strange voltages on one of the transistors. There are two pairs on operating from error voltage phases out of pins 14 and 15 on the PLL chip U13.  Measuring across R14 and 15 I saw only one phase was working.

I checked the transistors and found one shorted from collector to base.  I have not looked to see if it was  Q3 or Q4 but looking from the front of the radio its the once closest to the pll chip.

I pulled the logic board and to my surprise found a trace between base and collector on the transistor. Both sides of the charge pump have to be the same to operate correctly so this short was obviously wrong. I cut the trace and now the control voltage is rock stable and varies no more than .01 volts. WWV carrier looks very clean and straight and fuzz that was present before is gone.

This is easy to check. Just put ohm meter between center and right pin on transistor closest to PLL and if it reads a short you have the problem. My logic board is Rev E

This was a subtle problem but was causing me missed decodes on FT8. No decoding 100 %

Hope this  helps someone

Gary K4MT

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