[TenTec] Substitute semiconductors for Omni VI XTAL OSC.

[Ken Brown]

Hi Curt, Checking the voltages on Q1, Q2 and Q3 was definitely the thing to
do. Fortunately I have two Omni VI and I could compare the non-functional
one with the working one.

On the working one the voltage there was 5.5 V, but with the DMM probe
touching there the oscillation was also gone. So that was not really the
"normal" working voltage. The loading of the DMM lead no doubt killed the
oscillation and the lack of power detected by D25 made Q3 shut down to
raise the base voltage on Q1 to maximum available from R22, R24 voltage
divider.

The same thing should have happened with the non-working Omni VI yet the
voltage was only 0.3 V on the base of Q1.

I did an ohmmeter check of all resistors. I found R22 measured 10 k Ohms
instead of 4.7 k Ohms. As soon as I replaced R22 with a good resistor it
started working.

There was some sort of rubbery stuff on a lot of the parts on this board.
It was only on my newer Omni VI, the one that failed. No such stuff found
on the older Omni VI. There was also a white powdery looking substance on
many of the parts that had the rubbery stuff. It might have been mold,
fungus or metal corrosion. I think the rubbery stuff must have helped
moisture to stay there and either grow mold or fungus, or accelerate
corrosion. That in turn caused R22 to change resistance. Oh the joys of
maintaining electronic equipment in Hilo.

Thanks for the help. I may go ahead and order a couple of those varactors
just in case.

73 DE N6KB

On Fri, Jul 14, 2017 at 1:44 AM, Curt Benjamin via TenTec <
tentec at contesting.com> wrote:

> Ken, have you measured the voltage on each xistor and on the zener diode?
> Does it look normal? Also, does it look like there's voltage across the
> selected (by which band is selected on the front panel) steering/switching
> diode (D9-D18, etc.)?
> The parts you listed with the possible exception of the VHF transistor and
> varactor are garden-variety (although, kitsandparts.com has a suitable
> and cheap varactor replacement - MVAM109).
> Curt, K8AI
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