I've been playing around with a replacement for the ovenized oscillator
used by Ten Tec. Ten Tec was supplying a small board which used a 14 pin
DIP TTL 5 volt TXCO 20 MHz oscillator package. They were charging $150.00,
but the kit is no longer available.
The biggest problem is that *precision* TXCO 5-volt TTL level devices are
becoming scarce as 3.3 volt technology becomes the current standard. There
are packages available, but very few are adjustable or have spec's in the
+/- 2.5 ppm range, or better. 3.3 volt HCMOS packages may work with the
TTL levels needed to drive the Omni Vi; but someone else will have to
verify it.
Currently there are two 20.000 MHz packages being offered on eBay that have
some promise. Both are surplus, and proprietary parts, meaning the
manufacturers are not willing to release spec's. I am assuming that any
trimmable TXCO is probably at least +/- 2.5 ppm.
One is a 14-pin DIP TTL level package which works, but I have not been able
to find an adjustment tool that properly engages the trimmer cap. I used
one for a few months in one of my Omni VI rigs, but adjusting it was too
much of a hassle. Otherwise it interfaced in the same manner as the Ten Tec
replacement/upgrade kit.
The other is 14-pin package made by Ralston. This is a 5-volt device, but
the output is a clipped sinewave, which is not able to directly drive the
TTL logic in the Omni. However, it will directly drive the oscillator
chain in the radio, providing the two transistors are used as a buffer.
That involves removing the ovenized osc. and removing the 150 pF base to
emitter cap in the Colpitts osc. This is a proprietary part, but I suspect
it directly crosses to a 2.5 ppm TXCO they sell OEM.
it is a SMD package, but it is very easy to work with. I located a source
for a printed circuit board (three for under 5 bucks) that was designed to
be used with the Ralston TXCO. Total price for making three units should
be under 10 bucks, including the two SMD chip caps that are used on the
board.
If there is enough interest, I'll write up what I have done so far for the
Ten Tec Wiki, but that is a week or so down the line since I want to do
more testing. At worst, it will cost a few bucks to keep an old radio
running, and it is a reversible mod.
Pete k1zjh
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