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Drake 2880 Fine Tuning



Hi everyone,
I just wanted to share my experiences tuning my recently converted Drake
downconverter.

I failed to hear Dove after doing the IF and Crystal mod.  Several people have
mentioned that the mod doesn't have the exact offset expected, so I suspected
that I was listening too far from the actual frequency.  Combined with the
doppler and the narrow beamwidth of the Conifer 21 dish, I was probably just
tackling too much at once.  So, I set about to try to determine the actual
offset without having a steady 2.4 GHz signal source at my disposal.  

As a bit of background, the Drake converter has a crystal which is multiplied
X256 and subtracted from the recieved frequency to obtain the output,
otherwise known as the intermediate frequency (IF).  The mod attempts to do
the following: 2400-(8.8125*256) = 144 MHz.

I hooked up my frequency counter to a 10X high impedance probe (to avoid
loading the oscillator circuit and changing its frequency) and probed around
the crystal, attempting to find the crystal's fundamental frequency.  At the
little chip cap closest to the trimmer cap, I found a solid signal at 8.8133
MHz.  This is the cap to the right of the trimmer if the unit is oriented so
the N connecter is facing away from you.

Running the numbers (2400-F/256) showed that the 2400 MHz comes out as 143.795
MHz, similar to the frequency tested in the Feb '98 QST article.  Allowing for
doppler, this means that Dove will come out on 144.016 +/- .050, so for a good
portion of the pass the signal is out of the band, and my radio won't go out
of band!

The QST article says to adjust the trimmer and bring it onto frequency, but I
could not get enough range to bring it to the desired 8.8125 MHz.  Careful
tuning did allow approximately 8.81265 , bringing Dove just barely into the 2M
band at the lowest extreme of the doppler shift.  

To be honest, my main counter has not been recently calibrated and I have not
yet heard Dove, so there is possibly some error in the measurements.
Nevertheless, I did get similar readings on two different counters so I have
reasonable confidence in the results.  With a 256 multification factor, it
doesn't take much change to cause a big change in the operation of the
converter!

If you are doing the mod and your 2M rig won't go below 2M, I'd recommend
testing the actual frequency of the LO crystal and adjust as necessary.  If
you don't have a frequency counter, you can probably hear it on a general
coverage receiver (with the Drake's cover removed).  I haven't tried this yet,
but have on other crystal oscillators, so it should work.  If yours is lower
than expected, try tuning the crystal trimmer for the lowest oscillation
frequency possible (to move the signal up in the 2M band).  

Those who haven't yet purchased the crystal for this conversion might want to
consider a slightly lower crystal frequency.  After all, P3D and the current
satellites don't occupy even 1.5 MHz of the band, so if you adjusted the
frequency so that 2400 MHz came out on 145 MHz instead of 144, you'd stay more
centered in the 2M band and would never encounter this situation.
Bleedthrough from strong terrestial 2M signals may be more of a problem,
however.

So what's next?  I may attempt to add more capacitance (or inductance) to pull
the crystal closer to the desired 8.8125 MHz frequency, but it is usable as
is.  I'll let you know how this goes if I attempt it.

I'd be interested in hearing any comments or experiences of others doing this
modification.  How much adjustment did you get from the trimmer cap?  Were you
able to get the converter aligned so that 2.4 converts to 144 MHz?  Any
comments on adding more capacitance?

73,
John Oppen, KJ6HZ

Obligatory disclaimer:  These are simply the results of my testing.  Your
results may vary.  Do not attempt this if you are not comfortable probing
around inside RF circuitry with test equipment.  If you let the smoke out,
you're on your own!



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