IC-9700 Fan mod stabilizes TCXO

Another interesting post appeared today on the IC-9700 reflector. It describes how to improve frequency stability.

I have modified my IC-9700 to keep the fan continuously running at a reduced speed by fitting a 6.8V Zener diode across the appropriate switching transistor. (N.B. I have documented the process with pictures of where I installed the diode, and I am trying to get the file uploaded to the files section of this site.)
The fan now runs continuously. I can’t hear it in my shack because I have a number of other pieces of equipment which much noisier fans!
I have done some brief frequency stability tests on CW transmissions using a 1 minute on – 1 minute off cycle. Using an HP 53131A with GPS Disciplined external reference, observing the frequency to approx 0.1Hz resolution (N.B. This frequency counter uses reciprocal measurements so the gate sample time is approx 0.5Hz for this level of precision allowing rapid frequency fluctuations to be observed.)

Using Firmware 1.06

144MHz 20% power approx 1Hz variation during the transmit period
144MHz 100% power approx 1-2Hz variation during the transmit period
432MHz 20% power approx 2Hz variation during the transmit period
432MHz 100% power approx 5-6Hz variation during the transmit period
1296Hz 100% power approx 1-2Hz variation during the transmit period

The frequency variations during transmit periods were superimposed on a general frequency drift/wander.
I performed an automatic frequency adjustment of the transceiver before testing at each frequency/power level and the initial frequency accuracy was approx 1Hz each time.

These figures are significantly improvements on what I had previously observed with the un-modified transceiver.
I have tested at 20% power level on 144MHz and 432MHz because that is approximately the power required to drive my solid state amplifiers to full legal (UK) output.

This modification has improved the frequency stability of my new IC-9700 to approximately the same level as my old IC-910 (with CR-293 high stability crystal), and the adjustment of the reference oscillator doesn’t require the transceiver to be disassembled!

I do hope that Icom is able to implement a frequency disciplining facility, but maybe as an interim work around they might be able to implement a facility to specify an option to keep the fan running continuously during receive for users concerned about frequency stability.
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(from Stephen Tompsett forwarded by Poster)

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