I have the Codan 8525B running with full frequency selection now. It is making use of the original front panel buttons. I'm pretty pleased with it!
After failing to drive the existing PLLs, and popping part of one somehow, I went with an Arduino Nano driving an Si5351 triple digital VFO board.
The Arduino Nano and Si5351 are mounted on a board that replaces the original front.
As mentioned above, my attempts to drive the original PLLs was less than satisfactory, so they have been disabled, then the Si5351 board is used to generate the first mixer, second mixer and side-band frequencies.
To limit the mods to be fully reversible, the front is just the same, with a white .9" OLED display in the place where the 2 digit LED was. Also, each original VCO has a short from the gate to gnd to disable them, and the coupling resistor was cut at the VFO end and now has the Si5351 signals coupled to the cut resistor end with a 10nF cap instead.
There is one cut on the main PCB to disable the original USB/LSB signals.
All theses mods can be reversed quite well if needed.
I have not made use of the call tone option, and it was removed as the proto version was installed in that place. But there is no reason why it could not be incorporated if wanted.
The nano is up to 90% full, and I am looking into using a Teensy or Nucleo board that has the same pin out as the nano. Level shifters from 3.3V to 5V will be needed then.
This is a work in progress.
For those interested in more info, the files are below...
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