Ok job done. Details as follows for anyone else who comes across this thread. This should work for adding an OCXO to a Racal counter. Bliley module was £13 from Aliexpress.
I reused the analogue to TTL input circuitry and bypassed the doubler, stealing part of it to provide a stable buffer for the doubler and the main counter IC. This involved snipping out a 7400, resistor and capacitor and soldering a jump wire.
The existing module had an interface compatible with the new module but the new module needed a trimmer adding. Decided to knock up a PCB for this. Rather than wait for China I used a bit of FR4 and just hand drew it out for etching. Took some thinking and measuring then the holes were drilled then it was cleaned and then traced out.
![](https://i.imgur.com/GrBbzLY.jpg)
Into a sink of boiling water containing a tupperware tub full of ferric chloride. Yum. Poked around with a cocktail stick low tech style.
![](https://i.imgur.com/BK62zvw.jpg)
Out, cleaned and tinned (tinning isn't great as it was done with an iron and solder but it'll do):
![](https://i.imgur.com/hhkF8Od.jpg)
Populated. The trimmer is on the back so it's available in the "calibration hole" in the back of the unit.
![](https://i.imgur.com/3eGpcAg.jpg)
Now the board was modified. IC4 was removed and pins 1-6 jumped. C36 and R60 were removed. This effectively bypasses the doubler.
![](https://i.imgur.com/yHjQlz8.jpg)
Board then installed.
![](https://i.imgur.com/6XSOhev.jpg)
Does it work? Woohoo it did! Went through careful cal after this and is bang on the mark now.
![](https://i.imgur.com/VzXoIlB.jpg)
Quick test with the MFJ doohickey at 150MHz to check channel A.
![](https://i.imgur.com/KNd7lJk.jpg)