Racal 9915 new OCXO (was: Repairing off frequency Racal 9442 OCXO)

[bd139]

Got myself one of these in a counter and unfortunately it is off frequency and I can't adjust it out with the built in offset control. It's a neat little box that kicks out an unusual 5MHz. Starts off above frequency but settles at 50Hz below frequency. I can't adjust it into the correct range at all.

I have opened it up (it is designed to be opened unlike some ovens) and inspected it and nothing is obviously wrong. I slid a thermocouple inside and that's reading a solid 75.1oC constant temperature so temp is fine. Ergo this is likely the crystal has aged out of range of the adjustment or one of the oscillator caps is out of tolerance. Anyone know where to start with this one or if it's even worth it? It strikes me that as it drifts through the ideal operating point, it may be possible to reduce the temp to around 60 oC and it'll be spot on.

Option 2 is run the spare Bliley OCXO I have up in it and stick a /2 on the output and throw this in the trash.

Photos of the gubbins below.

[bd139]

Actually looking at the counter schematic it actually doubles the 5MHz frequency up to 10MHz in IC4 / IC5 so not a lot of clever stuff needs to go in it to fix. Basically jump the doubler and make up a little board for a NOS OCXO I have lying around to go on. Job done 

[bd139]

Decided to short C36 and just snip R60 out thus not switching IC5f. IC4 is incorrectly marked as IC6 above

[bd139]

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.



Into a sink of boiling water containing a tupperware tub full of ferric chloride. Yum. Poked around with a cocktail stick low tech style.



Out, cleaned and tinned (tinning isn't great as it was done with an iron and solder but it'll do):



Populated. The trimmer is on the back so it's available in the "calibration hole" in the back of the unit.



Now the board was modified. IC4 was removed and pins 1-6 jumped. C36 and R60 were removed. This effectively bypasses the doubler.



Board then installed.



Does it work? Woohoo it did! Went through careful cal after this and is bang on the mark now.



Quick test with the MFJ doohickey at 150MHz to check channel A.

[TheSteve]

Very nice!!! Gotta love the cheap surplus OCXO's.