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OCXO tuning screws not working on old equipment

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Gertjan:
Yes, those screws are covers  :)

Under the "Fine" cover screw you will find the trim capacitor you are looking for.

Be very gentl with it! On my Racal-Dana 1998 I only have to touch it for a change in frequency...
And I have to wait 10...15 minutes before the oscillator does not drift anymore.
So, this OCXO does take some time to fine-tune....

regards, Gertjan.

Thomas:
Yep, Gertjan is spot on.

Adjusting the screws didn't seem to do anything, and after a day or two I thought "sod it, I'm going in" and removed them.
THERE are the trimmers :o
NOW we're getting somewhere ;D

I really should have taken the hint, because my 1992 has brown fiber washers on the covers :-[

tverbeure:

--- Quote from: Thomas on July 07, 2023, 10:37:50 am ---Yep, Gertjan is spot on.

Adjusting the screws didn't seem to do anything, and after a day or two I thought "sod it, I'm going in" and removed them.
THERE are the trimmers :o
NOW we're getting somewhere ;D

I really should have taken the hint, because my 1992 has brown fiber washers on the covers :-[

--- End quote ---
Amazing! I'm starting to think that this could explain the symptoms of what I'm seeing!  ;D

Just my luck that the two counters that I'm trying to calibrate are using trimmer obfuscation techniques.

GigaJoe:
im measure Freq quite rare

and very dissatisfied with all internal oscillators.  as it need many hours to stabilize when turn ON, and even 8D direct counter most likely 2 last digits will be incorrect. 
cheap OCXO well insulated with Vref like LT1021-5 , after a reasonable time to age , gives -10 per year stability and no jitter.

high stability OCXO inside an equipment, required constant ON condition, so like meter off but not a reference. in my case usage are few times per year , and it doesn't make sense to keep it running. so i decided to build external 10M ref that constantly on,   well Insulated OCXO consume around 20ma on 12V  vs 120mA open space, and significantly lower jitter,  as a thermal loop control  do it much more precise, due to less heating loss.

as well as I noticed for quartz oscillators need approximately a week  to get stabilized (stop drifting) after extended offline period. so adjustment  better do after long period of ON or STANDBY .

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