[time-nuts] Ovenaire OCXO -- Final Diagnosis Summary

Frederick Bray fwbray at mminternet.com
Sun May 19 21:15:04 EDT 2013

I may not have mentioned that I think I figured out the underlying 
problem with this OCXO lies with the trimpot in the oven control 
circuit.  (I believe that someone mentioned this as a possibility, so I 
can't take credit for finding it on my own.)  Some of the other things I 
did helped, but adjustment of the trimpot was the final issue.  If the 
pot is misadjusted, the oscillator will wander all over the place as the 
oven temperature swings wildly.

I drilled a second hole in the Ovenaire case so that I could access the 
pot.  I also monitored the current to the unit with a DVM.  When the 
oscillator started wandering, I tweaked the trimpot until the current 
increased, indicating the oven was activated.  I kept adjusting it to 
the point that the frequency varies by 0.1 Hz on my GPSDO counter.  
Perhaps it is less than that, but I don't have enough digits to say.  (I 
know that for final adjustment, I will need to use a scope but the 
counter does let me see if the OCXO is wildly unstable.)

Now, it is clear that the adjustment is very touchy.  I am going to 
remount the assembly in the Cushman tonight or early tomorrow and see if 
handling it has made it unstable.  If so, next weekend I will tear it 
down again and figure out the value of the trimpot and the number of 
turns.  If I can find a trimpot with more turns, I will buy it.  
Otherwise, I will just get an exact replacement.

All of this has caused me to realize that the Ovenaire is probably still 
quite usable for what it was designed to do.  Although I considered 
installing the MV89 in its place, the Cushman power supply isn't 
adequate to handle it.

Thanks again to all.

Fred Bray

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