[time-nuts] Ovenaire OSC 49-61C

Frederick Bray fwbray at mminternet.com
Wed Jun 12 16:01:14 UTC 2013


Hi Mark,

I am not sure about the design of your OCXO but recently I repaired an 
Ovenaire from a Cushman 5510 service monitor.

I found that there was a bad trimmer pot in the oven control circuit.  
This wasn't apparent until I started monitoring the current drawn by the 
unit while trying to adjust the trimmer pot. I found that if I barely 
touched the adjustment screw, the current would vary greatly.  The 
temperature swing of the crystal made it impossible to adjust the 
oscillator trimmer cap properly.  These problems disappeared after I 
replaced the trimmer pot.

To get the new pot to a reasonably close starting, I measured the 
setting of the old one after I removed it and duplicated that on the new 
one before installing it.  I had to make an extra hole in the outer case 
to access the pot adjustment.  Although there was a hole in the internal 
foam to access the pot, there was no corresponding one in the outer 
can.  I adjusted the pot for the minimal variation in oven current while 
monitoring the output frequency for minimal frequency swing.

Now, based on observations conducted over a few days, the OCXO barely 
wanders and I would guess is within what Cushman required at the time 
the 5510 was built.

Good luck with yours.

Fred

On 6/12/2013 7:48 AM, Mark C. Stephens wrote:
> Anybody have service, schematics or experience for one of these OCXO?
> They are commonly seen in early HP 8566A spectrum analysers.
>
> It is out of adjustable range.
> Fortunately I had a spare OSC 49-61C floating about and replaced this off frequency reference with it.
> However the replacement tends to wander about a fair bit.
>
> By removing the 4 bottom screws and the 2 top screws, applying a little heat with a heat gun around the bottom to melt the glue, the oven came apart with little protest.
> I was able to carefully access the top 2 boards and there is no sign of component failure there so it maybe something inside the oven itself.
>
> I have stopped here until I get further information.
>
>
> -marki
>
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