[time-nuts] OXCO Issues

[J. L. Trantham]

Fred,

I've been thinking about your OCXO issue of the oven cycling.  I had a very
nice theory about what to look for and where to look.

However, if you have an issue with the external 12 VDC supply to the OCXO
assembly, that, clearly, needs to be resolved first.  Also, you could remove
the supply from the test equipment and attach a known stable bench supply
directly to the OCXO then look at current draw (cycling) and output
frequency stability.

If your OCXO is anything like the two that I have opened, the foam is easy
to pull out.  There is usually a piece of 'packing' (fiber) tape folded over
to allow you to pull the foam enclosed oscillator and oven assembly out of
the case.  I would like to see some pictures if you have any.

Can you share the model number, whether you are adjusting a cap or pot,
whether the adjustment is internal to the OCXO or external, whether it has a
'screw' cover to the adjustment(s), and whether it has one or two ('Coarse'
and 'Fine') adjustments?

All the problems I have had have been on the oscillator board or the output
board that is just inside the bottom cover of the OCXO.

I am not sure about how best to 'reseal' the unit when finished.  My most
recent one had not externally accessible adjustments, just an external pot
adjusting a 4 VDC source to the EFC control.  When resealing it, I turned it
on, let it heat up for an hour or so, then quickly disconnected it, applied
the 'Quik-Set' epoxy, then quickly reapplied power to let it heat up again.
This gave a small bubble or two out through the epoxy demonstrating an 'air
tight' assembly.  It dried with power applied.  When it cools, I'm sure
there is a 'vacuum' inside.  However, at operating temps, it should be at
'atmospheric' pressure.  I am about 20 feet above sea level.  I don't know
if 'sea level' internal pressure at operating temperature is critically
important or not but it seemed easy enough to achieve.

Good luck.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Frederick Bray
Sent: Saturday, May 11, 2013 12:22 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OXCO Issues

I looked inside the OCXO and didn't see anything obvious.  I didn't pull the
foam out, as I didn't want to destroy everything.

After I re-assembled everything.  I hung my DVM on the 12 volt line that
feeds both the oven and the oscillator.  I watched it vary by almost a 
volt.   I am now beginning to suspect that I should go into the main 
power supply and replace at least the cheaper electrolytic capacitors.  
There are a couple screw terminal units that are quite expensive so I am
hoping to avoid having to buy them.  I will also check for out of tolerance
resistors.  I found several on the other board of the power supply and I
suspect it is a question of heat since all the other resistors I have
checked at random elsewhere in the 5110 are fine.


On 5/10/2013 3:42 PM, Frederick Bray wrote:
> Thanks for the suggestion.
>
> I'll probably pull it and look inside. If it is something simple the 
> problem may be fixable.
>
> Fred
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