[time-nuts] OXCO Issues
lists at rtty.us
Sat May 11 14:28:14 EDT 2013
If the OCXO is putting out a solid signal, and it tunes up and down, the problem is not likely to be in the oscillator circuit. In this case, the heater / temperature control assembly is likely the issue. Most control failures melt the solder on the oven and then totally fail. In this case, it's *almost* keeping the right temperature. I'd bet on something in the feedback circuit on the control op-amp.
On May 11, 2013, at 10:14 AM, J. L. Trantham <jltran at att.net> wrote:
> 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.
> -----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.
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