[time-nuts] OXCO Issues

Sat May 11 20:59:23 UTC 2013

It seems to me that the only way to achieve the 'extreme' of temperature
stability is to supply heat at the same rate that it is leaving.  This would
result in the heater always being on but the temperature never varying.  I
suspect that there is some sort of variable current control to the 'heater'
(which is coils of wire on the two units I have opened) but I doubt it is
continuously variable.  Rather an 'on/off' approach at its most 'sensitive'
setting with the setting being 'Hi' when first turned on and 'Lo' once at
operating temperature so that the current is max when first turned on then
some lower value once at operating temperature .  The sensor will have some
sort of 'hysteresis', turning the heating circuit off as the temperature
rises through a certain point (which I suspect might be adjustable) then
turning the heating circuit on as the temperature falls below some point
below the 'off' temperature.  In a typical home thermostat, it is 2 or 3
degrees difference.  In these systems, it has to be significantly less than
that.

However, if your OCXO continues to 'cycle', resulting a fluctuating
frequency, and if you had a continuously variable heating system (as I think
exists in the 10811 but I don't know about the Ovenaire systems) either the
heating control is 'fixed' at 'Hi' or the sensor/control circuit has a
significant 'on/off' issue (large hysteresis value).  It could be that the
heating circuit is either fixed at 'Hi' or the controller can't 'fine tune'.

I have never done any 'reverse engineering' on the oven control circuit of
the Ovenaire's that I have opened but I would be interested in seeing what
that circuit looks like.

Good luck.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bob Camp
Sent: Saturday, May 11, 2013 1:28 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] OXCO Issues

Hi

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. 

Bob

On May 11, 2013, at 10:14 AM, J. L. Trantham <jltran at att.net> wrote:

> 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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