[time-nuts] Opening an Isotemp OCXO

[Bob Camp]

Hi

If the OCXO was designed for a ~70 C upper end temperature spec, then a ~90C crystal
would make sense. 

When you feed +12 into the oven control, you are increasing the effective gain of the control
loop (it has more power). The cycling you see is the loop going into oscillation. It’s the same thing that happens if
you put way to much insulation around an OCXO. 

I *hope* the 4 amps below is a typo. 20W into the beast is way to much. 0.4 A at 5V would be
2W. That is a fairly normal number for an OCXO like you have at room temperature. 

Bob


> On Oct 29, 2016, at 9:51 AM, Peter Reilley <preilley_454 at comcast.net> wrote:
> 
> More information;
> 
> I added a picture to the dropbox from my Flir IR camera.   The picture shows the copper block
> that the crystal is attached to running at about 200 F.   In the IR shot the copper block is to the
> right.    In most of the regular pictures it is toward the bottom of the picture.    This is with the
> unit (minus the S30 chip) running on 5 volts for more than 10 hours.   Is that too hot?
> 
> While running at 5 volts the current is constant at about 4. amps, no cycling.   At 12 volts
> it cycled between .9 to .1 amps.   I would not expect cycling for the temperature control
> of an OCXO.   I would expect a linear temperature control circuit.
> 
> I looked at the tantalum capacitor on the bottom of the board.   The marking is 39-10.
> Does that mean 39 uF and 10 volts?   If so then it must be a 5 volt unit.   The capacitor
> did not explode at 12 volts.
> 
> Dropbox link:
> https://www.dropbox.com/sh/52e9d1rva9kpb3w/AABmbIj1aK7Zk2J9SNMmu-JAa?dl=0
> 
> Pete.
> 
> On 10/18/2016 9:11 AM, Peter Reilley wrote:
>> I bought an Isotemp OCXO82-59 with a frequency of 10 MHz for a $3 at the MIT flea market.
>> As expected it was dead.   It heats up as expected but looking at the output with a scope there
>> is nothing.   However looking at the output with a spectrum analyzer I can see a faint 10 MHz
>> signal.   It seems that the oscillator is running but the output circuitry is dead.   Reasonable
>> assumption?
>> 
>> Anyway, has anyone had any luck unsoldering the tin case without destroying it?
>> 
>> Pete.
>> 
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