[time-nuts] HP 10544A Repair

Nigel Vander Houwen timenuts-nigelvh at nigelvh.com
Sat Feb 27 23:38:06 EST 2016


Bob,

I’m not sure what type is in it. It *is* a 10544A, not one of the 10811 devices. The references I’ve found indicate the 10544A being an AT type. If it’s of any use, the date codes on the parts in the oven control circuit indicate it was built in the latter part of ’77.

I can’t say I’ve got a whole lot of experience looking at this, but should I expect to see a curve along the values, of which there would either be a minimum or maximum? That would be fairly indicative.

Rick,

The thermistor was not an exact replacement, and I couldn’t find any specifications beyond 9.93KOhm @ 80C referenced on the schematic. I don’t know that the pot wasn’t turned during the original debugging of why the crystal wasn’t on frequency, so my best bet now unfortunately is to find the set point myself.

Nigel



> On Feb 27, 2016, at 19:39, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
> 
> If you replaced the thermistor with an exact replacement,
> then you shouldn't need to change the pot.  If you didn't
> replace the thermistor with an exact replacement, then you
> should carefully measure the resistance of the pot as you
> found it.  (I hope you did not fool with it already.)  From
> the pot resistance, and the thermistor curve specs of
> the original thermistor, you should be able to calculate
> the factory oven temperature set point.  You can then
> calculate the new pot resistance based on the specs
> for the replacement thermistor.
> 
> Trying to determine the turn over temperature as you
> described (which was done with an automated system
> in the E1938A) would be extremely cumbersome when
> done by hand.  It would be a last resort if you fooled
> with the pot.
> 
> Rick
> 
> On 2/27/2016 6:48 PM, Nigel Vander Houwen wrote:
>> Hello All,
>> 
>> Relatively new to the group, but thought I’d ask for a bit of advice. I have an old HP 10544A ovenized oscillator that was pull from some equipment some time ago because it got too far off frequency to phase lock with the external reference. It looks like the thermistor in the oven controller failed, and just wouldn’t ever turn the oven on.
>> 
>> I’ve replaced the thermistor, and the oven is working properly again, but I need to tune the temperature as appropriate for this crystal. I’ve read that this involves plotting crystal frequency vs temperature, so I’ll need to adjust the potentiometer inside, close it up again, wait for temperature to stabilize, and take a frequency measurement, and iterate.
>> 
>> If I’ve understood what I’ve read correctly, I’m looking for a flat in the frequency response of the crystal with regards to temperature.
>> 
>> I do have a HP 5345A referenced from a thunderbolt to do the frequency measurement.
>> 
>> Am I on the right track? Further insights? Etc?
>> 
>> Thanks all!
>> Nigel
>> K7NVH
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