[time-nuts] -hp- 10811 repair (Thermistor)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Jan 22 22:48:52 UTC 2009


Dan Rae wrote:
> The 10811-60111 that came in my 5370B counter turned out to have an open 
> circuit Thermistor in it.  This is a "non-replaceable" part in theory.
>
> However, nothing to lose, I have successfully substituted a Digikey Part 
> number: 317-1371 100 kOhm 1% Thermistor ( from Cantherm, part number: 
> MF51B104F3950 ).
>
> I measured the resistance of it at the target oven temperature, working 
> back from the selected value of R20 that was installed in mine, in this 
> case 0 Ohms for 84C.  In fact this thermistor must be very similar to 
> the one used originally, since I only needed to use a value of 110 Ohms 
> for R20 to get bridge balance, instead of the short that was in there 
> previously. 
>
> Good enough for me...
>
> Dan
>
>   
Dan

The real reasons that this part is labelled non replaceable are

1) its epoxied to the oven.

2) Setting R20 to the correct value actually requires plotting the OCXO
frequency vs R20 and selecting R20 so that the OCXO frequency is located
at the stationary point on the frequency vs R20 value plot. This
requires a high resolution frequency measurement setup with a highly
stable reference and takes considerable time as one has to allow the
oven temperature to settle between adjustments to R20. Drift due to
aging and ambient temperature changes limit the accuracy with which the
OCXO turnover temperature can be achieved.



Bruce






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