Wilko

 On 25 Jun 2025, at 22:55, Richard Karlquist <richard@karlquist.com>
 wrote:

 OCCAM'S RAZOR!

 On 2025-06-25 07:25, Wilko Bulte via time-nuts wrote:

 On Jun 19, 2025, at 3:51 AM, Wilko Bulte via time-nuts
 <[5]time-nuts@lists.febo.com> wrote:
 I would be interested in whatever resistance the thermistors measure
 at when cold. Just to get another data point.
 Thanks you Rick, for your comment on failure or drift being highly
 unlikely. The thermistors are mounted in the inner ring using some
 flexible (silicone?) compound, so mechanical stress induced problems
 I feel are unlikely.
 thanks, Wilko

 On 19 Jun 2025, at 02:19, glenlist via time-nuts
 <[6]time-nuts@lists.febo.com> wrote:
 I have 6 of these beasts, I could measure something for you if  that
 was helpful...

 On 18/06/2025 9:26 pm, wkb--- via time-nuts wrote:
 Hello Morris,
 The idea to tack a resistor across the 3 thermistors is an
 interesting one. I will give that a try, once the attic is habitable
 again (temperature wise).
 The upper resistor I think is R35 in the schematic, marked 1% BULK
 METAIL FOIL?  Problem is I do not have a component layout of the
 donut shaped PCB in the oven.
 In case someone on the list has that component layout, please let me
 know.
 I would also be interested in the value of the resistors at room
 temperature, in case anyone has a dead oven and can measure them I
 would appreciate that.

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Yes, absolutely! Can you advise on how to use NGOCOMM to program the oven temp setpoint to the correct temp, as indicated on the oven label? Thanks, Wilko On 25 Jun 2025, at 22:55, Richard Karlquist <richard@karlquist.com> wrote:  OCCAM'S RAZOR! --- Rick Karlquist N6RK On 2025-06-25 07:25, Wilko Bulte via time-nuts wrote: It appears I have swatted the bug/problem: the thermistor flex circuit proved to be intermittent. Some little hack later the oven seems to be doing its think like it should. For more details, I have updated the webpage at [1]https://people.freebsd.org/~wilko/HP-OCXO-E1938A/ Best, Wilko -----Original Message----- From: Bob Camp via time-nuts <[2]time-nuts@lists.febo.com> Sent: 19 June, 2025 14:16 To: Discussion of precise time and frequency measurement <[3]time-nuts@lists.febo.com> Cc: Bob Camp <[4]kb8tq@n1k.org> Subject: [time-nuts] Re: repair of a HP E1938A OCXO Hi NTC thermistors have a wonderful parameter on their spec sheets known as beta. It drives a formula that computes resistance over temperature. Wikipedia "thermistor" and it will give you all the grubby math details. The problem is that beta is not a tightly controlled parameter. The fabrication process on a thermistor can only control it to a limited degree. The net result is that you can buy a thermistor that is very accurate at (or near) one temperature. When you get well away from that temperature the values of a group of parts diverge. The values on different groups diverge even further. Since these are precision matched parts, you can't just replace one of them. You would have to replace the whole set. They also are not something you can buy on the open market. An OEM contracts with a thermistor outfit to buy a custom part. They then order them thousands at a time. They just might pay many thousands of dollars for each batch. There are alternative approaches, some manufacturers have a pretty large room full of "raw" thermistor material. They buy it in bulk and sort it. They then chop it up "as needed" and match the design parameters on the OCXO up with the material. This is more common with PTC than NTC materials. Simple debug answer in this case is to monitor the voltage at the junction of the thermistors and that resistor. See what it does as the unit heats up. For further fun, monitor the voltages across each thermistor as this happens. If you don't see anything strange, move on to the other parts of the circuit. Bob On Jun 19, 2025, at 3:51 AM, Wilko Bulte via time-nuts <[5]time-nuts@lists.febo.com> wrote: I would be interested in whatever resistance the thermistors measure at when cold. Just to get another data point. Thanks you Rick, for your comment on failure or drift being highly unlikely. The thermistors are mounted in the inner ring using some flexible (silicone?) compound, so mechanical stress induced problems I feel are unlikely. thanks, Wilko On 19 Jun 2025, at 02:19, glenlist via time-nuts <[6]time-nuts@lists.febo.com> wrote: I have 6 of these beasts, I could measure something for you if that was helpful... On 18/06/2025 9:26 pm, wkb--- via time-nuts wrote: Hello Morris, The idea to tack a resistor across the 3 thermistors is an interesting one. I will give that a try, once the attic is habitable again (temperature wise). The upper resistor I think is R35 in the schematic, marked 1% BULK METAIL FOIL? Problem is I do not have a component layout of the donut shaped PCB in the oven. In case someone on the list has that component layout, please let me know. I would also be interested in the value of the resistors at room temperature, in case anyone has a dead oven and can measure them I would appreciate that. _______________________________________________ time-nuts mailing list -- [7]time-nuts@lists.febo.com To unsubscribe send an email to [8]time-nuts-leave@lists.febo.com _______________________________________________ time-nuts mailing list -- [9]time-nuts@lists.febo.com To unsubscribe send an email to [10]time-nuts-leave@lists.febo.com _______________________________________________ time-nuts mailing list -- [11]time-nuts@lists.febo.com To unsubscribe send an email to [12]time-nuts-leave@lists.febo.com _______________________________________________ time-nuts mailing list -- [13]time-nuts@lists.febo.com To unsubscribe send an email to [14]time-nuts-leave@lists.febo.com References 1. https://people.freebsd.org/~wilko/HP-OCXO-E1938A/ 2. mailto:time-nuts@lists.febo.com 3. mailto:time-nuts@lists.febo.com 4. mailto:kb8tq@n1k.org 5. mailto:time-nuts@lists.febo.com 6. mailto:time-nuts@lists.febo.com 7. mailto:time-nuts@lists.febo.com 8. mailto:time-nuts-leave@lists.febo.com 9. mailto:time-nuts@lists.febo.com 10. mailto:time-nuts-leave@lists.febo.com 11. mailto:time-nuts@lists.febo.com 12. mailto:time-nuts-leave@lists.febo.com 13. mailto:time-nuts@lists.febo.com 14. mailto:time-nuts-leave@lists.febo.com