[time-nuts] Re: repair of an HP E1938A OCXO

[Wilko Bulte]

Hi,

"Fun in production", yes, exactly.

In the mists of time there was likely a manufacturing process/procedure that characterised the xtals as part of the oven assembly production. That resulted in the "x degree C" sticker on the oven.
The main PCB got married to an oven, and the PIC on that main PCB was duly programmed with the temperature indicated on the oven label. Doing it like this once could produce oven assy and main PCB assy in fully independent  production lines. 

In a repair process a similar thing might have happened: new oven assy mounted, main PCB setting updated based on label of new oven assy. Etc.

All speculation, but this is roughly what I have seen in production. i.e. when I was in hardware design (when dinosaurs roamed the planet, and all that...)

Cheers,
	Wilko

NB: I updated the web page with my latest progress.

-----Original Message-----
From: Bob Camp via time-nuts <time-nuts at lists.febo.com> 
Sent: 1 July, 2025 15:31
To: chris at chriscaudle.org; Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Bob Camp <kb8tq at n1k.org>
Subject: [time-nuts] Re: repair of an HP E1938A OCXO

Hi

If you want to map “some reading” on the thermistor to 103.0 C on the crystal blank, you need a way to know the crystal blank temperature. You do a sweep run of some sort. You log the thermistor bridge readings vs frequency. You then convert this to temperature based on the data on that specific crystal. 

The B mode is “linear” over temperature. It has a nice steep slope of frequency vs temperature.
This makes it ideal for checking oven temperature. Since you need a calibrated crystal and circuit changes, there are practical limits using it. 

One would assume this was done “way back when” on the HP design. The data probably got saved somewhere in the PIC code. It may well have been updated when thermistor vendors changed. 

If you want to use the labeled temperature on the crystal to set the oven, you need that mapping. Why do it this way? It could save you a lot of “fun” in production.

Long long ago folks would do a “turn hunt” on the crystal. They then would ship the part.
Eventually they found that you actually needed to do that turn hunt at multiple temperatures due to a range of issues. Every OCXO design is different and exactly how this or that one is processed will be a “that depends” sort of thing. 

Bob 

> On Jul 1, 2025, at 8:43 AM, Chris Caudle via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> On Thursday, June 26, 2025 3:47:48 PM Central Daylight Time Bob Camp 
> via time- nuts wrote:
>> There really is no practical way to get an accurate temperature 
>> sensor inside the oven / on the crystal, without upsetting the 
>> operation of the device.
> 
> Do you really need to measure the temperature independently?  I don't 
> fully understand the control scheme and the differences between C mode 
> and B mode, but if you get the oven to the right temperature, will 
> that not set the oscillator output back to within spec?  I recall the 
> original description was the measurement was with EFC connected to 0V, 
> so I guess you would need to know what the nominal value of the EFC should be for on-target frequency.
> Am I thinking too simplistically?  Is there a range of oven 
> temperatures where the output is close to nominal but the temperature coefficient is not optimal?
> 
> --
> Chris Caudle
> 
> 
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