hi

This week I received the broken HP E1938A OCXO that Morris shipped me all
the way from Australia. Sofar I have spent some time for a quick test,
as well as documenting opening of the oven in a picture series that can
be found here https://people.freebsd.org/~wilko/HP-OCXO-E1938A

For now I am not unhappy, as opening the 'hermetically sealed' oven proved
far easier than I had feared it would be. I plan to add more info/pictures
once I progress into the inner workings.

Input / comments are as always welcome.

Wilko

Hi all,

Wilko thanks very much for the update - keep them coming!

The hockey puck I sent you is the one from my Z3815A which had been running for about 20 years until the oven stopped heating. I eventually was able to obtain a replacement unit and substituted the hockey puck from it into my original mother board. I did that because I thought there may have been some customization in the original board as the replacement unit warmed up but did not lock in the Z3815A. The mother board I sent is the replacement one I received with the faulty hockey puck on it.  That explains the difference in labeling. The replacement hockey puck works perfectly in the Z3815A with its original motherboard which suggests that it’s running at the correct temperature.

The higher current draw behaviour for the first minute or so falling to 50 mA afterwards that Wilko has described what what I observed as well on both motherboards. That makes me think the problem is with the thermistors, or their connections, in the hockey puck. The heating elements themselves all show proper continuity when tested with an ohmmeter.

I have read somewhere that the series capacitor for the crystal was individually selected for each unit which probably explains why it’s specified on the oven’s label.

I hope that helps,

Morris

Thank you Morris! And thanks to Bob for noting the different temperature labels.

In the meantime I have dug a bit deeper (into the rabbit hole maybe?) and added some more pictures & measurements for the thermistor section. Some interesting things were found.
My current gut feeling is that one or more of the flex wires and/or their soldering have issues.

Anyway, have a look in case you are interested in this project:  https://people.freebsd.org/~wilko/HP-OCXO-E1938A/

Wilko

-----Original Message-----
From: Morris Odell via time-nuts time-nuts@lists.febo.com
Sent: 6 June, 2025 12:07
To: time-nuts@lists.febo.com
Cc: vilgotch1@gmail.com
Subject: [time-nuts] Re: repair of a HP E1938A OCXO

Hi all,

Wilko thanks very much for the update - keep them coming!

The hockey puck I sent you is the one from my Z3815A which had been running for about 20 years until the oven stopped heating. I eventually was able to obtain a replacement unit and substituted the hockey puck from it into my original mother board. I did that because I thought there may have been some customization in the original board as the replacement unit warmed up but did not lock in the Z3815A. The mother board I sent is the replacement one I received with the faulty hockey puck on it.  That explains the difference in labeling. The replacement hockey puck works perfectly in the Z3815A with its original motherboard which suggests that it’s running at the correct temperature.

The higher current draw behaviour for the first minute or so falling to 50 mA afterwards that Wilko has described what what I observed as well on both motherboards. That makes me think the problem is with the thermistors, or their connections, in the hockey puck. The heating elements themselves all show proper continuity when tested with an ohmmeter.

I have read somewhere that the series capacitor for the crystal was individually selected for each unit which probably explains why it’s specified on the oven’s label.

I hope that helps,

Morris

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