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Recently I found a used HP/Agilent 53131A counter on Ebay and the offer was too good to pass on. It arrived in good condition and fully working, but it lacked an OCXO timebase and the built-in TCXO is not very stable.

I started looking around for OCXO options and while you can buy them on Ebay, most of them are quite expensive (more than US$200).

While searching for DIY options I found some projects and one of them even provided Gerber files for download. Click here for the blog post describing the project.

I bought a used Morion MV89A OCXO on Ebay from a seller that had a good reputation, ordered PCBs from JLCPCB and the necessary parts from Mouser. I splurged and also populated the DAC and voltage reference to enable the automatic calibration, which is a lot more convenient than having to open up the counter for manually trimming the OCXO. The automatic calibration allows you to trim the timebase without having to remove the counter from the bench, you can keep it powered and the OCXO hot and in thermal equilibrium.

Gerber files and BOM can be found on Github. Two caveats regarding the BOM:

  • Don't populate R3. The BOM notes it as a 5k6 resistor but it should be "DNP". If you place R3, the LM361 comparator will have a wrong trigger level and there will be no clock output to the main board. This will cause a power-on self-test fail (GP-IB).
  • JP1 has to be soldered closed.

There is one other upgrade to be done, replacing the internal fan which has a very obnoxious whining sound which is especially irritating since the fan is always on when the counter is powered.