HP 53131A Hard Power Switch Modification

Following on from my previous post in relation to the HP 53131A Frequency Counter Teardown I have devised a reversible modification that replaces its soft power switch (where the switch mode PSU and the fan are running continuously when the unit is plugged into the wall outlet) to a hard power switch which properly turns the unit off from the front panel. The modification is simple to do, uses inexpensive parts and is completely invisible, it looks and feels like factory behaviour. It is will worth applying if you have one of these for bench use so you can leave it plugged into the wall without having the fan running and switch mode noise being generated when the unit is not in use. It’s hard to understand why HP did not include a hard power switch in the original design, you will see from my modification there is plenty of room, and it seems such an obvious thing to do. Anyway, I have created a video showing the details of the modification, if you have one of these frequency counters I hope you find the modification interesting and/or useful.

UPDATE: Having re-read the schematic again it appears that the unswitched +/-12v from the switch mode PSU are provided on the OCXO option connector J9. This would explain the design approach, the PSU will run the xtal oven even when switched off so when you are ready to use it, its instantly available. Thats fair enough except that many of these counters exist that do not have an OCXO option, and even with one fitted, I personally would be happy to know I have to switch it on in advance of using it.

NOTE: Please forgive the under-exposure on the first part of the video and the audio noise in a few places through it, I have a new camera setup and am getting used to it. The audio noise I believe comes from the phantom power not being liked by the wireless mic (I hope its that at least). The under-exposiure is just my misinterpretation of the zebra exposure indicator, you will see I get it right (or at least much better) in the second half of the video.

See you next time, and thanks for watching….

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4 thoughts on “HP 53131A Hard Power Switch Modification

  1. Gerry, that’s a neat mod. My (older) HP5316B also has an “always on” linear power supply, I guess to keep the optional Crystal Oven nice and ready. Also, there is no fan, so when plugged it it’s “just” wasting juice, if not actually anoying me. Previously warm-up time from true power-off was maybe 15 minutes, but now I use an external Rubidium reference, which locks withing 2-3 minutes so counter warm-up is not a problem. I tend to keep the counter on a power strip which I shut off when I leave my workbench, but i might take a look inside to see if a hard power switch is an option.

    Somebody pickier than me might sugest heatshrink over the IEC socket and switch solder lugs… (Speaking as someone who’d picked up a home-made amp that was switched off, but still plugged in, when I was about 15 years old. I learned a lot about mains AC and the coduction properties of human skin, that day… )

    • Hi Laurence, I did put heat shrink over the switch lugs, its clear so did not stand out on video, you will see it if you look hard enough. I did not put it on the IEC connector because it was not done originally – I was going to say “good enough for HP, good enough for me” but I guess the whole point of the mod would suggest otherwise 🙂 I should have used heat shrink on the IEC connector – its a fair cop…. Gerry

  2. Pingback: DIY HP/Agilent 53131A 010 High Stability Timebase Option | gerrysweeney.com

  3. Thanks for your video!

    BTW, the fan is 40x40x20mm, 12V, if anyone intends to swap out for a silent type like I do.

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