HP 53131A Frequency Counter Teardown and Spillage Clean-Up

One of the instruments I have is a HP 53131A Frequency Counter, and putting the positives of this unit aside, its by far the most annoying bit of test gear I have! Why? well for some reason, when you apply power to the thing the fan runs even when its switched off. This is because the power switch is a soft switch and with just the AC cord plugged in the fan runs and makes the annoying fan noise. This is a really poor design and certainly not one of HP’s shining examples of engineering….I like to be able to switch my stuff off without reaching around the back and having to pull the power chord or unplug from the wall outlet – just plain crappy! HP what where you thinking?

I thought I would tear it down and have a look and see why this is, and perhaps see if there is any possible modification I could come up with to improve on this. When I took it apart I found a real mess of dust and some kind of substance spillage so a full clean-up was needed which somewhat distracted me from the power switch problem.

While its in bits, I also do a quick run through of the major components of the circuit and have a quick look at the power supply and power switch circuits in some detail.

UPDATE: The clean-up also appears to have resolved the strange trigger problems I had seen previously with this counter. Well pleased…

Here is the Agilent 53131A Component Level Manual including schematics for the frequency counter

Thanks for watching…

This content is published under the Attribution-Noncommercial-Share Alike 3.0 Unported license.


File Description File size Downloads
pdf Agilent 53131A Component Level Manual Component Level Service Manual for this counter
3 MB 6142


  1. Gerry,
    If your going to have the ‘Benny hill’ music, where are the young dolly birds that go with it ?

  2. Hello again, Gerry, Hmmm, an interesting peek inside the Agilent 53131A. I *was* eyeing one of these up, but in truth there is nothing wrong with my trusty (and silent!) HP5316B. Maybe I’ll stick with it.

    I guess that little fan is only 12V – why not simply move it across to the 12V or (better) -12V switched rails? At maybe 100mA, the power supply should have no problem driving it. I’d imagine that the -12V rail may have more spare capacity than the +12V rail??

    I guess the smps is a stock unit, that served in other Agilent or HP or even other gear? hence the “spare” 9V line, not used in *this* application.

    The “L” shaped PCB intrigues me. I guess it came about, because the outside form factor of the overal instrument was set (tooling, compatibility with other equipment), and this version of the product simply needed less PCB real estate that the case offered. In production, they could probably “nest” to “L”-shaped boards together, so at least save some PCB and assembly costs, that way.

    I wonder if the mystery fluid was Coke or Coffee? 🙂

    1. Hi Laurence, not sure what you would achieve moving the fan to the other supply rail? The problem is the SMPS is running all the time. I think the “L” shaped PCB may well make space for options. I have been thinking about installing a rubidium 10Mhz reference into this counter, there seems to be plenty of space for it but that will be for another time. The fluid was definitely not water based, it was some kind of oil-based stuff. Gerry

      1. … I meant move the fan power to the (soft) switched side, so that at least the fan is only on when the unit is powered and ON. OR you could just unplug the fan and see if the counter objects!

        Sure, there is room for a Rubidium reference in there, but those things are POWER HOGS! I can (nearly) do TOAST on mine – it burns about 25W when running (FE-5680A). The existing power supply and themal management won’t handle that, of course.

        EEK! And I’ve just seen the latest prices on eBay for those counters, I think I’ll love my LED HP5316B a little longer…


        Any thoughts on that Maynuo serial/USB lead?

        1. Hi Lawrence, I understand what you are saying now. I think thats part of the design issue, the fan is required to circulate air around the PSU, even when its idle things run a little bit warm so putting the fan on the switched size of the 12v line would not be a good idea. Good point about the rubidium references, I do not have one so I did not know how hot they run, I guess I will not do that then 🙂 Gerry

  3. Hi Gerry,
    please tell me how to purchase with paypal the PCB for upgrading my HP53131a counter.
    Many thanks

  4. Hi Gerry.
    I believe I’ve seen similar grey goo! The goo was a foot.

    Some brands of those grey, “rubber” equipment feet are actually made of plastic which, over time, degrades and reverts to a more fluid form. Given sufficient time it flows very much like a liquid – but very slowly.

    1. Hi Hairy Pete, indeed you are correct, I have since modded a bunch of these counters and I have seen it time and time again, its the stick-on rubber spacer used in some counters, looks like the rubber breaks down into a liquid. Alcohol cleans it up nicely though. Thanks for the comment. Gerry

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