A quieter HP-53131A: how to remove whining fan noises for $1

The Hewlett-Packard HP53131A counter is plagued with a high pitched noise that never dies because this instrument only has soft power and never turns off its fan in order to preserve the accuracy of its internal timebase OCXO (oven controlled oscillator). This is not an issue for an industrial environment but on a lab workbench... Ugh. Many have complained, some even changing the soft-power button for a hard one. This solution is pretty awesome but presents two problems: the obvious loss in timebase accuracy (unless you leave your counter on for 48 hours every time before using it) and the fact it's still going to be noisy when switched on.

It turns out there's a two-step simpler though not as radical silencing option: change for a quieter fan and replace the mounting screw with vibration proof mounts.

The first part is rather obvious, and there's a few options for fans that are much quieter than the original HP part while retaining a decent air flow. For example The Sunon MF40201V2-1000U-A99 or if you are ready to sacrifice a bit of airflow the CUI Devices CFM-4020V-145-123. I have used the first one to replace the dead fan in the counter I picked at auction. But even with this new fan a very irritating whining noise remained at a frequency around 1800Hz (this of course depends on your fan).

It turns out that annoying-Orange grade of annoyance noise can be completely obliterated with a $1 solution. You will have noticed that the original fan is mounted using 2 crews out of 4 only, and the screw mounts are a little separated from the rest of the mounting flange. Both contribute to creating more vibrations and thus more noise than is necessary. Two products can help here: a rubber flange or rubber shock mounts that replace the screws.

I used the 'Spectroid' phone app to make (very) approximative measures of the relative sound levels of each option. Before any changes (screw mounts) the noise level at the peak was around -70dB at 1820Hz. With the rubber flange this was reduced to -75dB at 1555Hz. Most of the gain was in the whine that had already almost completely disappeared. I was not expecting such a big improvement for such a little hack. Finally, the rubber replacement for the mounting screws got completely rid of the whining noise (-80dB). The absolute value of those decibel numbers doesn't matter: they are in fact relative to the maximum level the phone can capture from its microphone. But what we're really interested in here is their relative value: the rubber flange reduces the annoying noise by 5dB, and the screw replacements by 10dB. In practice the gain is a lot more impressive than the number seems to tell. That the rubber screw replacements work better is expected as they completely isolate the fan, while the rubber flange only reduces vibrations in the compression direction (the screws still hold the fan in tension). Four of these little rubber fan mounts cost around $1, so no reason not to add this to your next Digikey order!

The new rubber fan mounts

After replacement of the fan mounts the whining noise remains under -80dB. Unfortunately I forgot to take a pic of the 'before' spectrum :(

Best of all, this also works for other HP products that have the same issue, like the 33120 signal generator and I suspect quite a few recent products that have a tendency to all have fans these days.

Great success!