HP 5335A quiet fan mod - using original fan

[SilverSolder]

The HP 5335a Universal Counter is a great instrument, but it is among the noisiest pieces of equipment due to its fan.

So I finally got round to taking the lid off the 5335a with the intention of replacing the fan with a quieter one.  But the old fan turned out to be a masterpiece of German over-engineering, a beautiful die cast Papst unit rated for 10 years continuous operation at full speed.  Let's see if we can civilize it?

A significant part of the noise from a fan actually comes from the torque pulses from the motor being coupled to the aluminum back plate, which acts like the sound board of a guitar to amplify it up.  The stronger the fan, the stronger the torque pulses.

An obvious solution is to mount the fan using vibration preventing grommets.  The problem is in the practical implementation - this type of grommet needs some space for the rubber to operate and absorb the vibration.  The smallest grommets that I could find that looked like they might work require a hole with a diameter of 0.3" and look like this:



An hour of quality time with a drill stand and two different end mills later, the fan mounting "ears" were modified to take the grommets around 2/3 of their circumference, set back into the thick flange of the fan:




When installed in the 5335A, the fan is now fully floating, about 1mm off the surface of the back plate, using some appropriately sized washers.



The results of this mod alone, was astounding!  -  it completely took the harsh edge off the sound, it became more like a "woosh" of rushing air than the sound of a loud vacuum cleaner that it had before.


However; the fan is still louder than it needs to be, pumping way too much air for the amount of heat generated in the unit and the temperature conditions in my lab area (never higher than 25C). 

I considered adding one of those PWM fan controllers for computer case fans to regulate the speed but thought better of it...  why would you put an electrically noisy thing like that inside your test equipment?  Powering the fan in situ using an external DC power supply showed that at around 15V, it was pleasantly quiet and still pumps more than enough air to keep the unit cool.   I considered running the fan off the +15V supply rail (instead of the 25V unfiltered supply that it uses from the factory)...  but again, why would you want to put an electrically noisy fan on the power rail of a test instrument?  -  I ended up keeping it simple:  a 150 ohm, 1W resistor in series with the positive lead, strategically placed in the air flow of the fan:



Winner, winner, chicken dinner!  The 5335A has now gone from being by far the noisiest test equipment I have, to one of the quietest - it is nearly silent - significantly better than many other instruments where the fans have been swapped without the use of vibration dampening grommets.

I will definitely be retrofitting other equipment with the rubber grommets! They are a bit of a pain to install, but they work extremely well.  The solid old Papst fan will hopefully last 20 years at the reduced RPM, so the work won't have to be re-done too soon...