ATS-2 Audio Test system - Fan replacement

The problem 

I purchased this excellent audio tester years ago, but it wasn’t until recently that I truly had a need for it. In my otherwise very quiet home laboratory, it didn’t take long for the cooling fan’s unpleasant, ear-straining buzz to start bothering me. It wasn't necessarily just the volume of the sound, but rather its harsh spectrum. There was nothing else to do but replace the propeller. The cooling cannot simply be removed, as the internal power dissipation can rise up to 70W in the worst-case scenario.

The ATS-2 Audio Test System  from Audio Precision

I want to emphasize that I do not recommend this modification to anyone who is not proficient in electronics. A novice could damage the device, and of course, the warranty is voided once the lid is opened, though such a concern is unlikely to persist with devices this old.

The reason

The original fan is quite small, a 40mm Sanyo Denki, model 109P0424F302. Here is a link to the datasheet:

Original fan: 109P0424F302

The original fan moves 8.62 CFM (cubic feet per minute) of air - at the nominal voltage of +24V. The ATS-2 supplies approximately +21.3V to the fan (at least in my unit), which means the fan is intentionally being run at a slight undervoltage, likely to mitigate noise, but this also reduces the airflow. As seen in the specifications, the noise level is about 30 dBA. Fans from that era utilized traditional, rather crude ball bearings, which explains the harshness of the sound.

The replacement

Noctua is well-known for its efficient and very quiet fans, so I turned my attention there. They have one 24VDC model in a 40mm size that moves 5.24 CFM, which is fairly close to the original flow:

Noctua +24V 40mm fan

However, I didn't want to risk the device overheating, as this new fan (with already a somewhat lower flow rate) would also operate at an undervoltage, resulting in even lower airflow compared to the original. Therefore, I decided instead to acquire Noctua's 12V version, which has nearly the same air displacement at its nominal voltage but a much lower noise level of only 17.9 dBA. Feeding it the nominal voltage to it (12V) guarantees that the fan runs according to the specification. While modifying the analyzer's PCB to output a true 24V might be possible, I did not consider it a wise approach, as it's best to leave the original circuitry of such a precision instrument untouched.

Noctua +12V 40mm - lowest noise fan

Implementation

Since this is a +12V fan being supplied by +21.3V, the voltage needs to be dropped. An 8.2V Zener diode would be a natural choice, but I don't really stock such "old-school" components in my home lab. Since this fan only consumes about 0.25W (20mA), it is easy and feasible to use a suitable series resistor to achieve the voltage drop.

 An 465 ohm resistor is required:  (21.3V-12V)/0.02A = 465 

I used a parallel connection of two resistors; 1kΩ and 820Ω, but one standard 470Ω resistor works just as well. The power dissipation in  the resistor is approximately 190mW, be sure to choose a resistor with a power rating of at least 1/4W. 

Resistors in series with the feed, dropping the voltage to a suitable +12V level (shrink tubing not yet positioned over the resistor and solder points. The old connector (to the right) from the old fan is reused.

The benefit of using a resistor is that polarity is irrelevant; it can be placed in series with either of the power wires. 

With this specific fan type, there is a yellow PWM feedback wire from the fan. Cut that wire and make sure its end is properly insulated, as it is not needed.

Mechanical

To gain access to the inside of the device, remove the screws securing the top lid. Some screws may be covered by warranty stickers, which must be peeled back to access them. The screws are located at the front and rear edges of the lid, as well as on the left and right sides. There is also one screw on the top of the lid, near the far-right edge.

I used M3 × 18 mm screws with M3 Nyloc (friction) nuts for the blower assembly to ensure the nuts do not vibrate loose. Do not overtighten the screws, as this would reduce the effectiveness of the anti-vibration pads in the fan corners. Be sure to reinstall the original metal fan protecting grille on the outside (not pictured). The 18 mm screws are long enough to accommodate it as well.

You need M3x18mm screws and M3 Nyloc nuts, 4 of each.
The old screws and nuts should be scrapped.

The connector on the Noctua fan’s cable does not fit the tester's PCB, but you can salvage the connector (along with a bit of cable) from the old fan. Make sure to insulate the soldered joints and the resistor properly with heat-shrink tubing. The wiring is simple; red goes to red, black goes to black. Keep the fan cable as far away from the audio sections as possible.

Finalized installation of the replacement fan, and a proper location for the fan cable.

Important Note: Orient the fan so that it pulls air from the inside and blows it out through the rear, maintaining the original airflow design. Most fans, including Noctua and Sanyo Denki, have small arrow markings embossed on one side of the frame; one indicates the direction of blade rotation, and the other indicates the direction of airflow.

Happy Analyzing! — Christian