Why?

I consider my FT-991A fan a bit noisy (or my environment is too silent) and I was considering replace it.

My FT-991A ships a Panasonic ASFN60371 fan. After checking its Specifications, I have found that its noise is rated at 30.5 dB(a).

Browsing in Digi-Key inventory, I have found a similar size and quieter 27 dB(a) SunOn fan.

By comparing the specs, I found that the SunOn fan has a bit more of air flow (23.5 CFM vs 21.8 in Panasonic). Another advantage; while the Panasonic is a ball-bearing fan rated 1.92W, the SunOn features a proprietary magnetic-levitation bearing, and is rated 900 mW, a bit less than half the power. Well, not that massive savings in terms of total power budget, but still an advantage. So, more airflow, less power consumption, less noise.

Fan Control

The FT-991A features an completely analog fan control, reacting in realtime to temperature conditions. By reading the FT-991A schematic:

1. The Temperature is measured in both V/U and HF boards by a LM20 sensor, providing THVU and THHF signals respectively.
2. Q4030 (a SPDT switch) selects either THVU or THHF signals, selected by the Main Unit HFB signal, based on the frequency that the transceiver is operating at the moment.
3. Q4030 COM signal feeds Q4032 LM2902 OpAmp with the TH signal (which also goes to the CPU), bias a few more transistors and finally delivers the power to the fan.

Important: There's no ground lead!! Both leads are controlled by the OpAmps.

The fan is a two-lead powered and has no tach (speed sensor) feedback.

Heavily redacted FT-991A Fan Control Schematic. Click to enlarge.

Replacing

The replacement is super straightforward.

1. Remove the top cover
2. Remove the shield (be extra CAREFUL to not strand the speaker cable)
3. Remove the fan's 4 screws and remove it
4. Install the new fan. In order to do something non-destructive in the old fan, use a female Jumper Connector as the power lead connector. Be sure to notice which pin goes to the black and red wires. Double check with your original fan.
5. Pay extra attention in the polarity!!!!!
6. Reassemble the radio, reverse order.

Impressions

On hand the new fan feels way lighter - Well, we're replacing a ball-bearing fan with a magnetic levitation fan.

During tests, I stressed the radio by doing FT-8 in FM band at full power (50 W) which is the most heat-stressful operation of the radio, in my use case.

One interesting finding is that I perceived that the original Panasonic fan has a noticeable wail tone. The SunOn fan makes noise, of course, but mostly sounding like 'wind', with no companion wail.

The step 1 fan rotation noise almost compares to the Panasonic. But as the radio gets hotter, the new fan is definitely more comfortable, especially by the lack of the wailing. The ramp-up ramp-down profiles are practically the same - didn't find any noticeable difference.

Oct 2021 update

More than two years later, radio is doing great, withstanding the Brazilian summer, 4 minute full-power 100W SSTV TX, RTTY, FT8 in full power in both V/HF bands and doing great. No degradation or whatsoever in fan (or radio) so far.

I found that it was starting to develop a wailing tone - but it was dust build-up in the fan blades. Remove the fan, give a good clean with a soft brush and enjoy a quieter operation again.

Properly selecting a fan

Selecting a fan by solely relying its advertised airflow is plain bad practice. Jim W8NRX brought to my attention that one should pay special attention to fan performance curves, because the fan will put some effort sucking the air through the vents, circulating through the radio and finally exhausting it - and not all fans behave the same when under load.

For the uninitiated, Static Pressure roughly translates to the effort that the fan puts to suck the air: The higher the static pressure, the more difficult to suck the air.

Considering the Panasonic fan datasheet Static Pressure x Airflow curves:

Panasonic fan Static Pressure vs Airflow curves

Now, from the SunOn fan datasheet:

SunOn fan Static Pressure vs Airflow curves

The selected SunOn fan for this project follows the curve number 2.

When comparing the curves, we can see that the SunOn curve number two outperforms in every aspect the Low and Middle curves of the Panasonic airflow curve - while Panasonic outperforms SunOn on its Standard (the top curve) while in high static pressures (high static pressure: congested environments, difficult to suck the air), while SunOn delivers a lot more air under lower static pressure.

It might be possible to infer that the SunOn will be on par with Panasonic fan performance when under heavy load, and exceedingly providing more airflow when the fan runs in lower speeds.

What about Noctua fans?

Noctua is a reputed company known for its silent fans. They sell a fan that fits the same factory size (60x60x25), in its NF-A6X25-FLX product. However, it is a no-no for the FT-991/A. Keep reading.

The original Panasonic blows 21.8 CFM, while the Noctua fan is rated 29,2 m³/h - which results in 17.18 CFM, which is way below the original fan. And finally, Noctua doesn't disclose airflow curves for their product.

I find these extremely concerning and is a blocker - and I do not recommend anyone using the Noctua fan in your 991s.

Hope that helps; 73 de PY2RAF.

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