How can I reduce fan noise on a Keysight E1401B/E1421B?

To lower the fan speed, follow these instructions:

The power supply for these products is Keysight 0950-2283. This modification will not affect warranty of the product.

The fan output voltage of this supply can be lowered by removing resistor R148 on the LRA board of the power supply. This will lower the fan voltage from a nominal 9.5 volts to 8.5 volts. This change will lower quiescent fan speed and noise emitted from fan. This change will not affect the circuit's response to increasing the fan speed as the internal air temperature increases.

1. Remove the power supply from the instrument.

2. Place the power supply on the bench with the ZYTEC label on bottom (markings on the supply panel appear upside down).

3. Orient the supply so that you are looking at the end opposite the input power connector.

Viewing the board on bottom is the LRA board. Locate the group of eight components behind 14 pin dip. The group has starting from panel 5 resistors - 2 capacitors - 1 resistor. The 4th resistor in the 5 resistor set is R148. This resistor has a white outline silksreen around it and is clearly distinct from others. This is R148.

4. Take a pair of cutters and clip resistor leads to remove resistor from pc board.

5. Replace power supply back into instrument.

Noise reduction is appreciable, but it has not been quantified.

Note: This modification is not possible on the E1421A.