There are two adjustable items on the board. Refer to description, layout picture and schematics

• Switch on the PA100, keep it in STBY state
  
• Select the TEMP indication with the METER switch
• Measure the DC voltage from the test point TP1 to the GND (use multimeter)
  
• Turn the TEMP trimmer R14 on the RF amplifier board until the test point TP1 voltage reading is 1.0 V ± 0.1 V
• Turn the CAL trimmer R29 on the analog control board until the reading is in the middle (50ºC) in the meter scale
  

This adjustment is not mandatory, because the default fan starting temperature is abt 55ºC. The adjustment is however recommended.

• Stay in the TEMP reading with the METER switch
• Simulate 50ºC temperature with the TEMP trimmer R14 on the RF amplifier board by keeping the temp reading in the middle scale (50ºC)
• Adjust the FAN trimmer R2 and monitor fan starting
• Leave the FAN trimmer at the fan starting threshold position 50ºC

If you like, you can set the fan starting temperature for a lower temperature level.