Please check the following picture

 
You need to use a screwdriver to turn the Yellow button.  And of course use a multimeter to see the value.
 
The Power supply for Printhead (Xaar 128Plus ) is 33V7.5A
 
Usually the voltage for the printheads is between 30V to 33.5V.  It depends on the INK, temeperature and Humidity.
 
First you need to set the voltage to a value.  For Example  31V.  Then printing the solid color picture. ( K  C  M Y  100%, or mixed 100% )
 
When you print the picture without nozzle clog ( After print run a printhead status test to see the nozzles ), then it means the voltage is in good value.
 
If you have nozzles clog, then you need to decrease or increase the value of printheads. ( First adjust amout is about 1V each, then 0.5V each, then 0.2V each )
 
After several times printing you will find the best Voltage for the printheads in your place.
 
Attention:  The voltage should not go over 34V.  Otherwise the Carriage board will be self protected . ( Can't print anything )