HP 5890 Series II Plus Reference Manual page 142

Detector Systems
Electron capture detector (ECD)
A very clean system may produce a value below the low end of 10 (100
Hz). To correct this condition, an adjustment is made to the present
potentiometer located on the ECD electronics board.
ECD Potentiometer Switch
ECD Potentiometer Adjustment
ECD Potentiometer Switch and Adjustment
1. Remove the right side of the panel.
2. Flip the switch up to the Adj. position.
3. Using a small screwdriver, turn the potentiometer clockwise until the
background level is at or slightly above 20 (200 Hz, low reading for
trace analysis).
4. Replace the right side of the panel.
142
Up: Adj
Down: Fixed