Amprobe AM-540 User Manual page 17

(for Frequency only)
2. Measuring Frequency by using AC Voltage function
Step 1: Turn the rotary switch to  position.
Step 2: Connect test leads to the circuit. Connect the common (COM) test
lead to the circuit before connecting the live lead (connecting diagram
refer to "Measuring AC Voltage").
Primary display shows AC voltage measurement reading.
Secondary display shows frequency measurement reading.
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