HP 8340A Service Manual page 150

PHASE LOCK INDICATOR
The input to the Phase Lock Indicator is the wire OR outputs of
the phase/frequency detector
input is ECL LOW
in this condition, the de voltage at the base of Q3 is lower than
that at the base of Q2 so Q3 is ON and Q2 is OFF. If the loop
unlocks, the input to the phase lock indicator consists of
varying width pulses, the average de value of which is about half
way between a logic LOW and HIGH. The voltage divider consisting
of R35 and Rl5 causes the voltage at Q2 base to be lower than
that at Q3 base, so Q2 turns ON indicating an unlock condition.
Model 8340A - Service
G
(Al6B pin 14 and Al6A pin 3). This
(approximately 3 volts)
Scans by HB9HCA and HB9FSX
when the loop is locked;