JVC RX-D Series Training Book page 36

PROTECTION CICUIT
4/8
PROTECTION 5
REG DET input:
This input detects the regulated voltages; A+5V_Vi, A+5V_DSP,
and +9V_Vi_Tu. Any of these three voltages missing can cause
one of three transistors (Q162, Q163 & Q161) to turn off and
give a LOW state input at pin 28 of IC501.