Pioneer PDP-503CMX Product Training Manual page 38

Address Resonance & V-Mid Assembly's
The two Address Resonance circuits are located next to each Sub
Address PCB and the V-Mid circuit is located just to the right of the
power supply PCB. In later production models the V-Mid assembly
will be located on the power supply bracket. These circuits are used
to protect the address driver IC's in the event of extreme picture
changes. An example would be if the entire picture was alternating
each cell black and white. This causes the IC's to overheat. The
Digital Video circuit will detect the rapid frequency change in the
addressing signals and will turn on the V-Mid clamp switch. The V-
Mid circuit will take the normally 60 volt address output signal and
lower it by half (30 volts). The voltage will return to normal once the
Digital Video circuit has determined the frequency to be an
acceptable rate.
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