Sony TVP-08, Training Manual

Page: 107 / 139

99
HV Regulation Control
Overview
The HV regulation control circuit uses a tap from the HV Block to keep the
High Voltage at a consistent level. It does this by setting up a reference
voltage and comparing that to the voltage from the HV Block tap. This
circuit also contains a +12 volt LVP that will lower the High Voltage if the
+12 volt line should lower.
Regulation Control
The HV Block has an output that is representative of the High Voltage and
rises and falls with the level of High Voltage. The tap from this block is
input to the D board at CN8006/3. This voltage, typically around 7.2 volts,
passes through R8174 to IC8007/5 Inverting Input. This portion of IC8007
is used as a buffer. Whatever voltage is input at IC8007/5 is output from
IC8007/7. This voltage is output to the HV Protect circuit and coupled
through R8128 to IC8007/9.
IC8007/10 Non-Inverting Input is a reference voltage that has been set at
the factory. It uses the +12 and +135 volt lines to set up a bias across the
B-E junction of Q8026. The conduction of the transistor, along with the
value of R8196 and R8201, set up the voltage to IC8007/10. R8201 may
or may not be present in the set. If it is, it will be mounted on the post of
CN8008. The High Voltage is adjusted for 31 KV at the factory. It should
be checked and adjusted according to page 57 of the service manual if
parts are changed in the High Voltage circuit.
The voltages at IC8007/9 and 10 are used to determine the High Voltage.
They should be nearly identical in value when the circuit is acting nor-
mally. This portion of IC8007 and its associated components are used as
a filter and gain control circuit. Its purpose is to reduce the input voltage
and filter the HV Block signal so that the set’s response to changes in
High Voltage are not too fast or too slow. Typically 6.9 volts is input and
1.5 volts is output. This voltage is sent to IC8007/3, which is a buffer, and
then output IC8007/1. It will then be used to vary the HV Regulation
PWM signal.
+12 High Voltage LVP
Q8025/B is connected to the +12 volt line. Q8025/E has 10 volts present
on it because D8039 holds this voltage. This keeps Q8025 OFF, which
keeps Q8024 OFF. If there should be a problem on the +12 volt line, this
would cause Q8025 to turn ON. If Q8025 turns ON, then Q8024 would
turn ON and cause the High Voltage to be lowered by placing 0 volts on
IC8002/3.