Sony TVP-08, Training Manual

Page: 23 / 139

15
Soft Start
Overview
A Soft Start circuit is necessary to keep the oscillator that drives the switch-
ing circuit above the normal operating frequency of the tuned circuit that
is in the switching supply circuit. If this frequency is not above the normal
operating frequency at start up, the voltage at the secondary could be-
come too high and cause damage to the set. The soft start circuit causes
the oscillator to start at a frequency high above the normal operating fre-
quency by holding the regulating voltage down at initial turn ON of the set.
This circuit is also activated if the Switching B+ voltage falls below a cer-
tain level.
Soft Start – Power ON
When Power ON is selected, the Vcc switch supplies voltage to IC6003
Oscillator. This oscillator is connected to the regulation line that begins to
develop voltage. This voltage is held LOW at Q6006/E while C6016 is
charging. Once C6016 is charged, the regulation line is controlled by
IC6005/4.
Q6005 provides a discharge path for C6016 when the set is turned OFF.
This is important because if C6016 is not completely discharged, the os-
cillator may output the normal operating frequency during Power ON. The
discharge path would be through Q6005/C-E junction. Q6005 is OFF
during the set’s operation because of the voltage applied to it from IC6011/
3. Be careful when measuring voltages at Q6005/B as this circuit is easily
loaded by a meter or a scope. It is best measured using a scope and a
10X probe.
Soft Start - LVP
The soft start circuit can also be activated if the voltage from D602/3
Switching B+ goes too low. When the voltage across R6007 drops below
12.6 volts, it will cause Q6002 to turn OFF. This causes Q6003 to turn
ON. When Q6003 is ON, the cathode of D6014 will be held at ground
potential. This is the same condition that occurs at turn ON, therefore the
oscillator will oscillate at a high frequency and this will reduce the output
voltages from the secondary supplies. If this occurs while the set is oper-
ating, it will shutdown. The set will act as if there was an AKB failure, the
Timer LED will flash five times, pause and then repeat.