Switch Mode Power Supply - Philips H8 Technical Training Manual

POWER SUPPLY
Switch Mode Supply (Figure 1)
The Power Supply is a Switching type supply.
The Power Supply begins operating as soon
as AC power is applied to the set. AC power
is applied to the Large Signal Board through
Fuse 1900 and applied to the Bridge Rectifier
6915. The rectified B+ voltage is applied to
the regulator switching IC, 7921, through Pins
6 and 3 of transformer 5912. Startup voltage
for the supply is applied to Pin 4 through
resistors 3914 and 3913. The Zener Diode
6921 prevents the voltage on Pin 4 from
exceeding 33 volts in case the Regulator IC is
not working. When the voltage on Pin 4
reaches 16 volts, the Oscillator inside the IC
begins operating. Once the Supply is in full
operation, the operating voltage for the supply
is supplied by Pin 8 of 5912 and is rectified by
6929. The Oscillator drives the internal
switching FET. When the FET is On, energy
is stored in 5912. When the FET is Off,
energy is transferred to the secondary of
5912. The Frequency and On time of the
internal switching FET is determined by the
Internal Comparator, the internal RC network,
and the Magnetizing Inductance of the
Transformer. The Internal RC network also
determines the Off time of the FET switch.
The On Time is determined by the ramping
voltage across 3924, 3923, the feedback
voltage from Pin 8 of the transformer, and the
voltage from the feedback opto-isolator. The
Reference Voltage for the supply is the VBAT
(135 volt source). This source is sampled
by 7971 which drives the feedback
opto-isolator 7929 to set the switch time of
comparator B. This controls the On time of
the internal switching FET to maintain the
VBAT voltage at the correct level. There
are four output voltages from the
transformer which are the VBAT voltage, 8
volts to the 5 and 3.3 volt regulators, 8
volts unregulated, and 27 volts for the
Audio Output circuit.
To troubleshoot the Power Supply, first
check the VBAT (135 volt source). If this
voltage is missing, check for a short on the
VBAT line. The most likely cause would be
a shorted Horizontal Output Transistor. If
the voltage is pulsing or is Low, check the
feedback circuit, 7971 and 7929. Also
check Fuse 1941 on the 8 volt unregulated
supply. This line supplies the operating
voltage for the feedback circuit. Next,
check the B+ voltage for 7921 on Pin 3. If
the drive signal is pulsing On and Off on
Pin 3, there may be a problem with the
feedback circuit. If there is no drive signal
on Pin 3, check for the operating voltage
on Pin 4. This voltage must ramp to 16
volts to start the supply. If it is missing or
is not ramping to 16 volts, check 3913,
3914, and 2919. Regulation problems
could be caused by resistors 3923 or 3924
changing value. If 7921 should fail, these
resistors should be changed by using
original parts only.
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