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Power Supply Block
The power supply consists of several stages. Except for the standby power
supply, the Main Micro IC3251 controls the operational sequence of the
remaining stages. The stages are listed in the order of operation:
•
Standby Power supply
•
Power ON stage
•
Picture tube Degaussing
•
Converter 2 stage
•
Converter 1 stage
•
Protection stage
Standby Power Supply
This stage is active as long as the TV is connected to AC. Therefore it
always supplies standby +12 volts and standby +5 volts.
Standby +12 volts is applied to the master power switch which is routed to
the AC relay's coil when the switch is pressed (latched ON). The +5 volts
powers the Main Micro IC3251 and infrared receiver so they can detect a
power ON command.
Power ON
When a power ON command is received by the Main Micro IC3251, it first
powers the AC relay RY5501. RY5501 feeds the converter 2 circuitry. It
then powers the Degaussing coil relay RY5503 and the converter 1 stage.
It is the converter 1 stage that makes +135Vdc for high voltage develop-
ment.
Picture Tube Degaussing
Degaussing is a common method color TV manufacturers use to elimi-
nate residual magnetism in the picture tube. The residual magnetism in
the picture tube acts as a magnet, causing the electron beam to be pulled
away from its target. AC current applied to the degaussing coil creates an
alternating magnetic field to disorient the tube's residual magnetism, as-
suring proper beam landing and color purity.
55
Waveform PSblk - Power Sequence
Channel 1 - AC relay output of Main micro IC3251
Channel 2 - Main relay output of Main micro IC3251
Channel 3 - DG output of Main micro IC3251 to RY5503
All waveforms are 5Vp-p; time base = 2 sec/div
This degaussing stage is powered with 60Hz AC immediately after the AC
relay is energized. The heavy current that passes through the two low
resistance degaussing coils also passes through the negative tempera-
ture coefficient thermistors. As these thermistors heat up from the cur-
rent flow, their resistance increases until the coil current and magnetic
field is negligible, concluding degaussing. As a precaution, the degauss-
ing coil relay is de-energized 9.5 seconds after being powered.
Converter 2 Stage
Converter 1 and converter 2 stages are similar. Their only differences are
how they first power up and what voltages they deliver to the TV.
After the AC power relay is energized, Converter 2 gets a kick-start volt-
age from Q5005. R5073 limits the amount of current through Q5005 to
start converter 2's oscillator. Once the converter is running, one output
(D5037) sustains the Vdd operating voltage. The higher converter voltage
reverse biases the kick-start diode D5014, blocking the start voltage from
transistor Q5005.
Regulation for the converter 2 stage is accomplished by sampling the +5
volt output and returning it to the oscillator. The sample voltage causes a
converter oscillator frequency change, which results in the voltage correc-
tion.
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