Philips DPTV400 Series, Training Manual

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AC Input (Figure 5)
AC power is applied to the to the AC Input panel via connector 1206. A series of spark gaps and
capacitors protect the set from damage due to excessive voltage spikes on the AC line. Filter choke
5000 prevents switch mode noise generated by the set from entering the AC power system, causing
interference with other electrical devices. Bridge 6000 produces RAW DC for the Main power sup-
ply located on the LSB. Bridge 6001 produces RAW DC for the Standby supply located on the AC
input panel.
Standby Power Supply (Figure 6)
The Standby supply is located on the AC Input panel. The Standby Supply provides the 5-volt
Standby voltage to the set in the Standby mode. When the set is turned On, it pr15, +9,
+3.5, +4.5 and +5.2-volt supplies.
Standby B+ is applied to the Standby Switching regulator, 7218 via Pins 6 and 4 of 5202. An inter-
nal switch in 7218 charges capacitor 2283 connected to Pin 1 of the IC. When the charge on 2283
reaches 5.8 volts, the internal switch switches to internal. The IC is now being powered by the
charge on capacitor 2283. The internal FET drives transformer 5202 until the charge on capacitor
2283 reaches 4.8 volts. The IC repeats the cycle until the 5-volt Standby voltage reaches the cor-
rect level. Shunt regulator 7212 then turns On, turning opto-isolator 7213 On. The operating volt-
age for 7218 is then supplied by the rectified Hot secondary voltage from Pin 1 of 5202. The inter-
nal regulator keeps Pin 1 at 5.8 volts. Regulation is accomplished by monitoring the 5-volt standby
voltage. If the 5-volt supply increases, shunt regulator 7212 will conduct more, causing the resis-
tance of the transistor inside 7218 to decrease. The sensing resistor, Re, inside 7218 will sense the
increase in current and reduce the On time of the internal FET, which will lower the 5-volt supply to
the correct level.
In Standby, the +5 volt, +4.5 volt, and 3.5-volt supplies are being applied to the set. The 3.5 and
4.5-volt supplies are switched on the Power Interface panel. The Standby 5-volt supply is applied to
the Processor located on the SSB. The +15, +9, and +5.2-volt sources are turned Off. When the
set is turned On, the Standby Line goes Low, turning transistor 7214 Off, turning 7205 On, which
turns 7215 On. This switches the +15-volt supply to the set. The +15-volt supply then switches
FIGURE 5 - AC INPUT
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