Surge Protection Circuit; Operating Voltages And Resistances - Sony CPD-G400 Training Manual

Circuit Operation
Degassing only occurs after the front panel power is pressed. The se-
quence of operation is as follows:
1. Press the power ON button (not shown).
2. 158V is applied to the power supply stage (not shown).
3. The power supply stage makes standby +5V for the CPU IC1001 and
reset IC1003 (not shown).
4. Once the CPU is reset, IC1001/pin 5 outputs a HIGH to activate the
degaussing circuit.
5. The CPU HIGH turns on relay driver digital transistor Q608 (has inter-
nal resistors to operate with 5V input).
6. Q608 grounds degaussing relay RY601, energizing it.
7. RY601's contacts close passing 120VAC through TH601 to the de-
gaussing coil around the bell of the picture tube. The AC demagne-
tizes the internal metal parts of the picture tube.
8. Within two seconds TH601 has increased resistance from 4.2 ohms
cold to a few 100k ohms hot.
9. In approximately six seconds after the power button was pressed,
CPU IC1001 discontinues the HIGH from pin 5 so the degaussing
relay contacts open, removing power from the degaussing coil.
10. CPU IC1001/pin 5 will remain LOW as long as there is standby +5V
applied to it (power switch is ON).

Surge Protection Circuit

At power ON this monitor consumes power in two places:
• Degaussing coil circuit = About 6 AC amps.
• Monitor power supply = About 0.5 to 1 amp depending upon the charge
of the filter capacitor (C611).
The surge protection circuit reduces the amount of power and voltage
fluctuations to the monitor's power supply until the degaussing circuit has
completed its task. This is accomplished by placing a current limiting
resistor (thermistor TH600) in series with the bridge rectifier D625. 1.8
sec after the power button is pressed, the current limiting resistor is jumped
out of the circuit.
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The sequence of operation is as follows:
1. Power ON button is pressed.
2. Heavy current goes to the degaussing circuit.
3. The monitor's power supply is current limited by TH600 (6.5 ohms
cold). Although delayed, the power supply voltages come up.
4. +15V is one of the power supply voltages. This voltage is applied to
the surge protection circuit at RY602 and C635 via R671.
5. At +15V, C635 takes about 0.2 milli-seconds to charge to 8.6V.
6. Zener diode D627 conducts, turning ON Q607.
7. Q607 in turn energizes the surge relay RY602.
8. RY602's contacts jumper (short) out current limiting thermistor TH600
so the monitor will have full power.

Operating Voltages and Resistances

Location
IC1001/pin 5 4.9V
Q608/Collector 0V
D628/Anode 2.3V
D627/Cathode 0V
Q607/base 0V
Q607/Collector 0V
R672/RY602 0V
Part
TH600 Surge Resistor
TH601 Degauss Thermistor
DGC Coil Degaussing
Circuit Voltages
At Power ON After 2 seconds
0V
11.8V
11.8V
8.6V
0.72V
0V
2.92V
Selected Part Values
Function Measured Resistance
6.5 ohms cold
4.2 ohms cold
15.6 ohms