Protection; Troubleshooting - Sony CPD-G400 Training Manual

will increase the HV. This LOW going pulse is applied to switching
MOSFET Q901's gate. The longer the LOW going pulse duration, the
longer Q901 stays turned ON. As Q901 stays ON longer, its output volt-
age increases. This results in a higher B+ voltage for Q902 that increases
the HV from flyback T901.
The regulating loop is closed when 9V from the FBT T901/pin 17 is ap-
plied to IC901/pin 11. A lower input voltage at pin 11 will cause Q901 to
conduct longer and increase the B+ and corresponding flyback voltage
until T901/pin 17 is at 9V again. RV901 adjusts the voltage at IC901/pin
11 and consequently the HV. In the aging mode, the HV is set to 27kV
+0.2kV.

Protection

IF the regulating is defective and the flyback voltage rises too high, the
CPU will enter the low power Active OFF Mode for protection. The CPU
monitors the +300V flyback output at T901/pin 8 via zener D912 to detect
excessive HV. Normally there is 19V at D912/cathode and 3V at D912/
Anode (16V zener). Excessive HV causes the cathode voltage to rise
proportionally. If the cathode rises to 21 volts, the CPU IC1001/pin 16 will
receive a trip voltage of 5V to activate the Active OFF Mode for protec-
tion. The front panel LED will blink at a 1 Hz rate to indicate excessive HV
has caused the monitor shutdown.

Troubleshooting

Checking the regulation loop:
Assuming you have replaced both Q901 and Q902 and there is still a
problem with the circuit, you can test this regulating circuit without HV by
following this procedure. First you will disable the HV, then use a power
supply to simulate regulation in the following steps:
1. Disable the HV by shorting Q902's Gate to Source (ground).
2. Place a scope probe on IC901/pins 3 and 5.
3. Turn the monitor ON without video input and move the front panel
control button to the left. This will activate the Aging Mode and keep
the monitor powered ON.
4. In the Aging Mode you should see a 9.6usec positive going pulse at
IC901/pin 5. The pin 5 pulse is present when there is input at pin 14
and IC901 is good. (Scope set to 2usec time base.)
59
5. Without HV, IC901/pin 3 should be LOW except for a 2msec positive
pulse. Pin 3 is LOW to try to increase the disabled HV.
,
1
2
T
3
C H 1 1 0 . 0 V =
C H 2 1 0 . 0 V = B W L
C H 3 ! 1 0 . 0 V = A L T M T B 2 . 0 0 u s - 0 . 2 0 d v c h 3 -
IC901 Waveforms
Name
Channel 1 H Drive input
Channel 2 PWM Output
Channel 3 MMV Output
Time base
6. Through a blocking diode for test equipment safety, apply voltage to
the cathode of D916 (IC901/pin 11). Watch the waveform at IC901/
pin 3. As you reach 9V on the external power supply, the LOW will
change into +5Vdc without a waveform. This proves the IC901 regu-
lator is OK.
7. The remaining parts in this regulating loop that must be manually
checked or replaced are:
• D913 16V zener
• RV901 100k ohm HV control measures 53k ohms, adjusted.
• The circuit foil path between FBT T901/pin 17 and IC901 can also be
checked when the external power supply is connected to D916/C.
Just check for voltage at destination T901/pin 17.
HV disabled, Aging Mode
Location Voltage/div
IC901/pin 14 10Vp-p
IC901/pin 3 10Vp-p
IC901/pin 5 10Vp-p
2usec/div