Toshiba 1997 G Series, Troubleshooting Manual

Page: 13 / 28

11
No Start-up:
A failure in one of the sub-circuits on the primary side
of the power supply can prevent IC 801 from switching
properly and starting-up. If IC801 is not operating, first
check for 160V on pin 1 of IC801. If this is missing,
check D801,C810,R810 and the relay. If the 160V is
present, refer to Figure 10 and check the following cir-
cuits:
Start-up Resistor
Check for a 16V start-up pulse at pin 9. If this is miss-
ing, check R68,1 the start-up resistor and D876 (not
shown).
Drive Circuit
Once IC801 starts running, the Drive Circuit supplys
pin 9 with a constant 17V. Therefore, if it fails, IC801
will not run properly. Check D864, R871, and C868.
LC Resonance
If either C870 or T862 opens, no current will flow
through the transformer. All secondary voltages, includ-
ing the Drive Circuit, will not be present. (See Drive
Circuit)
Latch Delay
9 15
1
14
12
2 3 4 5 7
8
11
VIN
(AC)
R810
C810
Q801
Vcc
V
B
V
IN
OUT
COM
GND
Css CONT
CT DT
R864
C869
C866
Q862
C862
R867
C867
R866
C874
R870
C870
#3
#2
T862
C865
C863 D875
C876
R861
R862 D862
D864
R871
TSD OVP
DELAY LATCH REF
OC
OSC
OSC
CONTROL
#4
#5
CD
OC
C868
D872
6
R868
START
Logic
Fmax
10
C873
D873
R872
Pre-amp supply
Latch Delay
Drive Circuit
Over current protect
Oscillator
Start-up Resistor
LC Resonance
To prevent IC801 from going into over-voltage protect
during start-up, the charging of C869 will temporarily
disable the latch. When a DC voltage is applied to the
cap, it acts like a short and it disables the latch. When it's
fully charged, it is an open to DC, and the latch will op-
erate normally. If the capacitor is open, IC801 will not
start-up. Check C869
Over Current Protect
If excessive current flows through the LC Resonance
Circuit, the over-current protect kicks in to protect
IC801. If R866 or R870 fail, it can give a false reading
and shut off the IC801.
Oscillator
C862 is a reference for the oscillator inside IC801. D872
is for protection. A problem here could damage IC801.
Pre-amp Supply
Internal to IC801 are two pre-amps that drive the MOS-
FETs. When pin 9 has voltage, 9V is supplied from pin
10 to pin 15. This 9V is used to power the internal pre-
amps. Check D873, R862, D862, D875, C873 and C863.
IC801
Check Pins 1,14, and 12 of IC801. If any one of them is
shorted to ground, the IC is bad.
Fig10