X-Ray Protect - Toshiba TP43H60 Service Training

X-Ray Protect

Refer to Figure3. The x-ray protect circuit senses
excessive high voltage. Pin 13 of Z801 compares
a voltage developed by the flyback transformer to
a reference voltage on pin 11 (approximately 25
volts). If the voltage on pin 13 increases to a value
GREATER than the voltage on pin 11, Z801 acti-
vates shutdown. To determine if x-ray radiation
protection is causing a shutdown condition, con-
nect a peak-response meter to pin 13. The normal
operating voltage is around 22V. If the voltage at
pin 13 rises above 25V, then excessive high volt-
age is most likely the cause of the shutdown. An-
other way the x-ray protect can trigger shutdown,
is by a loss of the 25V reference on pin 11. This
voltage is developed by the main power supply and
used for the audio amplifier and horizontal start-up
circuit. The 25V on pin 11 should also be checked
using the peak-response meter.
Troubleshooting Tips:
Problems with the horizontal output's resonance ca-
pacitors, flyback transformer, deflection yokes, an-
ode caps, loss of the 25V-reference voltage, or a
shorted CRT may trigger the shutdown.
• The CRTs are the most likely culprit with an x-
ray protection shutdown. Each CRT can be dis-
connected separately by disconnecting the drive
PC board. The television can operate with one
of the CRTs disconnected without damaging the
remaining CRTs or television. A CRT may inter-
mittently arc and cause intermittent shutdown.
LIGHTLY tapping on the neck of the CRT may
duplicate the symptom. Take caution when tap-
ping. Tapping too hard can damage the CRT.
• The sealant around an anode cap on one of the
CRTs may become leaky. If this occurs, the seal-
ant has to be removed, the area on the CRT
cleaned with denatured alcohol, and the anode
cap must be replaced. Also check the other end
of the anode lead for proper contact with dis-
tributor Z450. The anode lead may bend inside
the distributor block. The anode lead can be un-
plugged, straightened, and tined to reinforce its
integrity; otherwise, it should be replaced. This is
covered in service bulletin TV9610.
• A shorted secondary winding of the flyback trans-
former or distributor block can cause an increase
in the high voltage. A ringing check may indicate
a bad flyback transformer. However, replace-
ment of the flyback transformer or distributor
block may be the only way to determine for cer-
tain if they are bad.
• If the resonant capacitors C444 and C440 be-
come leaky, the resonant frequency of the LC
circuit formed by the capacitors and flyback
transformer, would change and may increase the
high voltage. However, this is very uncommon
with Toshiba televisions.
• A shorted audio amplifier may pull the reference
25V to ground causing the voltage on pin 11 to
drop below the voltage on pin 13, triggering shut-
down. Pin 11 must be checked with a peak-
response meter because the main power supply
is not operational after shutdown.
D885
R472 D471
C471
T461
X-Ray Protection
19
25V Reference
R889
11
13
22V Normal
> 25V Shutdown
Figure 3.
Z801