Over Current Protect - Toshiba TP43H60 Service Training

Main power supply & complete shutdown guide
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+36V Over Current Protect
Figure 5 is the circuit diagram for the +36 over
current protect. Resistor R7782 is the over current
sensing resistor. It monitors the current flow to the
convergence pump-up circuit. An increase in cur-
rent increases the voltage drop across R7782.
During normal operation, Q768 and Q758 are
turned OFF and Q757 is ON. Because Q757 is
turned ON, the voltage at its collector is 0V. A
slight increase in voltage across R7782 turns ON
Q768 and increases its collector voltage. Then,
Q758 turns ON, and its collector voltage drops to
ground and turns OFF Q757. The emitter-collec-
tor current of Q757 stops, and the voltage on the
collector rises to a logic HIGH (approximately 2.1V
or higher). The logic HIGH is applied to pin 14 of
Z801 through D7701, and shutdown takes place.
Because Q758 and Q757 are also controlled by
Q759, the collector of Q768 should be used as the
test point. A voltage of 5V or GREATER at this
point indicates the transistor is turning ON and ac-
tivating shutdown.
+36V
R7782
R7784
C7770
Q768
Q758
R7785
0V
Normal
>
5V
Shutdown
From
Q759
Figure 5.
+36V Over Current Protect
Load
R7783
Stand by
+5V
R7742
D7701
R7745
To pin 14
of Z801.
Q757
Troubleshooting Tips:
• If excess current is pulled from the power supply,
check the convergence output ICs (Q752 and
Q751 located on the convergence output PC
board) and the surrounding biasing resistors. The
digital convergence board can cause Q752 and
Q751 to work too hard and pull excess current.
If this is suspected, remove the digital conver-
gence board from the television with the televi-
sion unplugged. Plug the television back in and
turn it ON if necessary. If the television comes
ON*, the digital convergence board may be bad.
If the television still shuts down, Q752, Q751, or
their surrounding biasing circuits may be bad.
* The television can power up without the digital con-
vergence board in place, but the television will be out
of convergence. The raster bows in from all sides
because the horizontal and vertical scanning are not
going all the way to the edges of the CRTs. Do not
let the television run for an extended time in this con-
dition. If left in this condition long enough, it can burn
the phosphorous. If additional testing is required in
this condition, turn the contrast and brightness all the
way down to reduce the risk.
• The over current sensing resistor can increase in
value and cause a false or intermittent shutdown.
Make certain the current sensing resistor is the
proper value.
21

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