HOW TO TROUBLESHOOT - SONY TRINITRON CTV-25 [BA-4 Chassis TVs] - Protection circuits details
A loss of data, vertical drive, flyback generator signal or
+13Vdc power will cause the protection circuit to shut OFF the TV. The 30Vp-p
retrace pulse from IC541/pin 3 is used as an indication of vertical output
operation. This pulse is reduced to 5Vp-p and monitored by Micro IC001/pin 17.
After two seconds of missing pulses, IC001 will turn OFF the TV set and blink
the Timer light four times.
Troubleshooting
When the TV is shutdown and the Timer/Standby light
blinks four times, the problem is in the vertical or horizontal section. Start
the TV by pressing the Power button. You have two seconds to identify the
missing signal with your scope or voltmeter before the TV shuts down again. Here is a general checklist of items that
will cause the TV to shutdown:
Some causes for the Timer/Standby light to blink four times
and repeat
- No Data/Clock input (IC301/pin 34, 35). This causes NO IC301/pins 13-14 drive output.
- Missing positive or negative 13 voltage to power IC541 at pins 2 or 4. This voltage comes from the FBT.
- Loss of either or both vertical drive signals from IC301/pins 13 and/or 14.
- Loss of horizontal drive resulting in no FBT voltages.
- No fbt/boost output from IC541/pin 3
Automatic Brightness Limiting (ABL)
The purpose of this circuit is to prevent sudden bright
scenes from shortening the life of the picture tube. It does this automatically
by monitoring the picture tube’s current and then using this voltage to limit
the brightness of the picture. The
flyback transformer T504 (secondary) supplies the picture tube’s high voltage.
The ground end of the secondary at T504/pin 11 is current limited by a 100k ohm
resistor, R535. As the picture gets brighter, the high voltage current
increases, causing a voltage drop across R535. This ABL voltage decreases with
increasing brightness.
Voltage divider R533, R534 and R532 bias this ABL line with
a positive voltage that is applied to Jungle IC301/pin 3. A lower ABL voltage
decreases the level and gain of the RGB waveform. This is how ABL section uses
the Jungle IC to keep the brightness within a reasonable operating range.
OCP Protection
One part of the protection circuit is shown here in block
form. The over current Protection (OCP) stage shuts OFF the TV during a fault.
Q571 is used to monitor the current flowing through the FBT and horizontal
output transistor. If the current becomes excessive, Q571 conducts, grounding
out the DC voltage from the Jungle IC301/pin 18. This ground causes IC301/pin
34 to send data to Micro IC001/pin 37. The data instructs IC001 to shut OFF the
TV, allowing the horizontal output transistor to cool down.
Vertical Output Failure
Pulses from the vertical output IC are monitored for
activity by IC001. Two signals are
output when the vertical output stage is receiving the drive signal and
amplifying. The main drive signal goes to the vertical deflection yoke. The
other signal is made inside the vertical Output IC541’s flyback generator and
output pin 3. At pin 3 is a 30Vp-p pulse that is reduced by R549 and limited by
zener diode D001 to 5Vp-p. IC001/pin 17 receives and monitors this 5-volt
vertical pulse to prove the Vertical stage is operating.
If IC001 detects a loss of these vertical flyback pulses for
two seconds, IC001 will turn the TV OFF. As part of IC001’s diagnostic program,
after the set is turned OFF (but not unplugged), the Timer/Standby light will blink
four times, pause and repeat. This indicates the problem is a loss of vertical
signal. Unfortunately, +13Vdc that
powers the vertical IC541 comes from the horizontal stage (FBT). Therefore, a
horizontal failure will also cause the light to blink four times, pause and
repeat.
Excessive FBT Voltage
An open safety capacitor C508 or high B+ voltage can cause
the flyback transformer (FBT) to produce an undesirably high secondary voltage. The FBT’s secondary voltage at T504/pin 7 is
monitored for excessive signal level. This
stage consists of T504, D574, IC521, Q300, IC301 and IC001. Flyback pulses are rectified and compared to
a reference voltage to determine if they are excessive. The 120Vp-p T504/pin 7
pluses are rectified by D574 into 109.2Vdc. This voltage is reduced further to 8.76Vdc
by voltage divider resistors R583, R582 and R584 and applied to the + input of
comparator IC521/pin 5. As long as the voltage at pin 5 is no higher than the
9.7Vdc at the negative input pin 6, the output of IC521/pin 7 will remain LOW
(1.34Vdc). The Jungle IC detects the
comparator output. The LOW from comparator IC521/pin 7 is applied to Q300/base,
keeping it OFF as if it were not in the circuit. This allows the horizontal
pulses from Q502/collector and the internal voltage of 3.4Vdc to remain at
Jungle IC301/pin 18 for normal operation.
When the FBT voltage is excessive, the TV is required to
shutdown. An excessive FBT voltage level will cause IC521/pin 7 to output a
HIGH, which will turn ON Q300. When Q300 saturates, the 3.4volts from IC301/pin
18 are grounded. This ground immediately causes shutdown data to output
IC301/pin 35 to the Micro. The Micro receives this command and produces a HIGH
at pin 6 to shut off the TV. After the TV is shut OFF, the Timer/Standby light
blinks two times, pauses and repeats for as long as the set is plugged into
120Vac.
Excessive B+ current Failure
Leakage in the Horizontal output transistor Q502, shorts in
flyback transformer T504 or a short in a secondary winding would draw excessive
current from the B+ line. Excessive current drain will cause the TV to shut
down, allowing itself to cool off. Components
involved in this stage are R571, Q571, Q300, IC301 and IC001. All the current from the B+ source flows through
resistor R571. Transistor Q571’s
emitter-base junction is essentially placed across this resistor. Resistors
R574, R575, R572, and R573 pre-bias Q571. C571 across Q571/b-e prevents rapid
scene changes from turning Q571 ON and triggering the protection circuit. If R571 drops sufficient voltage to turn ON Q571, its
conduction will apply a positive voltage through D581 into the base of Q300,
turning it ON as well. When Q300 turns ON, the 3.4Vdc from IC301/pin 18 is
grounded out. This causes IC301 to send shutdown data from pin 34 to IC001/pin 37.
IC001 responds by shutting OFF the TV set when pin 6 is brought HIGH. After the
TV is shutdown, the Timer/Standby light blinks two times, pauses and repeats
for as long as the set is plugged into 120Vac.
During a failure that causes the TV to shutdown
or go into blanking, the Timer / Standby light D002 blinks, pauses and blinks
again. The number of times it blinks identifies the defective stage:
|
Standby Light Blinks
|
TV Symptom
|
Problem Circuits
|
|
2 times, pauses and repeats
|
Shutdown. TV powers OFF.
|
Excessive B+ current
demand or high FBT pulse amplitudes.
|
|
4 times, pauses and repeats
|
Set Turns OFF
|
Vertical Failure (may also be Horizontal Failure since the
vertical IC is powered by the
FBT.)
|
|
5 times, pauses and repeats.
|
1. Raster, but no video
2. Sound OK
|
White balance failure, weak picture tube or Low G2
voltage.
|
|
Continues to blink once a second
|
No or defective Jungle
IC301 communications.
|
No reply from an IC (data bus is busy -grounded or held
HIGH)
|
|
|
Timer / Standby light – blinks two times
The TV is shutting down because there is an excessive
current drawn by the Horizontal Output Transformer or Transistor. After
checking for shorts in these stages, test the protection trip circuitry.
Testing the Protection Circuitry
- Locate Q300/base. The collector lead of this surface mount transistor is by itself. If the collector is pointed up, the base lead is at the left.
- Monitor the DC voltage with a DVM (peak reading is preferred).
- Turn ON the TV.
- The normal voltage should be less than 0.5Vdc. If it is less than 0.5Vdc before the TV turns OFF again, there is a problem in the protection circuit at Q300 or IC301. Test Q300 for leakage or replace. Move your DVM probe to IC301/pin 18 and turn ON the TV. If this voltage stays below 2Vdc, Q300 is leaky. If above 2Vdc, IC301 may be defective.
- If the voltage at Q300’s base is HIGHER than 0.5Vdc, the problem may be in IC521, Q571, or in the circuitry.
- Place your DVM at IC521/pin 7 and turn ON the TV. The normal voltage should be less than 2Vdc. If it is higher, the problem is around IC521 or there is excessive FBT voltage from an open safety capacitor (C508) or high B+ voltage. Reduce the AC voltage and monitor the voltages at IC521/pins 5 &6. The voltage at IC521/pin 6 is fixed at the D573 zener voltage plus 0.6V (D572). The normal voltage at pin 5 should never exceed 10Vdc. If the voltage at IC521/pin 5 is HIGHER than 10Vdc, the problem is in the circuit’s safety capacitor or high B+ voltage. If the voltage at IC521/pin 5 is Lower than 10Vdc, the problem is about IC521.
- If none of the voltages around IC521 were high, the problem must be around Q571, a defective horizontal output transistor or transformer.
- Place your DVM at Q571/collector. Turn the TV ON. This voltage should not rise above 10Vdc. If this voltage is HIGH, suspect Q571, R571 and other resistors in the area. If these protection parts test OK, there is a heavy current demand on the B+ line. The problem may be a short at the FBT secondary windings.
Timer / Standby light – blinks four times
A vertical deflection failure or horizontal drive failure is
indicated.
Test the horizontal section by using one of these tricks:
* Place your scope probe by the FBT and turn ON the TV. You
will see the horizontal signal if the drive path from the Jungle IC to the FBT
is OK
* Listening to the frying sound at the yoke is another
indicator of horizontal signal arriving at the yoke
- Test the vertical stage by using the scope probe to monitor the vertical drive signal and the DVM to measure the positive and negative supply voltages to the vertical IC541 at turn ON.
- Test the Protection circuitry by placing your scope lead at the Vertical Output IC541/pin 3. You should see a 30Vp-p pulse there. If it is present, this pulse is not getting to IC001/pin 17. Replace zener D001 if shorted.
Timer / Standby light – blinks five times
This failure is caused by a problem in the video output IK
circuit. Turning up the screen control will let you see light on the screen so
you can make an evaluation of the problem.
