ELECTRONICS CORNER
My Repair Jobs
Job Number |
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0201-37 |
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Equipment type |
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Monitor |
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Maker + Model # |
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Made in |
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Comments |
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BAD |
1 |
Completely Dead |
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GOOD |
1 |
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Full repair description and observations |
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1 |
After cleaning the
Circuit from xs ust, I checked and followed the mains a.c. path, and was
found that it reached the rectifying diodes, and that 320V was formed across
the 220uF reservoir capacitor |
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2 |
I checked the outputs of
the chopper transformers and there was no voltage on any of the ouput
rectifying diodes. |
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3 |
I checked all
electrolytic capcitors in the primary which were found to be all good, but I
replaced them to be sure that they are not the cause of the problem. There
was 3 x100uF/25V and 1 uF 100V to be replaced. They
fault remained. |
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4 |
I also checked all
fusible and high wattage resistors, but they all gave the correct resistance
value |
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5 |
I started to follow the
circuit, and found that the dc voltage component was reaching the chopper
transistor (which I checked with diode test and was OK), but the chopper was
not switching off and on. |
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6 |
I looked at the
datasheet of the SMPS switching IC – TDA 4601 and understood that pin 9 –
which is the kickstart input voltage supply should
be over 8V to start the IC operating. On checking up, the voltage was found
to be just 4.5V |
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7 |
There were 2 possible sources which supplied the 9V to the IC :– i)
From the
chopper transformer via a rectifying diode ii)
From the
bridge rectifier network via a 4W, 10k resistor and 1n4007 diode. |
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8 |
I removed the diode from
the circuit, and the 4.5V wer still there. Hence the voltage must come from
the resistor. |
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9 |
The Resistor was now
eliminated from the circuit and the 4.5 volts were not found on pin 9 now.
However on testing the voltage on the resistor, it was found that there was
150V at one end and 149,8V on the other. The resistor was found to be goof
(9.9 Ohm) |
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10 |
All diodes in the
circuit were checked and found to be ok. Suspect fell on the IC itself, or a NTC thermistor which was just 26Ohms when
cold and did not warm up. However the latter was not in a critical part in
the circuit. |
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11 |
The resistor and diode
were soldered back in place and the TDA 4601 was removed. There was 96V on
pin 9 and 320V on pin 3. Hence the voltage inputs were reaching the IC, and
most probably the IC is faulty. |
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12 |
The Ic was replaced by a
TDA4600 instead of 4601 and the TV powered up and worked perfectly. |
Repair summary |
SWITCH MODE POWER SUPPLY
CHOPPER IC FAULTY |
Precautions, fouls and advises |
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1 |
The ac chasis may
contain a voltage of about 110V and can give a dangerous shock |
1 |
Voltage found on the switch mode power supply IC – TDA 4601, pin 9 |
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Voltages on IC when
faulty |
Voltages when IC was
removed |
Voltages when IC is
working well |
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Pin 9 |
4.5 V |
Pin 9 |
96 v |
Pin 9 |
13.2V |
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