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My Repair Jobs


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General information

 

Job Number

 

0201-37

 

 

 

 

Equipment type

 

Monitor

 

 

 

Maker + Model #

 

Hantarex  CT 28 EQ

 

 

 

Made in

 

Italy

 

 

 

 

Comments

 

 

 

 

 

Symptoms observed

 

BAD

1

Completely Dead

 

 

 

GOOD

1

 

 

 

Repair notes

 

Full repair description and observations

 

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

 

 

2

I checked the outputs of the chopper transformers and there was no voltage on any of the ouput rectifying diodes.

 

 

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.

 

 

4

I also checked all fusible and high wattage resistors, but they all gave the correct resistance value

 

 

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.

 

 

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

 

 

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.

 

 

8

I removed the diode from the circuit, and the 4.5V wer still there. Hence the voltage must come from the resistor.

 

 

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)

 

 

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.

 

 

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.

 

 

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

 

1

The ac chasis may contain a voltage of about 110V and can give a dangerous shock

 

 

 

 

 

Other measurements and technical comments

 

1

Voltage found on the switch mode power supply IC – TDA 4601, pin 9

 

Voltages on IC when faulty

Voltages when IC was removed

Voltages when IC is working well

 

 

 

 

 

 

Pin 9

4.5 V

Pin 9

96 v

Pin 9

13.2V

 

 

 

 

 

 

 

 

 

 

 




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