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General information

 

Job Number

 

0112-32

 

 

 

 

Equipment type

 

Monitor

 

 

 

Maker + Model #

 

Hantarex MTC 9000SR

 

 

 

Made in

 

Italy

 

 

 

 

Comments

 

 

 

 

 

 

Symptoms observed

 

BAD

1

Completely dead, not even standby LED light.

 

 

 

 

2

Power supply was giving  a strange ticking noise

 

 

 

GOOD

1

 

 

 

 

Repair notes

 

Full repair description and observations

 

1

No, or very low output voltages were being given by the chopper transformer

 

 

2

Chopper transistor – BU508 was found to be OK, and there was 320V d.c. on the collector

 

 

3

The circuit at the primary of the chopper was analysed to component level, and found a particular transistor BC368 was quite hot compared to others. It was found to be leaky.

 

 

4

On replacing the transistor, the tickling sound was not gven out, and the standby digit LEDs was lit. However they displayed the abnormal pattern ‘bb’. On pressing prog up to start the tv, it did not start up.

 

 

5

It was noted that the output voltages of the transformer were higher than expected. A voltage regulating pwere (2W) resistance across the high output was found to be high and was replaced. The replaced resistance broke open on turning the TV on. This caused the high voltage to go even higher.

 

 

6

The rectifying output diodes were disconnected from the cathode side and the output voltages were as follows: The 150 high Voltage was 400V, the 25V was 63V, and the 12V was 28V respectively.

 

 

7

All capacitors and transistors in the primary circuit of the switch mode were replaced, and all resistances and diodes were found to be ok. After all the rplacements were done, the output power was still high.

 

 

8

The chopper transformer was desoldered and  removed and suspected cracks gave suspect that the transformer was faulty, however it could neither be checked to confirm that it is faulty, nor found for sale from Hantarex agent and electronics shops.  All turns were low resistance (below 1 ohm) so probably there is an internal short.

 

 

9

After obtaining the circuit diagram from the Hantarex agent, Malta, the feedback circuit was tested. The voltage under normal operation should be 24V after the rectifying diode D10, but during the fault it was found to be 8V. This xs feedback caused the production of high voltages, and most probably it is due a fault in chopper transformer since all components in the feedback mechanism were tested and found to be OK.

 

 

 10

All conponents making part of the SMPS circuit was replaced or checked one by one which makes it more obvious that the transformer is the faulty component. Unfortunately, this transformer could not be found from anywhere and hence the monitor could not be fixed.

 

 

Repair summary

CHOPPER TRANSFORMER FEEDBACK COIL SHORTED

 

 

Precautions, fouls and advises

 

1

I checked a high voltage measurment of 450V with the digital multimeter being on the Ohms Function. This caused to damage the multimeter ohmeter and diode test functions! Never check voltages with the Multimeter being on OHMS selection.

 
 
Other measurements and technical comments

 

1

The feedback voltage after the rectifying vltge was 7.8V, where it should be 24V. This caused the SMPS to increas the voltage from 195 to 450V.

 

 

 




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