ELECTRONICS CORNER
My Repair Jobs
Job Number |
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0112-32 |
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Equipment type |
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Monitor |
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Maker + Model # |
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Hantarex MTC 9000SR |
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Made in |
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Comments |
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BAD |
1 |
Completely dead, not
even standby LED light. |
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2 |
Power supply was
giving a strange ticking noise |
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GOOD |
1 |
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Full repair description and observations |
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1 |
No, or very low output
voltages were being given by the chopper transformer |
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2 |
Chopper transistor –
BU508 was found to be OK, and there was 320V |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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9 |
After obtaining the
circuit diagram from the Hantarex agent, |
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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 |
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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. |
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. |