sansui dvd player repaired

No Power Problem In Sansui DVD Player Model 301K-Repaired

The complaint of the Sansui DVD player was no power. Judging by the power supply size it seems that it was not difficult to repair because it contains very less components. As usual the first thing to do was to check on the main fuse. The main fuse was blown and this suggested that there could be some shorted components in the primary/secondary side.

dvd player power supply

Bridge rectifier (4 x diodes) were checked on board and found to be good because the readings were almost the same. If you got different readings when checking on the four diodes then probably one or two of the bridge diodes is shorted. The power transistor was removed and checked and found to be shorted.

From my experience, if the power transistor or the power FET of a power supply is shorted then you have to check all the corresponding components. Resistors, diodes, capacitors and a transistor were checked and found only the transistor (C945) shorted and the rest of the components were tested good. Even the switch mode transformer and the Optoisolator IC were tested good too.

As for the secondary side, I just briefly scanned through the secondary diodes and all were found to be good. For your information, if the secondary diodes have problem, usually it would be easily detected by an analogue meter.

After spending sometimes on the power supply I concluded that the fuse, C945 transistor and the MJE13003 power transistor were the only bad components. I have the parts for the fuse and the C945 transistor but not the power transistor thus I have to buy it from the local electronics shop.

dvd player bad components

Now, all the components were replaced and the player was turned on-guess what? The power supply refused to work and measured zero volt at the output. All the components were checked and the bad components were replaced but why it still does not work. I took the power supply out and rechecked on the components that I had replaced but they were all good!

After careful inspection I found that the replacement power transistor base pin was link to the negative side (HOT ground) of the big filter capacitor. This can’t be because only the emitter pin of the power transistor that is suppose to link to the negative side and not the base pin. If you see the schematic diagram (other equipment schematic diagram) below the emitter is connected to Hot ground:

dvd player schematic diagram

So I reversed the transistor pin 1 and pin 3 at the back of the circuit board and adding wires as jumper I managed to connect the two pins in the respective circuit. This means pin 1 (emitter) is connected to the negative side and pin 3 (base) is connected to the circuitry and not negative side.

The reason I need to reverse the replacement transistor pins was because the original power transistor pin 1 is base pin 2 is collector and pin 3 is emitter.

DVD PLAYER TRANSISTOR

After it was done the power supply came back to life.

dvd player repair

Lesson learnt- A same part number for transistor could have different pins configuration-so beware. This only applies if the brand is different from the original one. In other words, if you use other brand of component, one has to make sure that the pins configuration is correct otherwise it would cause more problems than you can solve. Fortunately in my case the replacement power transistor did not blow otherwise I may have to recheck, troubleshoot and waste another hour or two to solve the problem.

Hope to see you next time for the coming interesting repair article.

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