Locating the problem
Next we can finally turn the chassis over and have a look at the boards. The logic board is tiny which is typical for these very inexpensive displays, while the power supply and inverter board take up most of the inside space. Firstly, we must unplug the cable that connects them (which is a little bit tricky as it can only be physically unplugged from the signal board) so that we can take some measurements on the power supply board.
Surprisingly, everything looks perfectly normal. But notice bare the board is, as the audio amp circuit isn’t populated with components on my particular one. Overall, there’s only a single transistor switcher and two output rails (+5 V and +13 V or so). For the inverter portion, it consists of only one small transformer with a single secondary winding.
The fact that it uses only a single swither should be our first clue. Bingo! Here’s our problem. The Alpha&Omega AO4828 (dual N-FET in SOP-8 package) was shorted. I replaced it, and the display sprang back to life.
I also replaced all the capacitors, because they were poor quality, and would have failed in a couple more years anyway. Most of the parts I used are visible in the picture, and everything is available from my stock. I Also think it prudent to add the unpopulated varistor in the input filtration circuit.
So here we are… And all it took was one dual transistor IC and some fresh caps.
Discussion
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