Bad transformer?

[Behemot]

I got board from Samsung 203B, recapped, voltages fine but no backlight. Fuse was replaced, transistors are good. Transformers measure resistance OK but CT-micro says inductance of one secondary is 5,97 H, the other 6,99 H. So I guess one transformer indeed is bad after all? It's 8,13 vs. 9,9 out-of-circuit, 1335 vs. 1350 ohm.

It is TM-203B, I found that most likely the TM-1221/TM-1222 would be replacement. It has 155 uH primary no idea how much the TM-203B has, but core has same size, it looks 100 % same (same color of plastics and so) and both have 1350ohm secondary. Only found it at Ali Express for 10,8 bucks a piece in a bulk of 5

[Jerryv]

Re: Bad transformer?

Does the backlight do anything? Blink on and off when you power up? Have you tried a test lamp?

I had a different Samsung with 4 lamps that would blink twice at power up and then nothing. I thought a lamp might be bad, but a test lamp showed one of the connectors would only light the test lamp partially. The secondary resistance was off a little on that winding so I replaced the transformer and it is working good ever since.

Jerry

[Behemot]

Re: Bad transformer?

That's it, it stays dark the whole time, same with other lamps.

[Jerryv]

Re: Bad transformer?

I don't think a bad transformer would keep it from trying to light and if one transformer is good the light connected to it would at least light briefly. Perhaps there is something wrong with your protection circuit so that it is preventing it from trying to light. Or the signal to turn on the lamps is not getting to the inverter.

Jerry

[Behemot]

Re: Bad transformer?

I only got the board itself without signal board but in 90 % cases the inverter goes to full without signal board connected. The signal board actually mostly provides driving for dimming and keeps backlight off, alowing to go on when you turn on the display.

[budm]

Re: Bad transformer?

The main board also provides BL_ON signal beside dimmimg control signal, unless the BL_ON line has pull-up resistor on the power supply board instead of on the main board.

http://elektrotanya.com/samsung_203b.../download.html
Per SCH of the main board, the pull-up resistor for the BL_ON line is on the main board not on the power supply board.

[Behemot]

Re: Bad transformer?

If I am looking right, there is 10k resistor from +5 V to BL_EN and from there there is also 100ohm resistor. That's no difference so let's say 10kohm between +5 V and BL_EN pin?

[budm]

Re: Bad transformer?

It does not matter on the value of the resistor as long as it can allow enough current to produce >2VDC at the BL_ON pin, the input impedance of the BL_ON pin on the power supply/inverter board will determine what the minimum current the input will need to produce >2VDC at the BL_ON pin.