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Electronics => Repair => Topic started by: dr_p on November 05, 2015, 11:53:28 pm

Title: Repairing a LED driver from a 32" TV
Post by: dr_p on November 05, 2015, 11:53:28 pm

Hi

Can you guys please help me out?! I'm stuck.
I'm trying to repair a 32" Philips LED TV that has no backlight. Everything else works.
I took out the power supply, and when powered on it makes an intermittent zum-zum-zum.
I uploaded a ZIP archive containing all the documentation - photos, schematic and also some oscilloscope captures.
The ZIP file is here: http://we.tl/zJ2z0ayQVh (http://we.tl/zJ2z0ayQVh)
I am using a grounded DS1054Z and using an isolating transformer for the board under test.

The 400V supply is stable, caps are ok.

I removed Q501 and IC500, both were short-circuit.

I connected the ground clip to the probe tip, placed it over Transformer TR300 (that poweres the logic board) and it seems to be working. Also +12VCC present and stable.

(https://lh3.googleusercontent.com/--MHUr-v094Q/Vjvi3QWYXgI/AAAAAAAADgg/0zYySaFv7xw/s800-Ic42/TR300%252520Ground_Wire_Loop_Over_Transformer.png)


Next I placed the same loop over TR200, and I got this pulsing:

(https://lh3.googleusercontent.com/-Zx8kNpNCh1c/Vjvi3GuayHI/AAAAAAAADgc/6j5yruIZMUE/s800-Ic42/TR200%252520Ground_Wire_Loop_Over_Transformer.png)


Now I scoped the output capacitors of TR200 - C211 and C212 (before / after the inductor). Both look like this:

(https://lh3.googleusercontent.com/-vh53XvWTj_Q/Vjviy122dAI/AAAAAAAADgs/EgUe7wled6c/s800-Ic42/C211%252520Output.png)


Next I scoped the transformer's driver - Q204, for Source to Drain, i got his:

(https://lh3.googleusercontent.com/-zNkMQYCM32o/Vjvi2TrhViI/AAAAAAAADgE/lSjoo2Po2Ro/s800-Ic42/Q204%252520MOSFET%252520Source-Drain.png)


Each "pulse" looks like this in detail:

(https://lh3.googleusercontent.com/-1fmDt_iL0KI/Vjvnl0XqOhI/AAAAAAAADhI/703wpv0UhuU/s800-Ic42/Q204%252520MOSFET%252520Source-Drain%252520%252528zoomed%252520in%252529.png)

Then I looked at IC201 (all measurements reffereced to it's GND - pin 1).

This is IC201 pin 1- pin7 (VDD)

(https://lh3.googleusercontent.com/-L2PNmELqOF0/Vjviz3Q7uZI/AAAAAAAADfc/ZsxcuHtqOQM/s800-Ic42/IC201%252520pin1-7%252520VDD.png)



This is IC201 pin 1- pin2 (Feedback)

(https://lh3.googleusercontent.com/-2dfzkzWmiz8/Vjvizhbn0OI/AAAAAAAADfQ/A9RogKN_r-Y/s800-Ic42/IC201%252520pin1-2%252520Feedback.png)



This is IC201 pin 1- pin5 (Temperature?)

(https://lh3.googleusercontent.com/-_8dfpmlWUq0/VjvizrCwkLI/AAAAAAAADfk/o9uu34Q4WdE/s800-Ic42/IC201%252520pin1-5%252520Temp.png)



This is IC201 pin 1- pin6 (Current Sense)

(https://lh3.googleusercontent.com/-f0gYyO39qc4/Vjvizw7iS4I/AAAAAAAADfY/2_Zvs6naC6E/s800-Ic42/IC201%252520pin1-6%252520Current_Sense.png)



This is IC201 pin 1- pin8 (Gate Driver Output)

(https://lh3.googleusercontent.com/-M7NFCdbyUE4/Vjvi0FdwBbI/AAAAAAAADfg/rJ2JpQH2NvY/s800-Ic42/IC201%252520pin1-8%252520Gate_Driver_Output.png)



I also scoped the current sense resistor R212:

(https://lh3.googleusercontent.com/-qBt9b8HOgpo/Vjvi2peFVkI/AAAAAAAADgU/O9vN4OI_dg0/s800-Ic42/R272%252520Current_Sense_Resistor.png)


Then the IC200 Optocoupler's diode:

(https://lh3.googleusercontent.com/-_JyAWpqLwYI/VjvizXuHv5I/AAAAAAAADfM/y5REq7DK4I8/s800-Ic42/IC200%252520pin1-2%252520Optocoupler_Diode.png)


And D205 (TL431) Anode - Reference :

(https://lh3.googleusercontent.com/-0QpTymDg9tI/VjvizHAAA9I/AAAAAAAADfo/LylV6O0uyF4/s800-Ic42/D205%252520%252528TL431%252529%252520anode-reference.png)


I can't, for the life of me, make sense of this - why is it restarting?
Can anyone please help?

Title: Re: Repairing a LED driver from a 32" TV
Post by: Rasz on November 06, 2015, 02:07:18 am

Quote from: dr_p on November 05, 2015, 11:53:28 pm

I uploaded a ZIP archive containing all the documentation - photos, schematic and also some oscilloscope captures.

do you really expect people to download zip archive?

Title: Re: Repairing a LED driver from a 32" TV
Post by: crispy_tofu on November 06, 2015, 04:42:16 am

I can't help (sorry), but there's quite a lot of pertinent information in the zip archive...  :-//

Title: Re: Repairing a LED driver from a 32" TV
Post by: tec5c on November 06, 2015, 05:37:16 am

Quote from: crispy_tofu on November 06, 2015, 04:42:16 am

I can't help (sorry), but there's quite a lot of pertinent information in the zip archive...  :-//

You mean impertinent...

Too much irrelevant info.

You found faulty components which you removed... and then you started probing with the unit on and can't make sense of what is happening? A pretty logical thing to do would be to test the surrounding circuitry of those shorted parts. Also, you didn't say if you had replaced them either.

If you can't find anything else faulty, replace them, see if they blow, if so start testing again, if not then I downloaded a zip file for no reason.