Using Standard LED Power Adapter To Replace And Fix Old CFL TV Backlight Problem
About 2 years ago I was checking an old Samsung T200 monitor that had a strange backlight problem. It was brought to me by an old friend.
This 20 inch monitor still had old cfl tubes and the screen went dark right after about a second every time it was switched on. The white power led indicated that the monitor was still switched on but the screen showed no active backlight after that.
Samsung manufactured this monitor in 2008 and it was put aside for a long time because I couldn’t find what was wrong with the 20 inch T200 monitor. Because I already had checked both the 5pin STV407D mosfets that drive the HV backlight transformers controlled by the 20 pin FAN7311 IC. And also both HV transformers where fine.
And probably the Mckinley 20.1” power board was fine too and so there was no need to buy another power board that in used condition was still available on Amazon for $45.
Until I yesterday rechecked the HV output with a couple of good cfl tubes I managed to disassemble out of another scrapped for parts monitor. And I noticed that this time again these good cfl test backlight tubes went dark after one second but several seconds later they fully lit up. Indicating that indeed the backlight controller with HV transformers and both mosfets worked splendidly but the original T200 cfl tubes must have gone bad somehow.
So today I also opened the old Samsung LCD panel to be able to remove both cfl tubes for further inspection. Removing them from the LCD was not hard. Opening the monitor housing was the hardest part which I already did about 2 years ago. Both old cfl tubes were just like clipped on onto the top and bottom sides of the screen. And by carefully removing the metal frame with back cover from the LCD panel plates and also unscrewing the 3 screws with which the LCD pcb is still attached to the metal cover, without breaking the still attached flex prints, we create enough space to carefully remove both cfl tubes with their reflectors from top and bottom side. Which also needs some freeing up on the cfl tube side wiring that is also holding the backplate that we need to remove before we can completely remove both cfl tubes.
Further tests with the original now removed cfl tubes indeed showed that the tubes poorly lit up and compared to my good cfl testtubes hardly were bright enough, now only showing a weak pink light. So this time new backlight leds are needed to fix the T200 backlight problem.
Above led strips show what type of leds that I am going to use to replace the now bad cfl tubes. They came from large strips I bought on Aliexpress in the past and were cut from the too large strips because I only need two strips of about 45 cm in length for the top and bottom side of the 20 inch T200 Samsung. These backlight led strips count about 17 leds per 10 cm. I originally planned to use these long strips in another monitor. But the bought step-down converter that was supposed to power up these backlight leds went up in smoke just after a few minutes of stepping down the about 25V input voltage to the lower led voltage needed. Although the module was said to operate up to about 30V of input voltage, the module went so hot immediately without any decent cooling that it was just a crap product. And it neither had any cooling block or fan. Therefore this time I wanted to try a different power adapter to feed the led strips which I will show here, that not only works excellent but also doesn’t cost much and can easily be a DIY circuit too. First I will show the now new working led strips I placed in series onto the led power adapter that I will show after that too.
First photo in the top right shows the blue-ish white led light that escapes from the top left side.
Above photo shows the bottom left side with also second new working led strip.
Above photo shows how after replacement both led strips are wired and come out of the left backside of the monitor. The 4 cfl HV output connectors are no longer needed and hopefully can be de-activated without any issues. I placed both strips in series, red1 +input, white1 to red2, white2 gnd.
And now I’ll show what simple and excellent led backlight power adapter I found working very well.
It uses a capacitor instead of a transformer and makes it a cheap and simple to make working led backlight power adapter. The only thing I will like to add for safety reasons is a cheap but trustworthy fuse to present a good circuit breaker in case there might arise some overcurrent problem. This way preventing any fire risks in the backlight circuit. Since the used leds are white 3V leds like blue leds are, this circuit is excellent suited for feeding our backlight circuit.
Next I’ll also show what voltage and current I measured over both led strips in series.
So that looks quite reasonable in measured voltage and current wise. These strips are made out of in groups of 3 placed leds in series. If we take the last 3 leds of this particular strip and place about 8V over them, they will also lit up all other groups of each 3 leds in series at also about 0.01 A of current.
When we also need to cut these strips into the necessary monitor size in inches, we have to cut them in the correct 3 led group length. A last group cut into a group of only 2 leds won’t lit up on the strip end anymore. In other words… only all complete 3 led groups will lit up afterwards.
I also measured the C1 capacitor and the e-cap used in the backlight power adapter circuit.
The e-cap was marked as 3.3uF (400V !) so it was in good condition. The 560nF capacitor (~250V) would be even better if it was a safety X2 type which it probably isn’t. My transistor tester probably is bit out of tune because it keeps nagging that I need to calibrate the device again. Anyway a safety fuse and a X2 capacitor make this simple led power supply circuit safer when needed.
Above the now with new led strips and new led power adapter circuit fixed monitor. Back in action.
Above my DIY vga tester with 12Mhz over oscillating cheap PIC, but good working monitor tester. This vga monitor tester can supply different test patterns with the 2 pushbutton up/down control.
It was made on a cheap pcb using a wiring pen to make all connections. Last but not least follows the solder side of the here used cheap but excellent working led backlight power adapter.
Both smaller resistors on this board are 470 kOhm (to discharge the caps after this circuit is de-activated). The larger one is 470 ohm, and the diodes are all 1N4007 which can handle up to 1000V!
And this small backlight power circuit can easily be mounted inside the monitor. At best glued and or with tie-wraps safely attached to the monitor case.
I think that after this fix this monitor will work again for many hours when leds and the used led power supply circuit keep working.
It at least will work much longer than the useless step-down led backlight power module with these led strips ever did! Because that crap step-down led backlight module never was true to its promised specs!
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous article on Investigating In-& Outdoor DVB-T2 Antennas
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20 Comments
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Waleed Rishmawi
September 24, 2022 at 10:22 pm
Wow, an impressive make over of worthless monitor into a LED specs one. Thanks for sharing. A lot of information provided and the details just overwhelming. Have a blessed day
Albert van Bemmelen
September 25, 2022 at 2:34 am
Thanks. Afterwards without any issue I also was able to de-activated the no longer needed 4 cfl HV outputs by removing the 20 pin FAN7311 controller chip and both HV transformers.
Arun Kumar
September 25, 2022 at 5:26 pm
Superb Sir,
Great job
Can you share the circuit diagram of the VGA tester.
Just enjoyed your article.
Thanks for sharing the experience
Albert van Bemmelen
September 25, 2022 at 10:49 pm
Sure Arun! Here is the link to version 2 of this monitor vga tester:
http://www.tvgenerator.narod.ru/vga_tester.htm
Parasuraman S
September 24, 2022 at 10:38 pm
Vow! Oh My God! Another herculean task and narrated in length to such an extent that I lost track quite often! But very interesting, probing and resolving work! Great!
Albert van Bemmelen
September 25, 2022 at 3:01 am
I will try to make it more interesting next time Parasuraman.
By-the-way: After the backlight modification was done, the standby function still worked, but when the monitor was switched off the backlight was still active. Showing just a white screen without any monitor data. I had searched for a 230VAC power connection on the power board to feed the backlight module that was also inactive in standby mode. But couldn't find one. And finding a helpful service manual or circuit sadly failed too. So left it as it is for now.
Parasuraman S
September 25, 2022 at 5:23 pm
Inverter boards are available in the market with power on or enable control. Please try that.
Albert van Bemmelen
September 25, 2022 at 10:55 pm
I know Parasuraman because I had a very bad experience with this completely overheated money worthless inverter.
GIFT ELIJAH
September 24, 2022 at 11:24 pm
Excellent repairs with an improved design for maximum performance and long lasting equipment. I love your never giving-up spirit even after two years Weldon Sir.
Please sir what is the voltage that powers that new small transformerless power module, and what is the output voltage of that module?
Albert van Bemmelen
September 25, 2022 at 2:02 am
The input is 230VAC straight taken from the monitor power input line. Add a small 230VAC fuse as mentioned in the article as overcurrent safety break. The module output over the led strips as load is just about 16.2VDC as was shown on the DMM. The total current through the ín series connected leds depends on the AC Xc resistor value of the H564J marked capacitor = 1/(2x pi x f x C) together with the 470 ohm resistor of the module. With both components the brightness of the leds is within limits also more or less selectable.
Important note! Always make sure that none of the leds in this circuit touch anything else in the monitor. Insulate the led strips where needed! Because as said the led strips are all still indirectly connected to the unsafe powerline with this led module. Therefore also only use this module in monitors that have earthed safety power cords! Better of course would always be to also use a small 230V to 230V transformer to safely provide the power for the led strips. Safety above everything else... but transformers are big and expensive and not always fit in the space left to mount them. Using a Variac to power your monitor or tv is still unsafe because its output is still connected to the primary power line like with the here used module.
This article showed that it is possible to fix a defect Samsung monitor with a small standard led module and replace the old bad cfl tubes with the right backlight led strips. And what failure was found in the Samsung T200 monitor.
Mohamed
September 25, 2022 at 2:52 am
From what I've read on the module cover in one of the pics, it says 3v DC.
Albert van Bemmelen
September 25, 2022 at 2:49 pm
The 3V is mentioned for 37 to 50 white leds in series maximum. But as showed the module also worked with the used 9 x 17 white stripleds with 153 leds! The 45 x 2 strips is in total 90 cm strip length in with 17 leds per 10 cm. About 8VDC per 3 series leds twice gave 16,2VDC. 153 leds total divided by 6 leds (the 2 x 3 series leds) gives 25,5 times 6 leds and so 51 times 3 series leds of about 8,1V each. The about 13 mA I measured on the 2A AC selector on my old TestLab DMM. Because on DC this DMM and also my TrueRMS UT890C DMM only showed ZERO mA!
Albert van Bemmelen
September 25, 2022 at 5:10 pm
PS: The about AC 0.013A or 13 mA was the current measured through both ledstrips on the DC output side of the led module. With only the backlight ledstrips active showing an empty white screen the true RMS AC current measured on the 230 input side of the Samsung T200 monitor was 43.9mA AC when measured with my UT890C DMM. Of course the DC current on the primary AC side measured was zero amps.
Paris Azis
September 25, 2022 at 12:22 am
Excellent modification Albert, very useful and well presented as well!
Yogesh Panchal
September 26, 2022 at 9:00 pm
Albert,
Good Modification!
BL-ON /Enable signal is the main reason for your Back light is still active ON after switched off because you have connected power module Direct to 220V.Most of Universal CCFL inverters are comes with 12 to 24 V Operating Voltage & available for Different size of the Screen.Replaced many none of them failure yet.
Albert van Bemmelen
September 27, 2022 at 3:36 am
Understood Yogesh. I know also Parasuraman was successful in using those too. But my previous used stepdown led power module had no coolerblock or fan and was supposed to power my longer ledstrips for another monitor. But became instantly hot and in a minute of 10 or more went up in smoke! So I have very little hope in using such modules to replace old cfl tubes again. It was promised to work up to 30V DC input voltages for my led strips but destroyed itself already at lower voltages around 20V DC. I tried to look up the useless module I had used to show it, but it probably no longer exists and was apparently also deleted from my order history list.
Albert van Bemmelen
October 1, 2022 at 12:59 am
addendum: I today found which led powermodule went up in smoke after just minutes of use at the right input voltage on my previous Aliexpress account! Here is the link to the included two 27 inch ledstrips that I re-used in shorter 20 inch (45cm) length for the repair in above article.
Parasuraman S
September 28, 2022 at 10:42 pm
My transistor tester probably is bit out of tune because it keeps nagging that I need to calibrate the device again :- Long press test, select self-test, short all the three leads and press test. It will become ok. After the process, it will show, remove probes. There is an instruction manual for it in pdf format. Kindly search for it.
Albert van Bemmelen
September 29, 2022 at 1:17 pm
I know Parasuraman, already did that several times in the past with the included calibration capacitor and the shorting leads but the thing is that it asks it again even after just changing the batteries. Which was the reason why I called it annoying. Also I already blew the avr processor on 2 of these identical digital meters by just measuring on a small e-cap capacitor that was still charged. Luckily I was able to program 2 new processors with the online found firmware to replace the avr mcu's and got both digital tester back working like new. So if you ever need the firmware bin file I have that too.
Lynn Blakely
October 3, 2022 at 6:52 am
Albert Thanks for the excellent repair, and enjoyed reading all the comments. Very well written article.