Unsolved Problem In PSU Of Samsung LEDTV-UA40EH5000PR Resulted In Change Of Board
This LED TV, which was a bit heavy due to metal body, was brought to me with the complaint that it became dead while watching it. After opening and cleaning inside thoroughly, when I applied power and checked, there was absolutely no secondary output and the power LED was blinking. The PSU number was BN44-422B and I could download the schematic from : https://elektrotanya.com/samsung_bn44-00422b_sch.zip/download.html
After studying the schematic properly, when I checked up, I could notice that the PFC voltage was not remaining steady after build-up. As mentioned by me earlier in a couple of articles, Samsung backlight turns on even without connecting the mother board.
The power on voltage controlled through an opto-coupler was present, the switching was taking place, but the voltage required for operation of the PFC control IC, FA5591N, at pin 8 was dropping drastically after reaching around 14V.
When the track to this IC was cut, the voltage was steady at around 14V. First I thought the IC was defective. On further check-up of the circuit I noticed that the VCC track was same for yet another Switching IC number, ICE1HS01G
When I cut the track to this IC and restored the track to pin 8 of FA5591N, and applied power, the PFC Voltage got built-up and the DC voltage was around 310V. The Switching IC therefore was short and I got a replacement for it. Now let us study what this switching IC does, as it is a bit complicated circuit. The extract of the primary and secondary stages are given in separate images below. The IC generates two pulses (LG) and (HG) which are fed to the primary of a mini transformer. There are two diodes back to back connected between the two outputs and the center of the two diodes is grounded. The pulse goes to the transformer through two current limiting resistors valued at 10 Ohms, which serves as shaping the wave form to its designed level. The secondary of this tiny transformer has two separate windings, which drive the two Mosfets connected in series to yet another large transformer to generate the required voltage for the LED.
The inputs to these Mosfets are through two 47 Ohm resistors, with two diodes connected in reverse, parallel to the resistors. There are two 10K resistors connected between the inputs to the Mosfets. The source of Mosfet one and Drain of Mosfet two are connected together to the primary of the second transformer, the return of which is grounded through one 0.033uF 1250V capacitor, and yet another cap 47pF, 1KV cap, with a 4.7K resistor to the ground, is tapped as Current Sense for the Switching IC, pin 2 through a diode, resistor etc. The source of the second Mosfet is grounded. Now, let us have a look at the schematic.
Sadly, even after replacing the IC, the switching did not take place! I combed every component that you can find in the primary and secondary and replaced the diodes, Mosfets, resistors and what not! The tiny transformer as well as the large one passed ring tests too. Then ordered for two more ICs from different sources, which were of different batch numbers. But nothing worked. By this time, days and weeks passed and the customer got pressure within the home side to go for a new one. Well, you can understand his and my predicament. He was a good friend and regular customer of mine and he had faith in me. So, I had no other go, but to order for a replacement from a reliable source. Now, let us look at this PSU:
After replacing the board, the TV worked very well, but the sound was distorted and poor, because the cone paper of both the tiny speakers housed in boxes was torn.
After replacing the speakers, the sound was good enough, though I cannot vouch for the quality of the speakers to be like the original.
I tested the TV for several hours for two days, before giving delivery to the customer. The mission was accomplished with a lot and lot of learning done and bulging satisfaction was accommodated by the collection bag adjusting its flexible neck! The old board was sent to one of my techie friends, who was in online chats several times giving me guidance, for a possible troubleshooting and restoration.
We had a lot of joint consultation and work carried out by me remotely to him. He has to hunt for the original IC from some salvaged boards. It is very sad that fake ICs in the market are affecting our service work very badly!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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Tayo
March 16, 2024 at 1:21 pm
Well done Parasuraman. Sometimes the only way to win is to admit defeat! Haha
THis reminds me of one of my very first attempts to fix my big Plasma TV with a defective PSU. I tried everything... literally everything to understand the issue and bring it back to 100% functionality but ultimately (after a couple of months of almost daily research and repair) I had to give up the ghost and order a replacement when it became available on the secondary market. I won by throwing in the towel. My mother still uses that TV to this very day... almost 15 years later!
Parasuraman S
March 16, 2024 at 11:33 pm
Yes, very much! There are many unknown factors that contribute and these are not easily traceable. Many thanks for sharing your experience and comments!
Rodrick Makanga
March 16, 2024 at 3:10 pm
Very true sir it is difficult to find replacements especially for original tv and hifi! Why didn't you go for a power module?
Parasuraman S
March 16, 2024 at 11:35 pm
Because of multiple SMPS working in the board and the input is created by two types of switching for the backlight power. Many thanks!
Yogesh Panchal
March 16, 2024 at 3:27 pm
Good attempt Sir,
try it with CA888 STR 5 wire PWM Module...
You will found some 8 pin PWM equivalent IC through this site.....
https://remont--aud-net.translate.goog/ic_power/?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Parasuraman S
March 16, 2024 at 11:37 pm
This has multiple SMPS and power factor correction in the circuit. Many thanks for the link and suggestions!
Yogesh Panchal
March 16, 2024 at 3:32 pm
Sir,
Have you checked any shorting in FB Circuit ,because datasheet stating...It is equipped with many fault protection functions such as FB short-circuit detection circuit which stops the operation when abnormal output voltage is detected.
Parasuraman S
March 16, 2024 at 11:38 pm
Yes, I have done. But nothing wrong was found!
Albert van Bemmelen
March 16, 2024 at 8:32 pm
Good repair and article! Soon I hope to fix a found cheap defect 32 inch 32LED8100C Salora tv. With the help of the books and download schematics from Kent Liew I hope to resolve the issue of this dead tv with not working but tested good led backlight. It now only starts when the power on button is pressed but immediately after that in a flash of a second it is off again. And sadly no service manual or schematic for this Salora can be found online.
Kent's Led TV book 8 was postponed and is in the final making and hopefully soon will be ready for download.
Parasuraman S
March 16, 2024 at 11:42 pm
Look for VGL, VGH, V_com, AVDD voltages on the scaler or T-con board. If they are present and disappears, the problem could be in the panel. If it does not come, use your thermal camera and find out which component is getting over heated. Just replace it. If it is a panel problem, you need to get expert advice to carry out CKVlines or LC1 & LC2 depending on the board. Sometimes masking the FFC cable would help. All the best.
Imoudu.O
March 17, 2024 at 4:45 am
Retreating from a battle line does not mean loss of battle.You won at the end....satisfaction getting into elastic bag...Lol.Thanks sir.
Parasuraman S
March 18, 2024 at 8:07 pm
Ha! Ha! Many thanks for your encouraging comments!
Waleed Rishmawi
March 18, 2024 at 9:23 pm
fake Ic's in the market is beyond my imagination. many time I have bought IC's that did not work and I kept for days looking for the fault and could not find any. wasted my time thinking it was a good ic. a lesson well learned to buy origianl parts from a trusted seller. we learn the hard way. thanks for sharing and have a blessed day.
Parasuraman S
March 19, 2024 at 11:54 am
Many thanks for your encouraging comments, dear friend!
Kelvin
March 22, 2024 at 11:34 pm
Well done sir
Parasuraman S
March 23, 2024 at 7:40 pm
Many thanks!