A Samsung LCD TV strange backlight problem can’t be solved, but it is working?
TV Model: Samsung LA40B530P7R LCD TV Symptom: No Display or call is as Display Darkness (no backlight) PSU P/N: BN44-00264A
I hear a friend said, he is facing a strange LCD TV backlight problem. Where he can’t find what’s wrong in power supply board (PSU), but when he just bypass the transistor pin base (B) to ground, the PSU is working now and their backlight is lit. So I suddenly remember that I also facing this problem before. But I can’t remember what the TV model is, but it is Samsung LCD TV too. But my friend TV is Samsung LA40B530P7R with PSU BN44-00264A.
When power on this TV the TV has sound but no display. Check their PSU secondary output are ok, but the PFC voltage just has 300V only. Normally, this PFC voltage should be around 390V! Refer to their PSU schematic diagram, found the PFC IC Vcc pin no voltage supply. This Vcc voltage is supply from QB801 pin emitter (E) . Measure the QB801 pin collector (C) has normal 18V, but the pin base (B) is very low till zero. Check the UB802 (KIA393F) protection IC and found pin-1 is 1.5V!
Because of this 1.5V and cause the QB802 and QB803 switch on. After that causing the QB801 pin-B voltage is drop to very low (zero) and QB801 is in switch off situation. Try to replace a new KIA393F IC still the same problem. Check the UB802 (KIA393F) IC corresponding components, all tested okay. So temporary use a wire solder to QB803 pin-B and another side solder to ground. So temporary bypass their protection. Power on the TV and it is working now! Their PFC voltage is back to normal, about 389V. After burn test this TV overnight, the TV still working fine. So the strange thing is here! What’s wrong about this Power Supply Unit (PSU)? Download this PSU BN44-00264A Schematic Diagram for more details.
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