Samsung UN50EH5300F BN44-00499A Power Supply Problem

[neilc6]

This TV was originally dead. I found the main fuse blown and a dead short across the main filter cap. Eventually traced the short to QL801 and QL802 which I replaced both (NCH, 500V, 4.5A). Now standby voltages are not stable. PFC voltage drifts 389-392V and both 13V and 5V have a slow oscillation (enough to turn on the TV for a few seconds and to read the peak voltage on my DMM). Don't have a scope. Closest schematic I can find online is for BN44-00498A.

[CapLeaker]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

unhook the main board from the psu and plug the tv in. See if the back lights come on and measure what the 12V and 5 V rails do.

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

When I first plugged in the board, I did the backlight test and they came on momentarily. I opened up the the panel and confirmed all the backlights are working. I did check a few caps near the shorted FETs and they tested good. I also powered the backlight from the led tester and the tv was showing the logo and rebooting but likely from the oscillating supply voltage. The voltages are being measured with the main board and backlights disconnected.

[R_J]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Check (or better, replace) cp812 and I would also check CS807 and the cap next to it (CS821?)

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

The small electrolytic caps CP812, CS807 and CS806 all test good out of circuit. Tested CP820, CL802 and CY8075 previously as good.

[CapLeaker]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Well, you ha e to figure out:

A: is there too much current being drawn
B: can the voltage rail deliver enough current

You could disconnect the suspicious voltage rail and power it from a different supply.

[dick_barton]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

The fastguide indicates a 5sec delay for the backlights to come on after the backlight "ON" signal is activated. What is the cycle time?

https://cdn.badcaps-static.com/pdfs/...3db525660a.pdf

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

I ordered a new board since it is difficult to troubleshoot without a schematic and without a scope. It is cycling on both 5V and 13V outputs with both the main board and backlights disconnected. Probably every 5 seconds. The fast track indicates delay is the bl-on signal from the main board. Interesting observation that the pulled up ps_on signal is a steady 3.3V.

[R_J]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Did you measure the voltage across CP812?

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Voltage on CP812 is not stable and cycling between 1 and 12V as well.

[R_J]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Thats the VCC for the pfc circuit, The source for that voltage is from the standby circuit, CS807 supplies the feedback VCC voltage for the standby ic, The elec. cap next to it, CS821 is the filter prior to the QS802 which is the VCC switch.
Check the voltage on CS821.

This schematic is similar, CS803(schematic)=CS807(on bn44-00499), CS821=CP812, CS808=CS821, QS802 is the same

[dick_barton]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

I ordered a new board since it is difficult to troubleshoot without a schematic and without a scope. It is cycling on both 5V and 13V outputs with both the main board and backlights disconnected. Probably every 5 seconds. The fast track indicates delay is the bl-on signal from the main board. Interesting observation that the pulled up ps_on signal is a steady 3.3V.

My thought is that if it's a 5sec cycle that it may well be a led problem which although they have checked out OK with the tester, may be failing due to the higher current supplied when being powered up by the power supply.

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

The large main filter cap is holding the charge so it is only varying slightly if the VCC for the PFC is cycling. Backlights aren't even connected so not affecting the circuit at all. Will check the caps on the PFC IC.

[neilc6]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Here is a closeup of that section. CS821 has pads for a chip cap but isn't populated.

[R_J]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

It might just be the picture but that one cap looks a bit dome'ed I had to guess I thought it was CS821, must be something else

[enigmaelectronica]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Hello, can you tell me wich codes or nomenclatures are for the Transistors (or mosfets) identified as like QL801 and QL802, thanks i have the same error...

[nomoresonys]

Re: Samsung UN50EH5300F BN44-00499A Power Supply Problem

Cap does look a little domed, you probably checked those 2 diodes there.