Samsung F7000 occasionally cycles and fails to switch on

[nath1977]

I have a UE40F7000 Samsung which occasionally fails to properly come out of standby. The relay clicks and the backlight flashes but then goes off again with a click, and repeats. I think its temperature related as leaving it overnight seems to allow it to switch on OK.

I assume its PSU related and had a look - it's a BN44-00632B. No signs of swollen caps or hot components.

I saw a great YouTube video on a possible PFC related issues and an underrated resistor https://youtu.be/DREofE4WIvg?feature=shared though a quick trace from the SPC7011F pin 7 lead me to a resistor and it measured ~10R as expected.

Any ideas where I need to focus my efforts? Thanks

[nath1977]

Does anyone have a diagram for a BN44-00632 - I have one for the similar 0622b but its not identical.

So after a little more poking around with all connectors disconnected I can see a steady 390vdc across the large electrolytic which seem healthy.

A5V is steady at 5.3vdc.
B5V is pulsing in time with the relay
13V is pulsing in time with the relay

Ive had a good look around for dry joints and reflowed some which look suspicious but no better.

Any pointers would be greatly appreciated. Thank you!

[Davi.p]

Check the recent thread of sordids666, the es8000 series, these series suffers of common faults..

[nath1977]

Thanks - that seems like a different issue and this one pulses without anything connected.

I've replaced all the electrolytics and or tested them out of circuit with an ESR meter and they're OK.

Where do I go next - what the likely culprit for the PSU failing to start and stabilise?

[nomoresonys]

Unplug cable from mainboard to powerboard at powerboard and recheck those voltages.

[Davi.p]

I have read on certain models same year and also 40" about a problem unfortunately of some turns shorted in main rails transformer, in another case was a problem with relay, this last is easy to check..

[nath1977]

Thanks for the replies folks. Further investigation has led me to both of the backlight driver IC's raising the FLT indication pin - this in turn is causing the relay to be opened. I can't see any issues with the Current sense and Over Voltage readings (based on little facts other than this doc).

Will keep digging....

[Davi.p]

The fault pin can't open directly the relay! Check better..

[nath1977]

The fault pin can't open directly the relay! Check better..

Correct, it drives the relay via a ERR_FLAG circuit which overrides the PWR_ON control thyristor.

The question is, why is FLT pin raised?

see this manual which is as close as I can get https://elektrotanya.com/samsung_bn4.../download.html

[Davi.p]

i saw psb photo, there's no err_flag, others mistery signals , but unplug MB from PSB and see if backlight stays on flawlessly, plus control the MB connetor output (A/B 5v, 13v) and lamps outputs (as the table printed on board)..