Samsung SyncMaster 943BM - backlight only for half a second

[Behemot]

Got this one with no 2 seconds to black but half a second to black. What I did so far:

- checked all caps
- checked inverter transformer
- checked inverter trasistor STU309DH
- checked fuse, output ceramic caps in the inverter
- checked votlages under operation, everything seems fine incl. BL_EN
- resoldered bunch of badly looking joints
- tried different CCFL tubes

The STU309DH has 7 V for a fraction of second on the drain than it slowly decreases to 0 V and again pulses to 7 V and goes down.

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Source connected?
Dark mage on screen visible with flashlight?
Power led change colour?... goes off.... stays on?

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

LED stays on, can see image with light so problem is clearly in the inverter. Time for pics I guess? Oh BTW, inverter controller is OZ9939GN.

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

BLON stays constant when lamps go off?

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Yes it is 2,9 V the whole time.

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

I guess it is picture time.
Have you tested the blue high voltage caps for leakage/shorts examined for cracks etc.

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

- checked fuse, output ceramic caps in the inverter

All of them measured +- 80 pF.

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Sorry - missed it.

[addytzu123]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Hi,
Just a thought - have you tried bypassing the protection circuit from pin 3?

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

I could I guess…that is some ordinary resistor, could be 0,5 W or 0,25 W? And what than, look for a smoke?

[addytzu123]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Well, it is 0,25W regular, and should enable the inverter to work with a "tired lamp"

[ek2oo2]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Is that safe? What if there's a shorted component in the inverter circuit?

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

As I said, you have to look for smoke than

[addytzu123]

Re: Samsung SyncMaster 943BM - backlight only for half a second

If you have had a shorted component, the fuse should have blown by now.
This trick can be used if you determined that the inverter is ok, and positively identify one CCFL with problem, and you leave it disconnected.
If you are willing to compromise, than you can consider this a temporary fix (dimmed brightness) i suppose.

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

I have not identified bad tubes. I have tried different tubes (still in display BTW ) and it does exactly the same. Don't think two completely different panels will both have bad tubes…

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

I thought it was a fix mainly for a flickering bulb for example a bulb on its way out, although I did get someone to try it recently on a half second to black , but it didnt work.
What is does is it it doesnt operate on the signal that says the bulbs have not lit within a certain time giving a half dud or dim bulb chance to light as it extends the time to infinity. I think it is ok for a short test with a removable bridge or on a machine that is otherwise scrap as I dont know what effect a half working bulb will have on the inverter transformer. Probably kill it.

[selldoor]

Re: Samsung SyncMaster 943BM - backlight only for half a second

Pictures? - Tried jumpstarting power board with a resistor or 3v battery
to see if the lights come on without the main board

[addytzu123]

Re: Samsung SyncMaster 943BM - backlight only for half a second

And beside pictures, can you post the voltages on pins 2, 5, 6 and 10 (respective to GND) of the OZ9939 to see what protection kicks in ?

[Behemot]

Re: Samsung SyncMaster 943BM - backlight only for half a second

5,13 v, 0,23 v, 0 v, 2,89 v

[stj]

Re: Samsung SyncMaster 943BM - backlight only for half a second

is IC101 supposed to be blown apart?


rule :1 - visual inspection!