Samsung 940NW - 2 seconds to black - now just black!

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

Budm,

Yes, a replacement transformer fixed it. Thanks you and everybody who has helped. This is the most awesome forum!

Kevin

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

Thanks Budm, I'll purchase a replacement and report whether it works.

[budm]

Re: Samsung 940NW - 2 seconds to black - now just black!

At this point I think you may have bad transformer.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

If they are dim then carefully check the sockets for burning/poor contacts then if you can be sure they are the same values ,one at a time swap C313 and C314 with the corresponding ones on the circuit to the other bulb.

I checked the sockets and they looked fine. All 4 blue capacitors are the same so I swapped them around, but the situation remains: dim on CN303 and off on CN304.

[selldoor]

Re: Samsung 940NW - 2 seconds to black - now just black!

As a separate note: Do these sockets output a pulse of 650V or a continuous voltage? Is it safe to use a meter that can measure up to 1000V? I can borrow such a meter, so if there's something I can measure I'd like do that first tomorrow, before swapping the capacitors.

Cant recommend that action - whilst the working voltage for many lamps is
less that 1000v the strike voltage on starting can be between 1500-2000v

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

I tried connecting each CCFL to each of the sockets, they all behave the same: CN301 and 302 seem OK, 303 very dim, 304 not on.

As a separate note: Do these sockets output a pulse of 650V or a continuous voltage? Is it safe to use a meter that can measure up to 1000V? I can borrow such a meter, so if there's something I can measure I'd like do that first tomorrow, before swapping the capacitors.

[selldoor]

Re: Samsung 940NW - 2 seconds to black - now just black!

Can you do the other half of that test and put the bulbs that were on CN301 & CN302 on to CN303 and CN304? - obviously they should stay dim but you never know.

If they are dim then carefully check the sockets for burning/poor contacts then if you can be sure they are the same values ,one at a time swap C313 and C314 with the corresponding ones on the circuit to the other bulb.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

They are lit brightly on the other two connectors, CN301 and CN302.

[budm]

Re: Samsung 940NW - 2 seconds to black - now just black!

So if you connect the CCFLs that show bad when connected to CN303/304, how do they look if you connect them to the other two connectors instead?

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

I've reflowed all the components carefully, but still no go.

Today I tested the panel on a dimmed room, and for the first time noticed that CCFLs on CN303 and CN304 are not totally dark as I believed last time: on CN303 the light is very dimly lit, on CN304 it doesn't light up.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

Both C313 and C314 are out of range. I'll try to touch up on all the solder joints ... the SMDs will take a while!

[retiredcaps]

Re: Samsung 940NW - 2 seconds to black - now just black!

What is the resistance measurements for the blue caps, C313 and C314. If you have a manual range multimeter, set it to its highest resistance range.

We are expecting both to measure out of range.

Something else you can try to reflow/resolder all the solder joints on CN303 and CN304 side (including the two caps mentioned above).

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

Ah, that makes sense. I hope it's not too difficult to find out what is wrong... any help is much appreciated!

[budm]

Re: Samsung 940NW - 2 seconds to black - now just black!

The transformer has common primary winding, the transformer drive to both set of lamps, each winding drives two pair of lamps, so if one pair does light up, then the primary driving circuit is working, one of the winding is not driving the lamps. Each lamp is connected to the little blue ballast caps. So something on the secondary winding that drive the connector 301/302 is not working.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

I've measured the transformer secondaries already and they were identical. But for sanity/double check: both windings read 990 ohms.

Sorry, it got buried in post 6. 990 ohms. But today when I measured them they read 998 ohms - both windings the same.

[budm]

Re: Samsung 940NW - 2 seconds to black - now just black!

Retiredcaps will want to see the voltages on every pin on the connector list the results with the Pin descriptions alongside. Also do the test twice once when plugged in
and then when switching on have your lead on the on/off pin and see if it changes.

Another quick test WITH POWER OFF for 5 mins. Meter on 2000ohms test S1 to S2 and S3 to S4 on attached pic

I still do not see the resistance readings that Selldoor has requested. I think you have open winding on the transformer.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

I tried each of the 4 CCFLs on CN301 - CN304, either alone or in pairs, but they don't turn on when plugged into CN303 or CN304, but they do when plugged into 301 or 302. Hope we're onto something... though I'm pretty clueless at this point with what is causing this? I looked at the manual Sabre504 posted but I don't really understand which is which - the component numbers seem to differ from the board.

[retiredcaps]

Re: Samsung 940NW - 2 seconds to black - now just black!

See my sig file for troubleshooting guides. Start with post #19 in my 2 seconds guide.

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

The recycler's junk pile yielded what I wanted for a very cheap price! So the good news is, I get video signal back. (I must've blown the clamp when juggling the circuit boards when plugged in...) The bad news is, 2 seconds to black still happens... Guys, what other diagnostics would you suggest?

[kktlam]

Re: Samsung 940NW - 2 seconds to black - now just black!

Thanks Retiredcaps. The monitor repair is not a rush, it can wait for sure. From where I live, there is an electronics recycler nearby that lets me look through their junk pile - so I'm actually thinking of doing that first... yeah $8 shipping for a $1 order is kind of steep!