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

[kktlam]

Dear experts,

I have been to many other hobbist/expert forums but only this one exhibits such helpfulness and wealth of knowledge! (Troubleshooting electronics remotely is a hard thing to do!) This is such a great forum and I congratulate and thank everybody who has contributed.

So here is another newbie's (typical) story:
My friend wanted me to help fix a monitor as titled, exhibiting the "2 seconds to black" symptom, with the video still visible under flashlight. I opened it up, replaced all the caps with new low-ESR ones, but it became worse: now the backlight doesn't even come on, and the video signal is gone! Only the blue power LED stays on for 2s and then flashes slowly at about 1 flash per second.

Now having "gotten my hands dirty" I want to get this monitor fixed, and so hoping you folks can give me some ideas! I have already done the following (according to the sticky thread):

- Replaced all the radial electrolytic caps except the main filter (big) capacitor
- Checked the resistance of all the MOSFETs (no shorts), fuses (no opens), and transformer secondaries (identical resistance); I "assume" all are OK.

Suggestions are welcome!! Attached are PSU board photos.

In advance, a big THANKS!

Kevin

[retiredcaps]

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

What are the DC voltages at CN2 (connector)?

[retiredcaps]

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

Did you check fuse F301?

[kktlam]

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

Hi RetiredCaps,

I measured F301 again just to be sure: it was continuous (0.05 ohm, using 200 ohm range.)

DC voltages at CN2, whether the logic board was connected, read:
15 V terminal: 13.49 V
5 V terminal: 5.13 V

Kevin

[selldoor]

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

[kktlam]

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

Thanks selldoor!!

It didn't occur to me (though I have certainly read this) to measure all pins for power off, on and then 2 seconds to black. Numbers listed below.

B-DIM: 3.48 V
15V: 14.40 V / 16.05 V / 14.40 V
5.1V: 15.2 V
A-DIM: 0.75 V
ON/OFF: 17 mV / 35 mV / 17 mV

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

[selldoor]

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

Have you a mistype on 5.1v - think its too late to edit

[retiredcaps]

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

ON/OFF: 17 mV / 35 mV / 17 mV

ON/OFF needs to be > 3V in order for the backlights to come on. The main board controls the ON/OFF functionality.

Post a picture of the main board. Look at my sig file for the troubleshooting guide link for how to test a voltage regulator. Report your readings exactly in the format as shown in the examples.

[kktlam]

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

Have you a mistype on 5.1v - think its too late to edit

Ahh, good catch. It is 5.12 V. I'm not very good with typing numbers

Retiredcaps, I'm going to read up on that and will report soon! Thanks!

Kevin

[kktlam]

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

The voltage regulator guide was interesting read. Somehow I didn't venture to this one but read the "2s to black" only.

The main board photo is attached. The readings are as follows:

IC600
1084-33
0703BL
PIN1: 0 V
PIN2: 3.28 V
PIN3: 5.12 V

IC602
18 <-- Edit: this number apparently tells us which voltage version.
APL1117
WIAB1
PIN1: 0 V
PIN2: 1.78 V
PIN3: 3.28 V

IC303
6F
709
PIN1: 3.28 V
PIN2: 0 V
PIN3: 3.15 V

While looking at the board closely, I found something VERY INTERESTING! The component D232 seems swollen, cracked and have brown/black stuff oozed from the bottom... photos also shown. I wonder what this is... I'm guessing 3 diodes in one package, perhaps?

[sabre504]

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

D232

diode-tvs;mmqa5v6t3,
5.32/5.6/5.88v,24w,s

[kktlam]

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

Impressive! Thanks sabre504.

[retiredcaps]

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

While looking at the board closely, I found something VERY INTERESTING! The component D232 seems swollen, cracked and have brown/black stuff oozed from the bottom... photos also shown. I wonder what this is... I'm guessing 3 diodes in one package, perhaps?

It definitely looks bad. You can test it in circuit to see what the readings are.

edit: sabre504 found it was I was searching around.

[retiredcaps]

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

You should have 8 readings for the test by following the datasheet provided by sabre504.

Use the diode test mode on your multimeter and report all 8 readings.

[kktlam]

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

I'm not entirely sure which 8 readings: do you mean forward/reverse measurements at each anode/cathode pair?

[retiredcaps]

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

I'm not entirely sure which 8 readings: do you mean forward/reverse measurements at each anode/cathode pair?

Yes. See datasheet for the pinouts.

[kktlam]

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

Here we go. The first number is red probe on PIN 2/5, the other number is reverse ie black probe on PIN 2/5

PIN2/5 to PIN1 = open,open
PIN2/5 to PIN3 = 046,047
PIN2/5 to PIN4 = 068,071
PIN2/5 to PIN6 = 832,open

I was reading the the multimeter's user manual: is it correct to interpret these numbers as diode at PIN1 open, diodes at PINs 3 & 4 shorted, and only diode at PIN6 is good?

[retiredcaps]

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

I was reading the the multimeter's user manual: is it correct to interpret these numbers as diode at PIN1 open, diodes at PINs 3 & 4 shorted, and only diode at PIN6 is good?

Correct.

[kktlam]

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

I'm thinking of replacing this diode package. Seems hard to find - may be I'll look for it in other broken monitors. Meanwhile, is there anything I should check? I'm a little worried that something else is causing this voltage limiter to blow - I think it acts like a fuse.

[retiredcaps]

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

Seems hard to find

This looks to be an equivalent

http://www.digikey.ca/product-detail...0-1-ND/2531527

Get 2 or 3 at 55 cents each.