DCLLCD DCL7A - quick flash of the back lights only

[jayoung]

I have a DCLLCD DCL7A that when turned on shows a very (and I mean VERY) quick flash of the back lights and then goes to a black screen. I have read several threads on badcaps.net concerning the LM004 Rev:3 inverter board. I have replaced all of the electrolytic caps with Panasonic FM / FR and have even swapped out the big capacitor with one of equal value. I have resoldered the pins on the two transformers and also the grey block capacitors near the transformers (one of the legs of one of the capacitors had a cracked joint).

I have measured the transistors as follows:
Q6:
0.599 kohms
1.930 kohms
1.212 kohms
Q7:
0.600 kohms
1.929 kohms
1.212 kohms
Q11:
174.6 ohms
128.5 ohms
90.6 ohms
Q12:
174.8 ohms
128.8 ohms
90.4 ohms
The voltages on the connector to the input board from top to bottom (in the overall picture of the boards):
0 VDC
4.84 VDC
4.84 VDC
0 VDC
4.87 VDC
4.87 VDC
4.87 VDC
0 VDC
0 VDC
4.03 VDC
0 VDC
0 VDC
no wire
0.657 VDC
4.72 VDC
The voltage across the main capacitor is 160.1 VDC
The voltage across the start-up capacitor is 13.73 VDC

Forgive me for the pictures since it's been raining here for the last two days and I couldn't take them outside in natural light. The back side of the input card is a little blurry and I can take it again if someone needs any information from it.

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

Forgot to mention, I have also tried 4 known good ccfl's with the same result.

[selldoor]

Re: DCLLCD DCL7A - quick flash of the back lights only

Try disconnecting the cable from the main board to the panel. Do the backlights then stay on?

Have you check the secondary windings on the inverter transformers

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

Disconnecting the cable from the main board to the panel makes no difference.

Attached are the measurements for the transformers in ohms:

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

I have measured the transistors as follows:
Q11:
174.6 ohms
128.5 ohms
90.6 ohms
Q12:
174.8 ohms
128.8 ohms
90.4 ohms

I regard the above readings are in the "gray" area. They are not 100% shorted, but they are a bit suspicious compared to Q7 and Q6.

Did you check fuse F1 to see if it is open?

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

See if Q9 and Q10 are shorted.

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

Did you check fuse F1 to see if it is open?

Maybe it's too late or my eyes aren't working, but I can't find F1...

See if Q9 and Q10 are shorted.

I don't see a label for Q9 or Q10, but if they are the ones below Q8, they don't appear to be shorted.
Q9:
2.733 kohms, 10.39 kohms, 12.11 kohms
Q10:
2.592 kohms, 10.46 kohms, 12.20 kohms

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

See circled.

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

See circled.

Fuse measures 0.3 ohms

Do I just check each pin to all other pins on Q9 & Q10?

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

Do I just check each pin to all other pins on Q9 & Q10?

See my 2 seconds to black guide starting with post #19, there is a procedure on how to test the mosfets.

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

See my 2 seconds to black guide starting with post #19, there is a procedure on how to test the mosfets.

I've used that document more times than I can count and I thank you for your work on that.

The datasheet can be seen here:



pins 1, 2, 3 are designated as S
pin 4 is designated as G
pins 5, 6, 7, 8 are designated as D

I've never checked one with more than 3 pins and this one is not designated with S1, G1, D1, S2, G2, D2, etc.

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

pins 1, 2, 3 are designated as S
pin 4 is designated as G
pins 5, 6, 7, 8 are designated as D

Then check pins 1-4, 4-5 and 1-5.

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

Attached are the measurements for the transformers in ohms:

Your readings for the 2 transformers are radically different. Can you doublecheck your measurements and posting?

Also, what is the part number of the transformer?

[Rtech]

Re: DCLLCD DCL7A - quick flash of the back lights only

Because the resistances of Q6 &Q7 are very different to Q11 & Q12,AND there is a significant difference in the transformer readings,as they are interlinked with the Q mosfets,then you really need to take the transformers out and measure them, and then decide if it is one of them or the pair of mosfets that are faulty.

[jayoung]

Re: DCLLCD DCL7A - quick flash of the back lights only

Then check pins 1-4, 4-5 and 1-5.

Q9:
1 -4: 9.88 kohms
4-5: 14.55 kohms
1-5: 1.91 kohms
Q10:
1 -4: 9.99 kohms
4-5: 10.56 kohms
1-5: 0.60 kohms

Your readings for the 2 transformers are radically different. Can you doublecheck your measurements and posting?

Also, what is the part number of the transformer?

I double checked the readings, they appear to be correct ( I did however have them labeled in reverse - Transformer 1 is actually Transformer 2 and Transformer 2 is actually Transformer 1)

The part number on the transformer is EPC-19

I regard the above readings are in the "gray" area. They are not 100% shorted, but they are a bit suspicious compared to Q7 and Q6.

I have a couple of other monitors to work on and I'm sure I will be ordering capacitors for those. Since the transistors are fairly inexpensive parts, I will probably add them to my order and replace them. Hopefully that will help in determining if the problem is with the transistors or the transformers.

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

The part number on the transformer is EPC-19

If you asking nicely, you might get this ebay seller to measure the resistance of epc-19.

http://www.ebay.com/itm/160917988500

I'm think I bought something from that seller before.

[Rtech]

Re: DCLLCD DCL7A - quick flash of the back lights only

I would suggest you have a look at Post 14.

[retiredcaps]

Re: DCLLCD DCL7A - quick flash of the back lights only

I would suggest you have a look at Post 14.

Yes, obviously remove the transformer and retest everything, but I was just suggesting that he ask the ebay seller for two reasons:

1) if epc-19 is bad, then he can compare that the replacement is indeed the correct part. Sometimes the part number, even though they are identical, is no guarantee they are the same.

2) we have a reference, however unlikely, in case both transformers are bad

I think we are both saying the same thing, but just from 2 different points of view.

[Rtech]

Re: DCLLCD DCL7A - quick flash of the back lights only

Yes, obviously remove the transformer and retest everything, but I was just suggesting that he ask the ebay seller for two reasons:

1) if epc-19 is bad, then he can compare that the replacement is indeed the correct part. Sometimes the part number, even though they are identical, is no guarantee they are the same.

2) we have a reference, however unlikely, in case both transformers are bad

I think we are both saying the same thing, but just from 2 different points of view.

You are right,and it is probably the same thing from two different directions.My concern was as yet it has not been shown to be either the transformer/s at fault OR the mosfets, or both,and until the former are removed and checked, as well as the measurements then on the mosfets with the transformers out,it really is hard to pinpoint the problem.

[UniT3d]

Re: DCLLCD DCL7A - quick flash of the back lights only

Hmmm .....

Pardon me for butting in ...

Rtech is spot on for noting the differences in readings between Q6/Q7 with Q11/Q12.

This is a royer circuit. Those Q6+Q7 or Q11+Q12 is a Rohm part 2sc4672 (available from Digikey). The way the circuit is interlinked, if Q11 is shorted, Q12 readings will also be affected (and vice-versa). The only way to confirm is to test off-circuit.

Also while you're at it, check the tuning polycap tolerance. If its out-of-specs, that new transistor replacement will bork again.