Gateway FPD1975W Power Supply (ILPI-021)

[spoonerj30]

Hello,

The backlight turns off after a few seconds. The symptoms are like the following thread:
https://www.badcaps.net/forum/showthread.php?t=13844

I've replaced all of the caps except the two Su'scon 47µf (red arrow in top pic). I am aware that Su'scon is in the list of bad caps but these two were not in stock at DigiKey at the time. I am going to replace these but I'm hopping that someone here has something to add.

None of the caps that were replaced looked blown. All of the replacements were Panasonic FM.

I have also read these related posts:
https://www.badcaps.net/forum/showthread.php?t=6940
https://www.badcaps.net/forum/showthread.php?t=13984

Thanks,
Jonathan

[jetadm123]

Re: Gateway FPD1975W Power Supply (ILPI-021)

The following is a potential cause for your "2 seconds to black problem":

1) Bad inverter transformer. Check the resistance of the secondaries (pins 7 and 8) of each inverter transformer. There should be no more than 3% difference between the two.

2) Bad CCFL wiring. Carefully check all the wiring and connections to the CCFL tubes to make they are not grounding out against the chassis.

3) Bad CCFL tube. Power down monitor before touching any CCFL wiring. If you have a spare CCFL tube, substitute it with one of the existing tubes. Do this for each tube to see if you can get all of them to light.

4) Overload sense circuitry could be bad. Major effort to troubleshoot this problem.

[spoonerj30]

Re: Gateway FPD1975W Power Supply (ILPI-021)

I am a novice so I am unsure of my results.

1. I used my multi-meter set at 200k Ω and with the red test lead touched pin 7 and with the black test lead touched pin 8 of one of the inverter transformers (T501, T502). The readout was 0.6. I repeated this on the second transformer. The readout was also 0.6.

So these results show that the resistance between pin 7 and 8 are 6% which is twice as high as the suggested readout of 3%.

2. I carefully checked the CCFL wiring and could not see any bare metal.

3. I do not have any spare CCFL tube.

I disassembled the LCD housing to get a good look at the CCFL wiring and noticed that the tubes have some discoloring at the ends (like a bad florescent bulb).

[jetadm123]

Re: Gateway FPD1975W Power Supply (ILPI-021)

I am a novice so I am unsure of my results.

1. I used my multi-meter set at 200k Ω and with the red test lead touched pin 7 and with the black test lead touched pin 8 of one of the inverter transformers (T501, T502). The readout was 0.6. I repeated this on the second transformer. The readout was also 0.6.

So these results show that the resistance between pin 7 and 8 are 6% which is twice as high as the suggested readout of 3%.

2. I carefully checked the CCFL wiring and could not see any bare metal.

3. I do not have any spare CCFL tube.

I disassembled the LCD housing to get a good look at the CCFL wiring and noticed that the tubes have some discoloring at the ends (like a bad florescent bulb).

I think we'll need better resolution on your resistance readings. Try setting your meter to 1K, since it looks like the resistance is in the 600 ohm range.

I also missed the part about you not replacing the 47uf caps. One of those caps is a startup cap. It could be the source of your problem. Try Panasonic FC caps as a replacement.

Some discoloration at the ends is normal. However, if there's a pinkish tinge while the CCFL is trying to start, that could indicate a bad CCFL.

[spoonerj30]

Re: Gateway FPD1975W Power Supply (ILPI-021)

Ah I see, it's 3% difference between the two readings.

My multi-meter doesn't have 1K so I set it to 2K with the following results:

T502 = 618Ω
T501 = 629Ω
Difference = 1.75%

If you would, could you please explain why the difference in this case matters?

Yes I am going to replace the two 47µf caps with Panasonic FMs.

[PlainBill]

Re: Gateway FPD1975W Power Supply (ILPI-021)

Ah I see, it's 3% difference between the two readings.

My multi-meter doesn't have 1K so I set it to 2K with the following results:

T502 = 618Ω
T501 = 629Ω
Difference = 1.75%

If you would, could you please explain why the difference in this case matters?

Yes I am going to replace the two 47µf caps with Panasonic FMs.

One popular failure mode in inverter transformers is shorted turns. These transformers are wound in several segments, each segment has multiple layers. The most likely short is between the start of one layer and the end of the layer above or below it. This represents the greatest voltage difference between adjacent turns.

Measuring a number of secondaries I noticed about a variation of about 2% on more than a dozen transformer secondaries that were all good. (These were type 4006A transformers used on a Darfon inverter). I attribute this variation to variations in the wire diameter, purity, etc. One more transformer in the set had a secondary that matched the others, and a second secondary that was about 4% lower than the lowest 'good' secondary. That transformer also failed a ring test, which confired shorted turns.

The 3% figure is somewhat arbitrary, but SO FAR I have not heard of anyone replacing a transformer that failed the 3% test and failed to repair the inverter.

Your readings indicate a variation of much less than 3%, accordingly the transformers are most likely good.

PlainBill

[spoonerj30]

Re: Gateway FPD1975W Power Supply (ILPI-021)

So this test is assuming that one of the transformers is good and using it as a baseline?

[PlainBill]

Re: Gateway FPD1975W Power Supply (ILPI-021)

So this test is assuming that one of the transformers is good and using it as a baseline?

Not the way I would have phrased it, but you are correct.

I would say 'The test assumes two transformers will not fail at the same time'. That's a fairly safe assumption, since one failing will cause the inverter to shut down. On the other hand, one 'fix' for 'two seconds to black' is to disable the protection circuitry. Still, if someone did that, it's a safe bet that the charred transformer is the bad one.

PlainBill

[spoonerj30]

Re: Gateway FPD1975W Power Supply (ILPI-021)

Okay, thank you PlainBill. Also thanks to jetadm123. I will report back once I've completed the final cap replacements.

[spoonerj30]

Re: Gateway FPD1975W Power Supply (ILPI-021)

After replacing the 47µf capacitors the monitor is still exhibiting the same symptoms. I'm not ready to give up on this. Please advise on how to proceed.

[alexanna]

Re: Gateway FPD1975W Power Supply (ILPI-021)

I disassembled the LCD housing to get a good look at the CCFL wiring and noticed that the tubes have some discoloring at the ends (like a bad florescent bulb).

If you still have the CCFL channels removed from the panel, remove any of the yellow tape from the ends where the small return wires are.
Then separate enough of the return wires from the metal channel.
Look for any discoloration of the wiring where there soldered to the CCFL.
I would also give a gentle tug on the wiring where it's soldered to the CCFL.
The wiring has a tendency to break some of the strands, and only a couple of strands of wire are making contact with the CCFL.