Samsung SyncMaster 940T blows inverter fuse

[alexanna]

Re: Samsung SyncMaster 940T blows inverter fuse

Also I have never used a ring tester,Do the tests you've done indicate any thing about the coils?
Al.

[alexanna]

Re: Samsung SyncMaster 940T blows inverter fuse

Not sure. I'm usually too busy pushing the power button to check, though it sounds a lot like the sound the transformers made when I only had one CCFL attached. Might be coming from there, might be somewhere else close by. I think I may also have seen a little bit of light coming from one of the transformers when the hissing started, though I may not have seen clearly as I was not looking right at it (might be arcing within the high voltage side of the transformer - possible that the input voltage to the transformer or switching frequency is wrong?).

The monitor should power on by itself when power is applied,Do as PlainBill suggests use a power strip to power on.
And in a dark room look at the ccfl transformers closely.
Al

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

I think I should have suggested that you use two bulbs hooked to one coil. Like you tried.
I was hoping to see something obvious like a coil completely shorted.
Any idea where the hissing is coming from?
Al.

Not sure. I'm usually too busy pushing the power button to check, though it sounds a lot like the sound the transformers made when I only had one CCFL attached. Might be coming from there, might be somewhere else close by. I think I may also have seen a little bit of light coming from one of the transformers when the hissing started, though I may not have seen clearly as I was not looking right at it (might be arcing within the high voltage side of the transformer - possible that the input voltage to the transformer or switching frequency is wrong?).

[alexanna]

Re: Samsung SyncMaster 940T blows inverter fuse

The inverter would not light only a single CCFL. I used a 19" wide CCFL along with the other lamp from the panel, and tried all 4 connectors. As long as I plugged in two lamps to each transformer, it was happy. All 4 connectors resulted in a nice, bright light. With just one lamp attached, the lamp did not light all the way across, was dim, and the inverter hissed a little.

I think I should have suggested that you use two bulbs hooked to one coil. Like you tried.
I was hoping to see something obvious like a coil completely shorted.
Any idea where the hissing is coming from?
Al.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

I don't think the higher voltage from the PSU could be causing the hissing. Perhaps you can try and feed the inverter 13v with your adjustable PSU (after all, this is what the monitor's PSU originally specifies as output, correct?)
....
I just noticed that no one has suggested this - have you tried measuring the resistance on the secondary side of the inverter's transformers? I think they should both show a reading that agrees with each other to within 3%. Since you have a ring tester, perhaps you can run a test with that as well. Also, you said you've checked many solder joints, but have you checked the ones on the inverter transformer and CCFL connectors? Lastly, look inside the CCFL connectors - do the pins inside look nice and shiny?

Same result at 13v. The voltage rating is printed on the front of the board - it's 13v. It says that at CN2, but I traced the source of the voltage and it's the same place the inverter gets its power from. I also thought I smelled the "hot electronics" smell just as the inverter hit 2.5A and started hissing, but I could have imagined it.

Resistance on the high voltage side of T301 is 1.322k ohm. On T302 it's 1.328k ohm, which is within 3% of T301.

Ring test results:

I also did a ring test on the transformers. The high voltage side of the primary transformer looks fine, but the low-voltage side doesn't light any LEDs. The high-voltage sides of the inverter transformers ring 2 LEDs on T301 and 3 LEDs on T302 (max of 8 LEDs on this tester. I'll try using the external power supply tomorrow; I'll have to pick a reasonable ground point also.

I added a little bit of solder to all pins of T301 and T302 (inverter transformers) as well as the nearby caps and all 4 connectors. I also removed the solder from the pins on the high voltage side of the transformers and added fresh solder. I took a look at the pins on the inside of the connectors, and they are all shiny.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

This is not going to tell what is wrong with the inverter it's just going to help find an area to check
Unhook all of the ccfls from the inverter,Use your test ccfl and one at a time hook to each of the ccfl connectors and then power on your going to see a 2 sec problem, but dose the flash look bright and normal on all of the connectors?
Al.

The inverter would not light only a single CCFL. I used a 19" wide CCFL along with the other lamp from the panel, and tried all 4 connectors. As long as I plugged in two lamps to each transformer, it was happy. All 4 connectors resulted in a nice, bright light. With just one lamp attached, the lamp did not light all the way across, was dim, and the inverter hissed a little.

[momaka]

Re: Samsung SyncMaster 940T blows inverter fuse

I don't think the higher voltage from the PSU could be causing the hissing. Perhaps you can try and feed the inverter 13v with your adjustable PSU (after all, this is what the monitor's PSU originally specifies as output, correct?)
....
I just noticed that no one has suggested this - have you tried measuring the resistance on the secondary side of the inverter's transformers? I think they should both show a reading that agrees with each other to within 3%. Since you have a ring tester, perhaps you can run a test with that as well. Also, you said you've checked many solder joints, but have you checked the ones on the inverter transformer and CCFL connectors? Lastly, look inside the CCFL connectors - do the pins inside look nice and shiny?

[alexanna]

Re: Samsung SyncMaster 940T blows inverter fuse

This is not going to tell what is wrong with the inverter it's just going to help find an area to check
Unhook all of the ccfls from the inverter,Use your test ccfl and one at a time hook to each of the ccfl connectors and then power on your going to see a 2 sec problem, but dose the flash look bright and normal on all of the connectors?
Al.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

whats the amperage rating on your supply?

It's rated at a maximum of 30v, 10A.

[shovenose]

Re: Samsung SyncMaster 940T blows inverter fuse

whats the amperage rating on your supply?

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

I pulled both components from the board and measured them. D102 tests fine on my multimeter's diode test mode, and R102 measures 35.91K, right where it's supposed to be.

I also did a ring test on the transformers. The high voltage side of the primary transformer looks fine, but the low-voltage side doesn't light any LEDs. The high-voltage sides of the inverter transformers ring 2 LEDs on T301 and 3 LEDs on T302 (max of 8 LEDs on this tester. I'll try using the external power supply tomorrow; I'll have to pick a reasonable ground point also.

I put everything back in the metal frame and used my power supply. The monitor output something like 14.8v while powered on, so I set my power supply to that voltage. Current through the inverter rises until it hits about 2.4A, at which point the thing starts to hiss and current starts rising quickly (I turn it off at this point). I tried reducing the voltage to 14v and it did the same thing. At this point, unless the power supply is generating the wrong voltage, it looks like the problem is in the inverter.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

D102 (the snubber diode) in conjunction with R102 and C106 suppress high voltage spikes induced by the transformer when the primary switcher turns off between pulses. If C106 was bad, I think there would be a lot more damage on the primary side. If you want, check D102 with your multimeter just in case, and also measure the resistance of R102 (note that R102 may appear to have lower resistance one way, but not the other, which is normal). If they both measure good, C106 is probably okay. Could be that the noise is there because the PSU is not fully loaded (because the inverter is not drawing any power). My SyncMaster 931B is pretty loud too, but PSU works fine.

At this point, I would go with the idea in your first post:

Use a 3A fuse (or close) just in case. If the fuse blows again, then it's likely not a PSU/ripple issue any more but rather a fault in the inverter.

I pulled both components from the board and measured them. D102 tests fine on my multimeter's diode test mode, and R102 measures 35.91K, right where it's supposed to be.

I also did a ring test on the transformers. The high voltage side of the primary transformer looks fine, but the low-voltage side doesn't light any LEDs. The high-voltage sides of the inverter transformers ring 2 LEDs on T301 and 3 LEDs on T302 (max of 8 LEDs on this tester. I'll try using the external power supply tomorrow; I'll have to pick a reasonable ground point also.

[momaka]

Re: Samsung SyncMaster 940T blows inverter fuse

D102 (the snubber diode) in conjunction with R102 and C106 suppress high voltage spikes induced by the transformer when the primary switcher turns off between pulses. If C106 was bad, I think there would be a lot more damage on the primary side. If you want, check D102 with your multimeter just in case, and also measure the resistance of R102 (note that R102 may appear to have lower resistance one way, but not the other, which is normal). If they both measure good, C106 is probably okay. Could be that the noise is there because the PSU is not fully loaded (because the inverter is not drawing any power). My SyncMaster 931B is pretty loud too, but PSU works fine.

At this point, I would go with the idea in your first post:

I was considering running the inverter off my adjustable regulated power supply by applying power where the fuse goes, but wasn't sure if that would be safe.

Use a 3A fuse (or close) just in case. If the fuse blows again, then it's likely not a PSU/ripple issue any more but rather a fault in the inverter.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

Try reinstaling the orginal cap c106 you might have had a poor solder joint.Moving it with the DVM lead may have ben making an breaking a circut.
Al.

I never removed it - I was waiting for confirmation that the part I bought would likely be a suitable replacement. I do this as a hobby and have no formal training, so I'd prefer to not blow anything up or catch anything on fire because I don't know what I'm doing

I removed the cap, turned it around, and put it back in. The squealing is still there, but almost inaudible. It changes pitch slightly right after I push the power button, then goes away completely after the inverter clicks on (yes, it has an audible "click" sound right when the backlight comes on). The monitor still ends up going out of control about half a minute after I turn it on.

[alexanna]

Re: Samsung SyncMaster 940T blows inverter fuse

Try reinstaling the orginal cap c106 you might have had a poor solder joint.Moving it with the DVM lead may have ben making an breaking a circut.
Al.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

On the solder side approximately 1/2 way between C116 and C106 there appears to be a gouge.
Might be a photo illusion...
If it is a gouge has it broken continuity on any traces?
.

There was a spatter of dark-brown flux residue around there. I thought it was a little weird, as I'm not used to seeing that on these boards (the other 3 didn't have it either). I cleaned it off with some MEK after I took the pictures. I checked again and didn't see any damage in that location.

[PCBONEZ]

Re: Samsung SyncMaster 940T blows inverter fuse

On the solder side approximately 1/2 way between C116 and C106 there appears to be a gouge.
Might be a photo illusion...
If it is a gouge has it broken continuity on any traces?
.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

It appears that C106 is generating the sound. If I poke it with a multimeter probe, the volume changes. It's attached to the transformer with some white goop (which I removed, didn't make any difference). It is labeled:

B
102K
1KV

The sound comes from that general area, and poking other components doesn't change anything. One end is attached to the + pin of the rectifier. The other end goes back to hot + through R102, a pin of the transformer through D102, and a pin of IC101 through D102 and BD102.

If I understand correctly, that cap is a 1nF 1KV ceramic cap. While I was at my local electronics supply store today, I picked up a couple of possible replacements. One is labeled .001M, 1KV, C and the other is labeled .001K, 1KV Z5F, MEXICO (I'm not sure about this one, but both were in the same bin). They're both orange and not blue, if that matters. I found other possible replacements on DigiKey, if this part actually needs replacing and my orange ones aren't suitable. I checked in-circuit to see if it's shorted, and it isn't.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

It'll be a little tricky. I've been running with everything screwed in to the metal housing to prevent shorts, and I'm using wires soldered to the back of the board to clip stuff in where the fuse goes. I'll have to come up with some way to flip everything over and still keep everything plugged in without shorting anything against the back of the LCD. Maybe I can rig something up later tonight.

It appears that C106 is generating the sound. If I poke it with a multimeter probe, the volume changes. It's attached to the transformer with some white goop (which I removed, didn't make any difference). It is labeled:

B
102K
1KV

The sound comes from that general area, and poking other components doesn't change anything. One end is attached to the + pin of the rectifier. The other end goes back to hot + through R102, a pin of the transformer through D102, and a pin of IC101 through D102 and BD102.

[stevenps]

Re: Samsung SyncMaster 940T blows inverter fuse

Can you use a toilet/paper towel roll, to see if the squeal is coming from any of those caps?

It'll be a little tricky. I've been running with everything screwed in to the metal housing to prevent shorts, and I'm using wires soldered to the back of the board to clip stuff in where the fuse goes. I'll have to come up with some way to flip everything over and still keep everything plugged in without shorting anything against the back of the LCD. Maybe I can rig something up later tonight.