Dell 2209W - no power at all - stripped down but can't see anything obvious

[retiredcaps]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

Are you asking me to do something specific? :O)
Am I to test continuity or try and visually trace?

Ohming something out means seeing if two are electrically joined together. So budm is asking you if the two are joined together (i.e. pin6 (vcc) of the 7575 connected to C957 Postive leg). I prefer to use continuity because my multimeter beeps and more importantly it shows the ohms reading at the same time. Additionally, I know the continuity threshold on my multimeter is less than 25 ohms.

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I have to stop using tech jargon. Sorry about that.

[Batz]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

By the way, is pin6 (vcc) of the 7575 connected to C957 Postive leg?

OK, no probs guys, just me getting confused. Yes it measures zero ohms.

OK I trace out the connection of the D952, it looks like the Cathode (banded sided) of D952 is connected to ZD501 Anode side?

I can't find any ZD501.

Are we getting close to a likely problem area? I'm not pushing just wondering :o)

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

We know that we are not getting VCC to pin 6 (per app notes of the IC, it looks like it is generatd from D952). The IC does get the kick start voltage to pin 8 (288v) as you indicate earlier.
From spec sheet:
"Connect this pin (8) to positive terminal of bulk capacitor to provide the startup current
for the controller. When Vcc voltage trips the UVLO(on), this HV loop will be off to
save the power loss on the startup circuit."

ZD501 is the one that sits between the two caps in the pictures cloase to the SMPS IC. C957 is the DC filter cap for the 7575 VCC pin6.

This is very complex board that contain the power supply and the logic board, pluse it is a double sided board (copper on both sides) which is very hard to trace out the copper connection.
So right now it can be bad 7575 taht does not start up even with the kick start voltage. You can try repalcing that IC, but for me, I would like to find out how that D502 fedd the DC to pin 6 o the SMPS IC, unless it is not but it gets the VCC from some where else.

[retiredcaps]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

ZD501 is the one that sits between the two caps in the pictures cloase to the SMPS IC.

I think budm means ZD951. It sits between C957 and C952.

While we are here, test ZD951 with your diode test function. Power off, unplug the TV, and wait 5 minutes for the caps to discharge. Place the leads on either side of the diode. Note the reading. Reverse the probes and take another reading. Report both.

[retiredcaps]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

So right now it can be bad 7575 taht does not start up even with the kick start voltage.

One simple test would be to see if VCC is shorted to GND.

Power off, unplug the TV, and wait 5 minutes for the caps to discharge.

Put your multimeter on ohms. Refer to 7575 pinout diagram. Measure the resistance between VCC (pin 6) and GND (pin 4). Report your reading.

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

Thank you retiredcaps, ZD951 as posted in post #35.

[Batz]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

Between the 2 pins on the IC = 6.4 ohms.

across diode zd951 we have 56kohms and 11.1kohms. In both cases the resistance starts high then settles down to the values given.

I hope I don't have to replace that tiny 7575 IC as it will be a tricky soldering experience!

[jetadm123]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

Between the 2 pins on the IC = 6.4 ohms.

across diode zd951 we have 56kohms and 11.1kohms. In both cases the resistance starts high then settles down to the values given.

I hope I don't have to replace that tiny 7575 IC as it will be a tricky soldering experience!

Looks like vcc is shorted to ground, which means you'll have to order a LD7575.

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I second that, unless there is a shorted out ZENER Diode some where on the board (not ZD951, it does not look like it from the picture) in parallel with the VCC pin to ground. You can try lifting the VCC pin and take a reading again.

[Batz]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

Hmm. Food for thought.

Will it be easy to get the correct LD7575, I mean is it a common part? I have an account here I think:

http://www.rapidonline.com/ but a quick search bring up nothing, they are one of the larger UK suppliers of electronic bits. Maybe my bad searching ...

Then how am I to go about replacing it without destroying the board?! Any tips for desoldering and resoldering these kind of things? I have replaced capacitors before but my technique then was grab them with long nosed pliers and wiggle/pull while I heated up first one solder join then the other. I have always found solder suckers/braid rather tricky to make work ...

@budm - can I test any other Zener diodes? What do you mean by lifting the VCC pin?

[selldoor]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

You can buy them on ebay - usual from china do worldwide search

To remove plainbill had this to say:
There are several approaches to removing SMD components, my favorite is a product called Chip-Quik. It forms a low melt point alloy that allows you to simply lift the IC. You must clean up the residue with solder braid. A kit runs about $15.

Another is to use a hot air rework station, but those cost a bit more.

A third approach requires a bit of dexterity and a pair of cutters with very small tips. Melt the solder on a pin, then clip the pin at the body of the IC. Repeat.

Its not as bad as through the hole - they are surface mounted so if you get most of the solder off with de-solder braid and lift gently its ok. You do have to be careful though so as not to tear the pads.

[Batz]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

OK, I've done some googling and there are some techniques I am happy to try, I'll grab some old PCB to have a play with first though!

I also found some of the required IC on eBay. No point ordering it until Monday so I'll wait to see if the consensus is that this is definitely the right next step or whether I need to do any more testing and/or order any more bits and pieces to replace.

Thanks everyone for the help so far!

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I still think you should lift up the VCC pin and see if you still get 6.4 Ohms, you have to do that anyway before installing the new IC just to be sure.

[selldoor]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

"@budm - can I test any other Zener diodes? What do you mean by lifting the VCC pin?"

In post 46 retired caps asked you to Measure the resistance between VCC (pin 6) and GND (pin 4). Report your reading.

you came back with" Between the 2 pins on the IC = 6.4 ohms."

budm wants you to unsolder pin 6 and either gently bend it up or slide something between it and the board and do the measurement again.

[retiredcaps]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

OK, I've done some googling and there are some techniques I am happy to try, I'll grab some old PCB to have a play with first though!

I like this smd solder video

http://www.youtube.com/watch?v=3NN7UGWYmBY

ChipQuik helps, but is expensive

http://www.youtube.com/watch?v=FTQqjggeklo

[Jasgriff]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I add solder to each side bridging all the pins then heat one side then the other nd just push it to the side. You have to have a good iron though and I have practiced on broken boards.

[budm]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I do the same as Jasgriff, use good wide soldering tip and 60 watts, work real well for me.
To lift one leg, I use X-acto knife to lift the pin while applying heat to the joint. Practice makes perfect.

[retiredcaps]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I add solder to each side bridging all the pins then heat one side then the other nd just push it to the side. You have to have a good iron though and I have practiced on broken boards.

Yes, that is certainly cheaper than use ChipQuik. Pictures at

http://www.infidigm.net/articles/solder/#desoldering

[Batz]

Re: Dell 2209W - no power at all - stripped down but can't see anything obvious

I have tried to lift only pin 6 but I am really struggling because of the other tall components nearby. I can't get the x-acto knife in at a low enough angle. So I think the only way I am going to measure the resistance between the 2 pins is to completely remove the IC. If I do this will I be able to test the resistance OK or does it need to be partially on the board?

Is this type of IC a common part to fail? If I replace it what chance the board suddenly works?! Or will I need to replace other chips too?