Electronics > Repair

Samsung PS51D8000FV power supply clicking

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BoringName:
The plot thickens.

So I couldn't get it to come back on. Just clicking every time no matter what I did. I had an idea that maybe the VS_DET pin on chip MCV14A might also measure the voltage and cut out if it's too high or something. Which would explain why higher voltages previously would click when lower voltages didn't.

So I soldered the ends of a 56k resistor where a metal wire is used to bypass some traces which goes to VS_DET. Being a dumbass I forgot to remove the metal wire so really the resistor wouldn't do anything but I figured I would turn it on anyway before turning the iron back on. The thing stayed on. I measured the resistance across it and it was zero so the resistor was doing nothing.... I removed the resistor and added some more solder to the wire.

It now stays on with VS at 203 volts.... if I turn it up any higher it will click off but it seems to be stable at 203 and VA at 58. So weird. The solder didn't look bad on the wire before I messed with it. I can't get my head around how that could have made it better. Obviously it's not fixed if it clicks at higher voltages but it's very strange.

I'm tempted to put it down to 200 volts to give a buffer and see if it works in the TV. Maybe not turn the brightness up to high and it will be ok?

edit: It powers up in the TV alright but now there is a lot of red where there shouldn't be. From what I have read that is probably due to the voltage being too low :( I tried getting the voltage higher but it just started clicking. The TV picture was fine before it started the clicking rubbish, hopefully something else hasn't broken....I guess I'll keep playing around with the board and try and get it stable at 205+

BoringName:
Inspired by the small amount of progress I made re-soldering a bridging wire, I figured I would try that some more. I re-soldered all the bridging wires in the VS/VA section of the board. Including the trace to the VS_DET pin on chip MCV14A. That trace was mainly bridging wires but also included half a dozen SMD resistors.

I can now get up to 215 volts now before it clicks which is well above the 205 required by the TV.

Power supply repaired. (well, repaired enough for me)

But the increased voltage didn't fix the red shadows on the TV, I tried upping the voltage to 210 and it made zero difference. I tried re-seating all the ribbon cables to the logic board and now the shadows are yellow instead of red..... I'm sure it's just an issue with the ribbon cables so I will keep working on that.

Anyway, the issue this thread was started for is fixed.

I couldn't find a schematic for BN44-00446A anywhere but the board seems to be a mash up for the following 2 schematics that are readily available - BN44-00333A and BN44-00508B

The values for the components don't match but they helped me work out the layout of the board. Especially with the microcontrollers.

In the end this is effectively all I did.
- Replaced all the smaller electrolytic capacitors - The board used to stay on for a little while if it hadn't been turned on for a day, after changing these it just clicked all the time.
- Replaced 1 small blue MKT capacitor near the VS pot - I don't think this really did anything.
- I didn't replace any of the 450 volt 68uF or 250 volt 180uF capacitors. - They tested ok with an ESR meter and most of the smaller caps I replaced did too.
- Refreshed the solder on all the transformers - I did this very early on and it didn't do anything.
- Refreshed the solder on all the bridging wires in the VA/VS section and along the trace to the VS_DET pin on MCV14A - This seemed to be what fixed it.

I don't know why that fixed it. One thing I did notice though is after refreshing the solder in the VS section, the VS Pot was more sensitive. Smaller turns increased the voltage a lot more.

Hopefully my suffering helps someone else in future. Although this TV is that old there probably isn't many left.

I'm just fixing it for a mate to put in his shed  :-DD He is going to owe me a few beers if I can sort out this red/yellow shadow problem!

edit: I cleaned the ribbon cable connectors between the main board and the logic board and the shadows are now black :-D

BoringName:

--- Quote ---I can now get up to 215 volts now before it clicks which is well above the 205 required by the TV.
--- End quote ---

Just a final update on this.

After a week or two my mate told me the TV started clicking again. It took me a while to get around to looking at it.

I started off turning the VS voltage down to minimum and it powered up ok. I noticed that adjusting the voltage would result in it clicking at different voltages the more I moved it around. So I gave the trimpot a clean and I was able to get it up to 222 volts (highest it would go) and it stayed on for a quite a while. Eventually it started clicking again but it highlighted the trimpot being the issue. Looking back, every fix I tried involved adjusting the voltage levels to test it. So any improvements I got were from manipulating the trimpot and not whatever else I did.

The strange part is it appeared to be working perfectly fine, it measured the correct resistances and turning the trimpot adjusted the voltage smoothly. There were no skips or jumps. I had previously refreshed the solder on it.

So I replaced the trimpot with a new one and now everything is working perfectly. Just bizarre really. I guess it was doing something I couldn't see on the multi-meter but it was enough for the power supply to go into error mode and reset.

BoringName:
And it's died again. Lasted a week with a lot of use.

Guess that explains why the trimpot tested just fine.

Only thing I can think of now is a component in the area of the trimpot is intermittently failing and whenever I put heat through that area with the soldering iron it's bringing it back to life for a little while.

It's mainly just a bunch of SMD resistors. I think I'll just leave it off for a couple of days and then test all the components in the area.

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