Barco LCN-42 No Power

[budm]

Re: Barco LCN-42 No Power

That looks like the Source resistor for the MOSFET next to it, more likely to have Source to Drain shorted out, since the voltage drop on this resistor will be use for the Overcurrent feedback for the SMPS IC, if the value is too high then it will cause the power supply to shutdown too earlier, if it is too low, then it will not go into shutdown when the current is already exceed the current limit of the MOSFET and other power component. I would think it will be about 0.20 Ohm MOX type 1W (you can calculate the Wattage if you know how much current is expected to flow through this resistor). By the way, I can see a lot of failings solder joints.

[retiredcaps]

Re: Barco LCN-42 No Power

The middle band cant be silver according to this website.

Yes, I figured that out in post #19, but I guess you didn't see the post before post #20?

4th band looks brown.

[Jasgriff]

Re: Barco LCN-42 No Power

That looks like the Source resistor for the MOSFET next to it, more likely to have Source to Drain shorted out, since the voltage drop on this resistor will be use for the Overcurrent feedback for the SMPS IC, if the value is too high then it will cause the power supply to shutdown too earlier, if it is too low, then it will not go into shutdown when the current is already exceed the current limit of the MOSFET and other power component. I would think it will be about 0.20 Ohm MOX type 1W (you can calculate the Wattage if you know how much current is expected to flow through this resistor). By the way, I can see a lot of failings solder joints.

Yes I noticed that they looked bad when I reviewed the pictures. I will go over the whole board and give them a touch up.

Do you think that resistor is that low considering the bands that can be seen?

[Jasgriff]

Re: Barco LCN-42 No Power

Yes, I figured that out in post #19, but I guess you didn't see the post before post #20?

4th band looks brown.

Long time since I was in college learning the bands off by heart lol.

[budm]

Re: Barco LCN-42 No Power

If it is the Source resistor, I would think so since most of the power supply I worked on has Source resistor from 0.1~0.47 Ohm range. That 3rd band with silver color (it looks silver to me from the pictures)

[Jasgriff]

Re: Barco LCN-42 No Power

If it is the Source resistor, I would think so since most of the power supply I worked on has Source resistor from 0.1~0.47 Ohm range. That 3rd band with silver color (it looks silver to me from the pictures)

According to the calculator it cant be silver on a 5 band resistor?

http://www.maplin.co.uk/search?criteria=resistor

Here is where I will be going tomorrow to get a couple of fuses. There is no sign of a resistor that looks like its going to do the job. Its a poor version of radio shack.

[budm]

Re: Barco LCN-42 No Power

I Know, I will look through my spare boards and see if they have one of these five band power resistor and see what the value should be. Does it look Gray or Silver to you? If it is gray, then that will be real odd ball value.

[Jasgriff]

Re: Barco LCN-42 No Power

It definitely looks more silver than grey its got a shine to it.

Q203 is also shorted. One of the legs of that connects to the resistor so that could have been what caused it?

[budm]

Re: Barco LCN-42 No Power

Q203 is the power MOSFET mounted on the heat sink right? The Drain voltage is probably in the 100VDC or higher, so when it shorted out that will voltage will be developed on that resistor and blow up that resistor, the current feedback circuit is connected to the Source of the MOSFET will be present at the Current feedback input pin of the SMPS IC also and could have damaged the input pin of the IC as well, I would Ohm out the SMPS IC also. Do you know what the IC number is for this section of this circuits? The Gate connection should be hooked up to some resistors and Diode back to the SMPS IC output put drive, check that pin also.
See some example PDF, PIN3 is the CS (CURRENT SENSE) pin.

[retiredcaps]

Re: Barco LCN-42 No Power

By the way, I can see a lot of failings solder joints.

I noticed that too and thought I put in a post regarding that, but for some reason it didn't show up. Maybe I forgot to hit submit.

[Jasgriff]

Re: Barco LCN-42 No Power

Yes its mounted directly onto the heat sink.

13778 pjhq it looks like it says on the IC

I have included another pic of the area from the Mosfet and the resistor. Take a look at Q202 that had also been damaged.

[budm]

Re: Barco LCN-42 No Power

It may use the Half-bridge drive circuits if there are two power MOSFET's driving the power transformer.

[Jasgriff]

Re: Barco LCN-42 No Power

There are 3 Mosfets directly connected to the heatsinks. 2 of them are together and then one of them on their own that's the blown one.

The others don't looks shorted.

[Jasgriff]

Re: Barco LCN-42 No Power

The control IC looks like it might be 1377B

And the 3 SMD resistors look to have taken heat too.

I have attached in image to show what's going where. Excuse the flash its really dark at the moment.

[budm]

Re: Barco LCN-42 No Power

Some kind of conductor may have touched that B+ trace and the chassis?
This power supply has PFC Voltage booster circuits.
I would like to see pictures of each sections so I can try to trace out the PS_ON and VCC for each SMPS, PFC IC s.

[Jasgriff]

Re: Barco LCN-42 No Power

I have pulled the Power Mosfet that has shorted.

11N80C3 Ordered.

Fuse - FT8AH250VP - These are the nearest I can find will they be alright?

http://www.ebay.co.uk/itm/8-Amp-8A-T...item35a4a11674

How can I find out what value the Q202 Transistor was?

[tom66]

Re: Barco LCN-42 No Power

Controller is an NCP1377B:

Check pins 4-5, 4-6 and 4-8 for short circuits - all should appear >10kohm if chip is okay.

[Jasgriff]

Re: Barco LCN-42 No Power

Some kind of conductor may have touched that B+ trace and the chassis?

Not sure?

This is exactly as I bought it.

Do you think that the Q202 Transistor popping might have saved the IC?

[Jasgriff]

Re: Barco LCN-42 No Power

Controller is an NCP1377B:

Check pins 4-5, 4-6 and 4-8 for short circuits - all should appear >10kohm if chip is okay.

They are all over 20k. It looks like the Transistor took the grunt of it.

[budm]

Re: Barco LCN-42 No Power

I still to find out the designators of those transistors you are talking about, I cannot easily see them in the pictures so I have no ideas what their functions are. Can you mark the pictures?
Oh I see the Q202 now, what is the output voltage of this power supply as printed on the DC output connector of this section?