Barco LCN-42 No Power

[BigDaddyAdy]

Re: Barco LCN-42 No Power

Hello guys,

I know it's an old thread, but here is my story:

I have two barco's with the same defect:
No 18V output.

I found that mosfet 11n80c3 burnt, the 18ohm power resistor exploded, the 2 33ohm resistors r203 and r204 burnt, r202(100ohm) burnt.

I replaced the mosfet with 11n60c3 (i have lots of them), the 18ohm replaced with 22ohm, r203, r204 eith 33ohm, r202 on one board with 180ohm, on the other with 100ohm.

Ok, now i have one working board, and one not.
On the working board is the 180ohm resistor and i also replaced ic 201, although it apeared to be good.

Now, on the other board i replaced r202 with 100ohm but haven't replaced ic201. First time i tested the board everything was fine, the power board was working well. After i mounted the board on the rack, on power on it blew again (the mosfet, r202, r203, r204) i thought because i put a 0.25w resistor on r202. Replaced again everything, this time a 0.5w 100ohm resistor, tested, it worked. Mounted on the chassis, it worked, turned it off and back on, it worked, turned it off and back on, it blew again. This time it also blew the 2 smd transistors q201 and q202 and r202 survived.

Now i am thinking of replacing the ic (maybe that is the culprit).

Can anyone help me with those two transistors?
Will these FMMT493 and 593 work?

[Jasgriff]

Re: Barco LCN-42 No Power

Right,

I decided to take the IC off again and this time it removed pad 8.

I have taken away the top coat to try and see exactly where Pin 8 connected to and it looks like it was not connected in any way?

So it looks like pin 7 and pin 8 were not used at all.

[retiredcaps]

Re: Barco LCN-42 No Power

And I see R206 in that same area, but the angle of the picture blocks the color band.

[retiredcaps]

Re: Barco LCN-42 No Power

when I get home I will have another look. I just remember looking at that pin when there was no chip there thinking why is the hv pin not connected.

Hmm, I had another look once I got multiple windows open and every photo rotated properly. It is a long way from pin 8 to those top side resistors I mentioned earlier.

What is covered underneath the glue around the J43 area?

[Jasgriff]

Re: Barco LCN-42 No Power

when I get home I will have another look. I just remember looking at that pin when there was no chip there thinking why is the hv pin not connected.

[retiredcaps]

Re: Barco LCN-42 No Power

Retired caps,

Can you put a circle around where you think pin 8 is connected please I can't see which resistor you mean.

I'm looking at the datasheet which recommends using a resistor in series. I have found that the "typical application" listed in the datasheet is a good proxy for the real board. However, that is not always the case.

If pin 8 (HV) has continuity to the positive leg of the large capacitor, then there is no resistor. If pin 8 has no continuity, then that suggests there is a resistor in series.

The most likely candidates for this resistor in series with pin 8 (HV) would be on the front side. Mabye R4, R5, R110, R112. Unless there is a SMD resistor on the backside that says 683?

PS. Regulars will know I'm still weak at reading schematics so I could also be way off?

[Jasgriff]

Re: Barco LCN-42 No Power

Retired caps,

Can you put a circle around where you think pin 8 is connected please I can't see which resistor you mean.

Budm,

I only found the short after you removing jumpers like you suggested that helped me pin point the problem. The problem yesterday was I had a few hours of messing about with it before you guys were even out of bed so I have already put the mosfet back in as I was too excited. I will take it back out though whilst I find the source of the hv pin.

I have come home to my mums for the weekend so I'm not going to be able to access the etc for a few days. I have still managed to find myself things to fix though! My central locking wasn't opening the boot of my car so I fixed that and my old dear had a Philips 26 inch TV with a permanent red line down it so far I have fixed them both :-)

[budm]

Re: Barco LCN-42 No Power

That is why I built a fume extractor after 35 years of sucking up the solder fumes, I use the bath room venting fan and got the charcoal filter and a light dimmer for controlling the speed. Ah, there is nothing like the smell of solder fume in the morning!
Oh, where was it again?
I sure like to know who generated this darn VCC.
I will have to look at NCP1377 data sheet again to remember what it is for.
Good job for finding the short, Jasgriff.
Oh by the way, you put that jumper back in place that I had you remove to find out why we lost the VCC? I would also leave the MOSFET out this time. And the two transistors and resistors you replaced are still good?
That pin 8 is the kick start pin, so I would think some kind of power connection should be there to kick start it.

[retiredcaps]

Re: Barco LCN-42 No Power

I am struggling to find where the HIV pin is attached too. I remember when I took the chip off it looked like it wasn't connected to anywhere. Do you think looking at the diagram that it definitely should be connected up?

Well, I wrote the post after desoldering a single capacitor so I could be high on solder fumes because I didn't use a fan for one capacitor, but figure 1 on page 2, shows a resistor in series with pin 8. That resistor looks like it is connected to the positive leg of the big filter capacitor.

If you can't find a resistor, then you should have direct continuity from the HV pin to the large capacitor then.

Note to self. Always use the fan regardless how long I'm soldering.

[Jasgriff]

Re: Barco LCN-42 No Power

I am struggling to find where the HIV pin is attached too. I remember when I took the chip off it looked like it wasn't connected to anywhere. Do you think looking at the diagram that it definitely should be connected up?

[retiredcaps]

Re: Barco LCN-42 No Power

I also noticed that there is less than 0.5V on the HV line?

Is the resistor that is in series with HV pin 8 open? According to page 12 of the NCP1377 datasheet, it is typically a 6.8k/0.5W resistor.

[Jasgriff]

Re: Barco LCN-42 No Power

New IC fitted and Mosfet back in.

I am getting just short of 14v on the VCC pin now but nothing on DRV.

I also noticed that there is less than 0.5V on the HV line? I am sure that this isnt correct according to the Datasheet.

[Jasgriff]

Re: Barco LCN-42 No Power

Right,

I found out what it was!

The new IC that I replaced became shorted on the DRV pin and the VCC pin one I disconnected that part of the circuit I get 14 volts back and then the 24v line was up and running!

I have now replaced the IC but left out the mosfet so I am going to test the IC to see what voltages I have going across it then I might throw in another mosfet to see how we get on.

P.s I have replaced the CS resistor that was previously blown. I am thinking that did not help with the operation of the IC and I am really hoping that by replacing that the IC will do its job and drive the mosfet.

[Jasgriff]

Re: Barco LCN-42 No Power

Right trying to trace back where this 13v should be coming from. I cannot work out from the number what IC3 is or what it does. There doesn't seem to be any significant voltage going into it to give 12.55v out on the marked pin?

Does anybody know what it is so I can continue to trace?

[budm]

Re: Barco LCN-42 No Power

Boy that is strange, you may be right that the VCC is coming from somewhere else. It is usually generated by the winding of the always on power supply then it will be fed to switched transistor to be turn on when it receives the PS_ON signal.
Right now we know that the 18v, 24v are not working so it can not be generated by those two transformer. You may have to trace the line, it should end at the Cathode of the Diode and DC filter cap some where.

[Jasgriff]

Re: Barco LCN-42 No Power

I have attached a pic to show the jumper that I removed and the black line shows where the vcc is coming from im getting about 8v over the black line.

If you can see C304 at the bottom there is 12v there so it seems that this is where the 13v should be coming from.

[Jasgriff]

Re: Barco LCN-42 No Power

I would like to try this to see if the 18v power supply section is loading down the 13VCC LINE, there is jumper wire in the middle of the picture shown with red line, lift one end up and see if the 13vdc comes back.

Its done the opposite.

I have disconnected the left hand side of the jumper and now I have 0v on the left hand track that goes back to the sub board but 6.2v on the right hand side that leads back to the replaced chip?

Maybe the vcc comes from somewhere else and that feeds back up to the sub board?

[Jasgriff]

Re: Barco LCN-42 No Power

Geez, 250V AC is barely enough to meet 220-240V AC regions.

And there is no way it can measure the PFC DC voltage of any modern electronic device with a limit of 250V DC.

At the moment it cant read anything as I blew the fuse by touching the wrong connection

[Jasgriff]

Re: Barco LCN-42 No Power

I would like to try this to see if the 18v power supply section is loading down the 13VCC LINE, there is jumper wire in the middle of the picture shown with red line, lift one end up and see if the 13vdc comes back.

I will try it now.

[retiredcaps]

Re: Barco LCN-42 No Power

The UNI-T one is a cheap one.

Geez, 250V AC is barely enough to meet 220-240V AC regions.

And there is no way it can measure the PFC DC voltage of any modern electronic device with a limit of 250V DC.