Barco LCN-42 No Power

[Jasgriff]

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?

Hopefully this diagram helps to show you how the pins are connected.

[Jasgriff]

Re: Barco LCN-42 No Power

The Power Supply Gives out 5v 18v and 24v if that's what you mean?

[budm]

Re: Barco LCN-42 No Power

So the PS1 as I redlined is for the 18v and PS2 is for the 24vdc?
So at this point it looks like you will have to replaced those bad caps and bad transistors, fuse, and resistors to find out what going on.

[Jasgriff]

Re: Barco LCN-42 No Power

This link shows the power boards connections and outputs. I think this is what you mean?

http://www.fspgroupusa.com/download/...7-3M01_DIA.pdf

The Mosfet has been ordered. (What paste do I use between this and the heatsink? CPU paste?)

I was thinking of ordering these fuses? They are the nearest ones I can find?
http://www.ebay.co.uk/itm/8-Amp-8A-T...item35a4a11674

Any idea what the blown SMD transistor might be? the top has blown off it.

Still not sure on the resistor value that needs to be ordered.

Shall I just replace the 2 bulging caps for now?

[budm]

Re: Barco LCN-42 No Power

I use the white heat sink compound for the power devices. Not sure if the CPU paste is electrical conductive or not, I will not use it if it is.
One way to find out if the transistor is NPN or PNP is to trace out the Emitter and Collector to see how B+ is connected to the Emitter or Collector, I can try tracing out the traces in the pictures but it will be real time consuming. I would still try the 0.20 Ohm for that Source resistor.
The Emitter of the Q202 is connected to a small value resistor (R203) which is Connected to the Gate of the shorted MOSFET, so the Gate to Drain may have shorted out and take out the Q202, I would be surprise if the Gate resistor is not damaged, a lot of current must have flown through Q202 from Collector through Emitter, through R203 to Gate.
If you look at the middle pin (Collector) of that transistor, the trace goes under the IC, you need to find out which pin or where it go to.
Notes: I look at the traces again, Q201 Emitter is connected to the R216? which is also connected to the Gate and the Base is connected through resistor network to the SMPS IC pin 5 Drive output. Q202 looks like it is connected to pin 4 of the SMPS IC which is Ground. So it looks like the function of Q202 is to limit the Gate drive voltage by shunting it through R203 and E to C to Ground.

[Jasgriff]

Re: Barco LCN-42 No Power

I can trace it. Is B+ the + of the big capacitor?

I didn't see the update that you made to one of your pictures. I know what you mean about the different circuits now.

[budm]

Re: Barco LCN-42 No Power

Please my Notes on post 45 see if I am correct, that the Collector of Q202 is connected to pin4.

[Jasgriff]

Re: Barco LCN-42 No Power

I use the white heat sink compound for the power devices. Not sure if the CPU paste is electrical conductive or not, I will not use it if it is.
One way to find out if the transistor is NPN or PNP is to trace out the Emitter and Collector to see how B+ is connected to the Emitter or Collector, I can try tracing out the traces in the pictures but it will be real time consuming. I would still try the 0.20 Ohm for that Source resistor.
The Emitter of the Q202 is connected to a small value resistor (R203) which is Connected to the Gate of the shorted MOSFET, so the Gate to Drain may have shorted out and take out the Q202, I would be surprise if the Gate resistor is not damaged, a lot of current must have flown through Q202 from Collector through Emitter, through R203 to Gate.
If you look at the middle pin (Collector) of that transistor, the trace goes under the IC, you need to find out which pin or where it go to.

The middle pin of the transistor goes to pin number 4 and then on to the negative bin of the big capacitor which is GND?

Does the attached picture with the lines make sense?

Those resistors do look burnt on the top too. I can replace them again I can only read one of them which says 303.

[budm]

Re: Barco LCN-42 No Power

OK, that is what I figure from the picture, the Collector is attached to Ground and the Emitter is connected to the resistor then to the Gate, the Gate needs Positive drive voltage so that means this Q202 is PNP type. So any PNP with Vceo rating of 100v and 100 ma IC or more should be OK, beta to be around 50-200 in this typical rating transistor.
30K resistor.
You should also check the Q201 and the other Gate resistor as well since they are hooked up almost the same way as Q202.
I will be surprised if the output pin 5 is OK if the Q202 blown its top, and Q201 and the resistor is OK, I hope they are OK.

[Jasgriff]

Re: Barco LCN-42 No Power

OK, that is what I figure from the picture, the Collector is attached to Ground and the Emitter is connected to the resistor then to the Gate, the Gate needs Positive drive voltage so that means this Q202 is PNP type. So any PNP with Vceo rating of 100v and 100 ma IC or more should be OK, beta to be around 50-200 in this typical rating transistor.
30K resistor.
You should also check the Q201 and the other Gate resistor as well since they are hooked up almost the same way as Q202.

For the Transistors to be working they should be open from collector to emitter? I am testing them and they aren't either way I try.

I have just re read retired caps transistor testing guide and it says if any are below 30 OHMs then they are bad but none of mine are near 30 ohms.

[budm]

Re: Barco LCN-42 No Power

Collector to Emitter should show open circuits (either NPN or NPN).
Base and Emitter should read like diode.
Base and Collector should read like diode.
I think this is how the drive circuits are hooked up.

[Jasgriff]

Re: Barco LCN-42 No Power

201 39.5K B to E
201 29K B to C
201 10K C to E

202 50K B to E
202 19K B to C
202 31K C to E

[budm]

Re: Barco LCN-42 No Power

Are you reading using Diode test function? Q202 should have shown bad reading when it has its top blown off like that, remove Q202 and test again, it may read open circuits if it is blown open not blown shorted.

[Jasgriff]

Re: Barco LCN-42 No Power

Are you reading using Diode test function? Q202 should have shown bad reading when it has its top blown off like that, remove Q202 and test again, it may read open circuits if it is blown open not blown shorted.

Took it off and used the diode function the Display just says OL every way that I try it.

[budm]

Re: Barco LCN-42 No Power

That is what I expected, it is blown open circuits. Now you will have to look for something close to the original one.

[retiredcaps]

Re: Barco LCN-42 No Power

I wrote the tips in my guide with the beginner in mind and tried to avoid using terms like base, collector, and emitter (drain, source, gate, etc). So that is why I use words like pins 1, 2, and 3 and use a simple resistance test.

Clearly, it doesn't work in all situations and in the next revision, I will have to consider adding a diode test.

Carry on.

[Jasgriff]

Re: Barco LCN-42 No Power

That is what I expected, it is blown open circuits. Now you will have to look for something close to the original one.

Yeah the transistors are going to be a bit tougher to find I think.

I have removed the 2 damaged SMD resistors they dont work out of circuit either. I will replace those with 30K SMD's

Its getting late here now so I will continue my search for the PNP 100v transistors tomorrow.

Thanks very much for you help guys as always and thanks for your patience!

[Jasgriff]

Re: Barco LCN-42 No Power

Quick update on where I am at with this one.

Ordered some 1 Watt 0.22 OHM metal film resistors and 3 Watt 0.22 OHM wire wound resistors. 1 to replace the blown and the others for spares.

Ordered a batch of T8AH fuses in case the board blows another.

Ordered the replacement Power Mosfet 11N80C3

Ordered some 33k SMD resistors to replace the 2 blown ones that connect to the transistors.

Ordered a bottle of Isopropyl Alcohol for getting rid of the flux and gunk.

Hopefully a few bits will arrive tomorrow so I can start working on getting it going again.

I have also had a good luck at the transistors today I noticed that they both start with a number '2' I have also emailed the people who make the Monitor and the people who make the board to see if they can help we with a diagram. Not holding out much hope but its worth a try.

[budm]

Re: Barco LCN-42 No Power

This what I do when I do not have the surface mounted transistor (or to lazy to order them). This is from Viewsonic TV power supply board.
By the way, the function of that Q202 is to discharge the Gate capacitor (inside the MOSFET) when the SMPS IC output goes low.

[Jasgriff]

Re: Barco LCN-42 No Power

Yeah I thought of doing that if I broke the pads when I took the old one off. It looks ok just needs a clean.

I have been reading your diagrams for a while today. It seems that most power supply's work in a very similar way?

I take it that when the SMPS IC goes low then the transistor connects the mosfets gate to ground which cuts off the power supply?