Barco LCN-42 No Power

[budm]

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.

[Jasgriff]

Re: Barco LCN-42 No Power

Just done a little more testing to try and work out whats going on.

It seems when I turn it on some power is supplied to the mosfet that I have marked with the letter A. This starts off at 75v or so but then quickly drops down to nothing. The same happened to the transistor which I think might be driving it? That transistor starts off at just short of 6v but then slowly drops down under 1 vault.

[Jasgriff]

Re: Barco LCN-42 No Power

The UNI-T one is a cheap one. I think it has probes and an on off switch and that's about it

[retiredcaps]

Re: Barco LCN-42 No Power

You meter AC frequency bandwidth is too lo for it to measure any AC in the 30~50KHz range.

I think I saw a Uni-T multimeter in one of Jasgriff's pictures as well. If correct, does that Uni-T have a bar graph?

[budm]

Re: Barco LCN-42 No Power

You meter AC frequency bandwidth is too lo for it to measure any AC in the 30~50KHz range.

[Jasgriff]

Re: Barco LCN-42 No Power

I thought retiredcaps would get to the answer quicker than me. I bet you didnt even have to look it up!

Here is the setting that I would have though was right.

[budm]

Re: Barco LCN-42 No Power

That is too low, that why I keep my 87 for poor man scope (I do have 4 scopes).

[retiredcaps]

Re: Barco LCN-42 No Power

I am not sure what the AC frequency bandwidth of your 175 is.

45Hz to 500Hz for 600mV to 600V according to manual page 12. For range 1000V it is 500Hz to 1kHz.

Not as wide as your Fluke 87 which goes from 50Hz to 20kHz.

[budm]

Re: Barco LCN-42 No Power

You just put it on AC mode and you should see the Graph bar moving up and down. You can try that on a known working SMPS IC out put pin. I am not sure what the AC frequency bandwidth of your 175 is.

[Jasgriff]

Re: Barco LCN-42 No Power

I will have a read on the net to see how I can scope the signal with this Fluke.

[retiredcaps]

Re: Barco LCN-42 No Power

He has a Fluke 175. It is True RMS and has an analog bar graph.

[budm]

Re: Barco LCN-42 No Power

Yes, an OSCILLOSCOPE. Do you have Fluke meter with bar grahp display?

[Jasgriff]

Re: Barco LCN-42 No Power

What scope lol.

am I going to need to buy an oscilloscope?

[budm]

Re: Barco LCN-42 No Power

I would leave the MOSFET out, you have to use the scope to look for the Gate drive signal.

[Jasgriff]

Re: Barco LCN-42 No Power

Right managed to move a few monitors that were fixed so that they are now out of the way for me to focus on my outstanding jobs so thought I would crack on with this one first....

I have removed the mosfet so that there is no power at all getting through to the transformer to cause more damage.

I have also replaced the 2 bad caps up on the top left. I knew they wouldn't make any difference to the problems I have at the moment but I though I might as well get them done. The ones that I took out were well goosed.

I am just going to double check that the voltages I had last time are still there just in case they did make a difference.

[Jasgriff]

Re: Barco LCN-42 No Power

Do you think its worth taking the mosfet back out again for now?

How can I tell if that 2.5v is pulsing or not?

do you mean the diodes on the sub board?

[budm]

Re: Barco LCN-42 No Power

Becareful if you are getting 2.5vdc steady to the Gate, that will turn on (slightly) the MOSFET, the MOSFET and the transformer will not like that since it supposes to be pulsing the Gate. I still cannot figure where that 13 v is generated from, and right now the PFC Voltage boost is proably not working either. By the way, did you check the MOSFET'S and Diode in the PFC section to see if they are OK?

[Jasgriff]

Re: Barco LCN-42 No Power

No still not getting 13vdc. Im getting about 2.5v out of the ic to the gate of the mosfet that previously blew. I am going to get a few more readings off it in a minute to give you an idea od what voltage is where e.g at the end of that resistor network.

I use paint to make markings on pictures. I find it best to zoom in to make your markings this makes them clearer when you save them.

[budm]

Re: Barco LCN-42 No Power

The PFC boosted voltage is also fed another resistor network. Did you get the 13vdc back that was treading low on the SMPS IC Vcc pin?
By the way, what do you to make the marking on the pictures on post #148, I like to use that instead of using the PDF.

[Jasgriff]

Re: Barco LCN-42 No Power

I replaced the blown mosfet on this today and the blown SMD resistor that connected to the CS pin of the SMPS IC. The great news is that it didnt blow this time.