Barco LCN-42 No Power

[budm]

Re: Barco LCN-42 No Power

Since the SMPS IC was not running (there was no 18vdc on the output side) and no drive to the Gate of the MOSFET, then I do not see why the MOSFET will just conduct by itself. That is strange.

[Jasgriff]

Re: Barco LCN-42 No Power

I have another Resistor and mosfet but i'm thinking that it will just blow again?

Yeah its very strange.

Right Can you see the R202 resistor. If that was blown would that cause the Mosfet to overload somehow?

[budm]

Re: Barco LCN-42 No Power

I think what we can do is to leave out the MOSFET, since we are still getting the VCC to the SMPS that drive that blown MOSFET, then we see why we are not getting the Gate drive output from that IC.

[Jasgriff]

Re: Barco LCN-42 No Power

Will I need to replace the resistor or can I leave that out too?

[selldoor]

Re: Barco LCN-42 No Power

Hi - if you are clutching at straws can I throw in J34 again. Whilst you checked it back along the trace to check it was working, did you check that no solder had burnt through to the track it was jumping.Perhaps test from jumper to R202 - cant see where other end goes - does it reach IC201? ( I see in later pics J34 has been cleaned up.)

You also mention thermal paste early on. Did you think B8? had been replaced using thermal paste? and if sodoes it also need an isolation strip and has it got one?

Talking of soldering being cleaned up there is one pin that looks worse - see pic might be worth checking that it is not connecting across to the parallel trace.

[Jasgriff]

Re: Barco LCN-42 No Power

Right,

Tried it with no resistor and no mosfet. I still have 5v but now the 24v has gone off aswell as the 18v.

I have traced R202 back to the IC and it connects to pin number 3 which is the CS pin. I am guessing that because there is no voltage getting through that resistor that the IC would not run at all? Maybe causing the the mosfet to short?

I am getting roughly 5v where that blown resistor was so I am thinking of replacing that and then seeing If I can test to see what the IC is doing further.

Do we think that a 3k resistor will be ok to replace the blown one at r202?

[selldoor]

Re: Barco LCN-42 No Power

No good asking me - too "in depth". - ill leave that one for Budm. Can I just clarify what I was asking. Can you do a resistance check between J34 and R202 or pin 3 in case the trace where it goesunder J34 is damaged and has been soldered to J34.
Also did you visually trace R202 to pin 3 or did you ohm it -again in case it is broken under J34

[Jasgriff]

Re: Barco LCN-42 No Power

No good asking me - too "in depth". - ill leave that one for Budm. Can I just clarify what I was asking. Can you do a resistance check between J34 and R202 or pin 3 in case the trace where it goesunder J34 is damaged and has been soldered to J34.
Also did you visually trace R202 to pin 3 or did you ohm it -again in case it is broken under J34

J34 is 0.02 Ohms and R202 to Pin 3 is 0.2 Ohms. I tested it with the multimeter.

I also checked to see if J34 is connected to Pin 3 or R202 and it not linked at all.

[retiredcaps]

Re: Barco LCN-42 No Power

I am now wondering if the filter cap has blown?

Put your Fluke 177 in capacitance mode and measure the big cap in circuit. I have measured it this way a few times and almost every big cap filter will read a bit low compared to its rating.

[Jasgriff]

Re: Barco LCN-42 No Power

Put your Fluke 177 in capacitance mode and measure the big cap in circuit. I have measured it this way a few times and almost every big cap filter will read a bit low compared to its rating.

I took it out and measured it. It measured a little lower but I later realised that a fuse had also blown in the plug so I changed that and put the cap back in.

Now without the mosfet in im still getting 330v on the big cap.

[budm]

Re: Barco LCN-42 No Power

So are you still getting the VCC at pin 6? R202 is the current sensing the voltage drop on the Source resistor of the MOSFET, if voltage drop on the Source resistor is too high due to too much current draw by the MOSFET, it will shutdown the output of the SMPS IC.
I think we need to get the 24vdc running again first, you have no 24vdc with the PS_ON applied, right?
Please see the PDF, if you do not have that voltage, then it may explain why the 24vdc PS is dead also.
By the way, do you remember the DC reading on the main cap without PS_ON and with PS_ON applied?

[Jasgriff]

Re: Barco LCN-42 No Power

Ok going to have a little look at these voltages....

330V across the big cap with the 5v jumper on.

330v across the big cap without the 5v jumper in place.

8.53v on those test points on your picture. It seems to be creeping up slightly every couple of seconds.

I have no 12v with PS_ON applied.

I am thinking of soldering another resistor in but leaving out the mosfet to see if that brings back the 12v supply.

[budm]

Re: Barco LCN-42 No Power

We should have about 13vdc at those two points instead of 8.53, we were getting the 13v after you replaced the MOSTFET and the SAMPS FOR THE 18VDC power supply section. This low voltage of 8.53vdc may explain why the 24vdc power supply stops working. We need to trace that trace out teo see where it goes to.
By the way, your power supply only has 24vdc, 18vdc and 5vdc outputs. Which 12vdc out put are you referring to?

[Jasgriff]

Re: Barco LCN-42 No Power

Thats was a typo sorry. I was thinking of 12v because I was tracing the cables yesterday and the 24v cables split off into pairs. So I was thinking 12v. Its definately 24v. I will have a look to see where that power comes from now.

[budm]

Re: Barco LCN-42 No Power

By the way that trace should go directly to the VCC pin of the SMPC IC you had replaced, please verify that for me, an at the Vcc pin, there is a ZENER diode connected to it also, ZD201.

[Jasgriff]

Re: Barco LCN-42 No Power

Here is the trace it goes back to two transistors.

[Jasgriff]

Re: Barco LCN-42 No Power

By the way that trace should go directly to the VCC pin of the SMPC IC you had replaced, please verify that for me, an at the Vcc pin, there is a ZENER diode connected to it also, ZD201.

Yes that test point goes back to VCC on Pin 6 and it goes through the zener diode. It tested at about 12 Ohms between the test point and the VCC pin.

[budm]

Re: Barco LCN-42 No Power

Can you please send me the same picture without the marking? by the way , you are getting good at taking the pictures.

[budm]

Re: Barco LCN-42 No Power

Do you mean you read 12 Ohms across that Zener diode both ways (reverse the test probes)? By the way, the Cathode side (Black) end should be connected to pin6, the Anode end should be connected to Ground.

[Jasgriff]

Re: Barco LCN-42 No Power

Here is the picture without the markings.

Yeah I managed to get my proper camera back. Makes a big difference taking pictures in the day light too.