Help me troubleshoot this power supply. FSP043

[JonathanAnon]

Okay just to describe in detail, in case I'm misleading..

- Turn on the power supply.
- Measure output voltage on 12v rail as 11.82V
- I connect the ON/OFF pin to the +12V for a split second, and the backlight flickers on then goes immediately off.
- Measure the output voltage on the 12v rail again as 11.82V

[ben7]

Originally Posted by JonathanAnon

Okay just to describe in detail, in case I'm misleading..

- Turn on the power supply.
- Measure output voltage on 12v rail as 11.82V
- I connect the ON/OFF pin to the +12V for a split second, and the backlight flickers on then goes immediately off.
- Measure the output voltage on the 12v rail again as 11.82V

Ok, the inverter circuit itself seems to be sensing over-current and shutting off, instead of making the power supply shutoff.

We should start by checking the MOSFETs that drive the transformer.

What is the part number on them? They are the two little 8-pin (4 on each side) chips on the bottom of the PSU board, they are near the inverter transformer.

[JonathanAnon]

I found those two ICs... They are both..

SP8K3

I'll be back tomorrow, thanks for your help..

[ben7]

Originally Posted by JonathanAnon

I found those two ICs... They are both..

SP8K3

I'll be back tomorrow, thanks for your help..

Tomorrow you can measure the resistance from G to S, and from D to S, to see if any of them are shorted. Check all 4 of them (2 in each chip).

[JonathanAnon]

SP8K3 Chip number 1

(Red to black)
Pin2 TR1 Gate to Pin1 TR1 Source: 22.9K
Pin7 TR1 Drain to Pin1 TR1 Source: 1.
Pin8 TR1 Drain to Pin1 TR1 Source: 1.

Pin4 TR2 Gate to Pin3 TR2 Source: 23K
Pin5 TR2 Drain to Pin3 TR2 Source: 1.
Pin6 TR2 Drain to Pin3 TR2 Source: 1.

SP8K3 Chip number 2

(Red to black)
Pin2 TR1 Gate to Pin1 TR1 Source: 30.7K
Pin7 TR1 Drain to Pin1 TR1 Source: 1.
Pin8 TR1 Drain to Pin1 TR1 Source: 1.

Pin4 TR2 Gate to Pin3 TR2 Source: 24.6K
Pin5 TR2 Drain to Pin3 TR2 Source: 1.
Pin6 TR2 Drain to Pin3 TR2 Source: 1.

[ben7]

The third one is interesting b/c its gate to source resistance is 30k, whereas the others read around 23K.

IDK if that means its bad or not

Hm, try measuring the resistance from D to G. - probe on the gate, + probe on the drain.

[JonathanAnon]

I've checked Drains to Gate, and they all read as "1."

I'm just wondering should I put the pins of the MM across the one filter cap at the start of the inverter circuit to see if it is getting the right voltage.. there's meant to be a 12V feed from the main transformer... I'm still wondering maybe it's receiving 11.78V (same as is available across the 12V DC output rails), and this is not enough??

[ben7]

Originally Posted by JonathanAnon

I've checked Drains to Gate, and they all read as "1."

I'm just wondering should I put the pins of the MM across the one filter cap at the start of the inverter circuit to see if it is getting the right voltage.. there's meant to be a 12V feed from the main transformer... I'm still wondering maybe it's receiving 11.78V (same as is available across the 12V DC output rails), and this is not enough??

Go ahead and measure the voltage! Try to measure it while you jump the BL_ON pin to +12v with the resistor. You might need to use some alligator clips to connect the meter to the power supply for hands-free use.

Maybe because the voltage is lower, to output the same amount of power the inverter circuit has to draw more current, but the inverter control chip thinks its drawing too much current, and shuts it off.

[JonathanAnon]

Voltage across the first filter electrolytic cap on the inverter board is 11.80V, same as the 12v DC outputs. I have to turn the board upside to measure this so I cant test the BL_ON at the same time.

[ben7]

Originally Posted by JonathanAnon

Voltage across the first filter electrolytic cap on the inverter board is 11.80V, same as the 12v DC outputs. I have to turn the board upside to measure this so I cant test the BL_ON at the same time.

Does the voltage fluctuate at all when the backlight flickers?

[selldoor]

Jon
Did you say earlier that you tried this panel (ccfls) on a working power supply
and main board. I have looked but cant just spot it.

[JonathanAnon]

Just to reiterate, I have tried some tests on the bits of this non working monitor with a working monitor of the same model:

The Backlight worked fine when I used it with another working monitor... i.e. I took the backlight out of the working monitor, put the current backlight in, and it worked seamlessly. So the backlight is verified as good.

The Video Board DID NOT work when I tried it with the power board and backlight of the working monitor. There was no display at all. So the video board is not working.

Power Board. I did not think it was wise to the power board in the working monitor in case it would cause damage, and I would be left looking very foolish with two broken monitors.. I did take note that the 12V rails on the power board of the working monitor were 12.88v and the 12v rails on the power board of the non working monitor were 11.8V.

My train of thought when I started the thread was to get the non working power board to have the same voltage readings across it's rails as the working power board. And then source/buy a working video board.

[ben7]

Originally Posted by JonathanAnon

Just to reiterate, I have tried some tests on the bits of this non working monitor with a working monitor of the same model:

The Backlight worked fine when I used it with another working monitor... i.e. I took the backlight out of the working monitor, put the current backlight in, and it worked seamlessly. So the backlight is verified as good.

The Video Board DID NOT work when I tried it with the power board and backlight of the working monitor. There was no display at all. So the video board is not working.

Power Board. I did not think it was wise to the power board in the working monitor in case it would cause damage, and I would be left looking very foolish with two broken monitors.. I did take note that the 12V rails on the power board of the working monitor were 12.88v and the 12v rails on the power board of the non working monitor were 11.8V.

My train of thought when I started the thread was to get the non working power board to have the same voltage readings across it's rails as the working power board. And then source/buy a working video board.

Ok, so on the power supply, most likely we will have to adjust the output voltage. It appears to be a TL431 regulated psu. Can you get better pics of the power supply area?

[JonathanAnon]

Photo of power board.
http://omg.wthax.org/powerboardoverhead.jpg

This is the schematic from my original post..
http://omg.wthax.org/FSP043.jpg

[ben7]

Originally Posted by JonathanAnon

Photo of power board.
http://omg.wthax.org/powerboardoverhead.jpg

This is the schematic from my original post..
http://omg.wthax.org/FSP043.jpg

Can we have a better picture of the bottom? There are some tiny resistors on the bottom which I want to read.

[JonathanAnon]

Bottom Left:

http://omg.wthax.org/powerboard_bottomleft.jpg

Bottom Right:

http://omg.wthax.org/powerboard_bottomright.jpg

The bits are packed quite tight together so it's hard to get the detail without taking the picture from directly above the component.

[ben7]

Originally Posted by JonathanAnon

Bottom Left:

http://omg.wthax.org/powerboard_bottomleft.jpg

Bottom Right:

http://omg.wthax.org/powerboard_bottomright.jpg

The bits are packed quite tight together so it's hard to get the detail without taking the picture from directly above the component.

No, I meant the underside of the board, the "bottom".

[JonathanAnon]

http://omg.wthax.org/back_board.jpg

btw, am I wrong in thinking that measuring the voltages across the caps all the way along (both ceramic and electrolytic) is a good way of tracing the signal across the board? What I was thinking of doing was measuring the voltages across all the caps and enter the information on to the schematic (with photoshop)... It should be pretty easy to diagnose with that shouldnt it??

[JonathanAnon]

Hi (if anybody's still listening )

I've been toying around with again and I think I've found the area where the problem is... I've been going through my book on LCD repair and it says that there should be 300V across the filter cap (mine has 330V) so we're good up to that point.

However, the next step is to measure the voltage between the negative pin of the filter cap and the VCC of the switching MOSFET... This is meant to be between 16V and 20V, but mine is only 10.6V.. gonna check all the resistors and diodes in that part of the circuit.

[ben7]

Originally Posted by JonathanAnon

Hi (if anybody's still listening )

I've been toying around with again and I think I've found the area where the problem is... I've been going through my book on LCD repair and it says that there should be 300V across the filter cap (mine has 330V) so we're good up to that point.

However, the next step is to measure the voltage between the negative pin of the filter cap and the VCC of the switching MOSFET... This is meant to be between 16V and 20V, but mine is only 10.6V.. gonna check all the resistors and diodes in that part of the circuit.

Hm, are any other voltages on the secondary out of whack?