Toshiba Satellite P100 Backlight Problem

[DarthChrisB]

Hi!

I have a Toshiba Satellite P100 with a non-working backlight. The image is faintly visible, so the screen and graphics card are working. I've already replaced the backlight bulb, inverter and lcd-cable with no result. After testing the pins on the inverter when powered on i found out that pin 2 has 12 volts but pin 1 has 0 volts. While supplying 9 volts to pin 1 with a battery the light turns on. That means there's a problem with the power supply from the mainboard. I couldn't find any inverter schematic, so I don't know whether it is really the case that pin 1 AND 2 are for input voltages.

I can't post links yet, so for the p100 schematics please use google to search for "repairalaptop p100 schematics", it's probably the top result.
The LVDS-schematic is on page 14
I also have a maintenance manual if needed.

How can I trace down the error? Which components do I need to test?

Thanks!

 

[NYJimbo]

I cant recall how this works on the older Toshibas but do you know if you would be seeing VIN on that pin if say the "lid switch" was closed ? I know ENA decides if its on or off but I don't remember if you would still get the VIN or if the switch would kill the voltage in. Does this model have a lid switch or mechanism and can you tell if its not in the "shut off" position ?

The P100 is pretty old so I don't know if the lid switch would make it sleep and kill all output to the LCD or if it would just make the LCD go dark.

Sorry, I cant be more help, too many machines in the shop to think about this one.

 

[katz]

Good call, Jimbo.

Try jumping that lid switch (if it has one) & see if things brighten up for you.

 

[DarthChrisB]

I cant recall how this works on the older Toshibas but do you know if you would be seeing VIN on that pin if say the "lid switch" was closed ? I know ENA decides if its on or off but I don't remember if you would still get the VIN or if the switch would kill the voltage in. Does this model have a lid switch or mechanism and can you tell if its not in the "shut off" position ?

The P100 is pretty old so I don't know if the lid switch would make it sleep and kill all output to the LCD or if it would just make the LCD go dark.

Sorry, I cant be more help, too many machines in the shop to think about this one.

I can't boot it up right now (it's disassembled). But I tested the lid switch (when on external monitor) and it worked perfectly. It's a mechanical switch and when I pressed it once (it doesn't have open/closed states) windows went into standby mode and when I pressed it again, it woke up successfully. But the light wouldn't come up on the internal screen.

I just tested two capacitors (c26, c25) that read +5V and +3V respectively on the schematic page 14, but when i tested them, I read 12 and 8 volts resp. I don't know what to do with it.

 

[mr m]

A couple of operational issues here.

You can't reliably test a cap without taking it out of a circuit. Plus a standard DMM is pretty useless for testing caps even with a capacitance setting. An analog meter will show a cap charging or discharging but all that tells you is it isn't totally blown but won't tell you if it's in spec. Some ESR meters can test an in-circuit cap but the good ones are expensive and I always pulled the component for the most accurate readings.

That 2 pin connector on the inverter is AC OUTPUT to fire the CCFL, not input. It takes anywhere from 5-20 V DC input to energize the inverter and it outputs around 1,000 V AC at startup and less while running.

With your symptoms it's usually the inverter that is bad. Then the backlight, cable or lid switch. You can still have a bad LCD panel. Rare, but I've seen it.

Even though the external monitor looked OK there are control lines from the mobo that tell the inverter to fire. Again, rare but I've seen mobo's cause dim displays.

You remembered to check the lid switch but did you check the brightness control?

 

[DarthChrisB]

A couple of operational issues here.

That 2 pin connector on the inverter is AC OUTPUT to fire the CCFL, not input. It takes anywhere from 5-20 V DC input to energize the inverter and it outputs around 1,000 V AC at startup and less while running.

You misread something. I wrote pin 2 not 2-pin, so what I meant is that pin 2 on the input connector (5-pin connector) reads 12 volts, but pin 1 on the same input connector reads 0 volts. When I supply power to pin 1 with a 9v-Battery the light turns on.

With your symptoms it's usually the inverter that is bad. Then the backlight, cable or lid switch. You can still have a bad LCD panel. Rare, but I've seen it.

All of the mentioned parts are ok.

Even though the external monitor looked OK there are control lines from the mobo that tell the inverter to fire. Again, rare but I've seen mobo's cause dim displays.

Now we're where we started from. I've already concluded in a previous post that the mobo doesn't supply enough voltage to the inverter. The problem I have is I don't know which line on the mobo is supposed to supply the voltage and I also couldn't figure out how much voltage exactly it needs.

You remembered to check the lid switch but did you check the brightness control?

Does it even matter? The light goes on when I supply voltage to pin 1 so I suppose the brightness control is ok.
One problem I have though, is that I can't use the FN-combinations at the moment (no Windows installed and they won't work in BIOS), so I can't change brightness right now.

Btw. the input voltages of the inverter read as follows:
pin 1: 0v
pin 2: 11v-12v
pin 3: 4v-5v
pin 4: 4v-5v
pin 5: 0v

Thanks for any help!

 

[NYJimbo]

Btw. the input voltages of the inverter read as follows:
pin 1: 0v
pin 2: 11v-12v
pin 3: 4v-5v
pin 4: 4v-5v
pin 5: 0v

Thanks for any help!

Did you check which are ground ? At least one must be, sometimes you have two. Its possible 1 and 5 are ground. Usually you have VIN, ENA, ADJ and grounds.

 

[mr m]

Back in the day I chip-chased but now with these multi-layered boards and no scope or logic analyzer all I do (except for DC jacks) is swap parts. I'd replace a cable or inverter but not a CCFL unless it's my laptop. Probably not worth fixing for a client but a great challenge for yourself.

According to your link the schematic shows the inverter troubleshooting path starts at the North Bridge and on to the LCD panel. No surprise there but a lot to test!

It might not be a voltage issue but a clock or control signal at fault. That 6 pin MOSFET at U4 (on page 14, LVDS) like all transistors is prone to fail. Try using a magnifying glass and really scrutinize the mobo components for a physical sign of failure. Maybe you'll get lucky.

If your sure it's not the LCD panel, inverter, CCFL or LDVS cable it must be the mobo. If you figure this out let us know. My head is spinning! Good luck, brother.

 

[DarthChrisB]

It might not be a voltage issue but a clock or control signal at fault. That 6 pin MOSFET at U4 (on page 14, LVDS) like all transistors is prone to fail. Try using a magnifying glass and really scrutinize the mobo components for a physical sign of failure. Maybe you'll get lucky.

Please correct me if I'm wrong, but this MOSFET at U4 controlls LCDVCC and from my understanding this is the power for the LCD panel, not the backlight.

I had an old but working Acer notebook with a similar inverter. What I did is I used the p100 inverter with the Acer to see what the voltages would be in working condition. The backlight of the Acer worked perfectly with my p100 inverter and here are the pin-assignments and input voltages:

pin 1: VIN (42V)
pin 2: not connected (11V)
pin 3: on/off (0=off, 7=on)
pin 4: brightness (0=low, 7=high
pin 5: GND

So now we know with certainty that only VIN at pin 1 is wrong. Am I right that VIN is INVCC0 from the LVDS connecter side (page 14)?

 

[mr m]

Please correct me if I'm wrong, but this MOSFET at U4 controlls LCDVCC and from my understanding this is the power for the LCD panel, not the backlight.

So now we know with certainty that only VIN at pin 1 is wrong. Am I right that VIN is INVCC0 from the LVDS connecter side (page 14)?

Yes to first question. BTW, Vcc is always a positive voltage.

I'm not sure about question 2. I was under the impression that the input to typical laptop inverters should be between 5-20 V DC. So I don't understand why there is 42V present at pin 1 in the Acer. Also, where is the inverter on page 14? It should be included in the LCD panel. I don't see either the 5 pin input or 2 pin output connectors let alone the transformer/coil used to create the AC output.

Sorry I can't be of more help. It's been 30 years since I've done stuff like this and there were no LCD's back then, just CRT's.

You definitely get an A for effort!