T61 inverter board problem

[wap32]

Hi,

I was replacing the LCD on my T61 and while it was powered on I decided to switch the LCD's for comparison.

Something must have gone wrong because now I don't get any backlight or power/bat/AC/etc indicator leds.
The machine still boots and I can see the image on the screen, just no backlight.
I tried both screens, and another LCD cable I had lying around, no change.

Do you guys think something on the board could have fried, or is it just a matter of switching the inverter?

[richk]

I think you burned a fuse on the motherboard

[wap32]

Thanks for the quick reply.

Where are the fuses located on the board, and where can I get replacements?

[richk]

They are soldered and not easy to find. I replace them by moving them from dead boards (boards with other problems). On most T61 boards (every FRU is different) the fuse is right next to the LCD connector (under some black plastic) and has a "P" on it. (there are more than one fuse involved, but that is the one I would test first.) They are not easily replaced without special equipment. People with less skill have had some luck soldering a good fuse on top of the bad one. This is not a job for a $10 soldering iron.

[wap32]

Well, that sucks.

I'll have to test all the fuses in the board to see if they're blown.
I might order some reasonable-valued SMD fuses from eBay to replace them.

You mentioned one of the fuses has 'P' on it, but some have 'N', right?.
Is there any sure way to spot a fuse on the board?

In the meantime I guess I'll order a new inverter anyway.

[richk]

The letter is the a key for the fuse attributes (current rating, fast or slow-burn) Try the one I pointed you at. That is probably the one.

[wap32]

You were quite right. I found at least one blown fuse.

For future reference to others, in the general area of the LCD connector I found:

F17 - Borwn/Gold color, marked 'B'
F7 - Pale green color, marked 'P'
F14 - Green color, marked 'M' with a logo
and a component without number, same color as F7, marked 'F'.

It turns out you were spot on, F7 (marked 'P') is blown.
The other component marked 'F' also doesn't pass the continuity test, but I'm not sure if it is a fuse.

I have taken a photo of the LCD connector area, it's here.

Replacing F7 is doable, the fuse is big enough.
However, if that 'F' is really a fuse, replacing it will be much more difficult.

Also found out that the F1 fuse (marked 'N') on the inverter is good, which makes me think that the inverter might be ok.
Probably it was just some current spike from changing the lamps while the inverter was on that blew up the fuses on the board.

I'm tempted to just bridge the fuses and see if it works, but I don't want to risk blowing up the board for good.

EDIT:
For reference to others, I've been checking datasheets for SMD fuses and code 'F' means 0.5A rating.
Code 'P' means 3A rating on 3 of the 4 datasheets I checked, and stands for 1.5A on the other.

EDIT2:
I've ordered replacements for F7 (marked 'P') and the 'F' fuse, SMD size 1206 3A rating and SMD size 0603 0.5A rating, respectively.

[jimmy274]

I have some more bad news - fuses are being installed so they could protect Power MOSFETs and driving MOSFETs. However, usually, when spike is too big for a fuse to protect electronics by blowing, some driving MOSFETs also burn. I can safely say that if you fail to solve the problem by changing the fuses alone, it's a power/driving mosfet problem. I would take a look at Q85 (the black ic with 8 pins near the LCD connector) and some others that are nearby and look the same.
However, don't check on them yet - they are very ESD sensitive and you could burn them by touching them. So, check if the fuses are ok, if they are, you're good to go. If not, post back, and I'll tell you which mosfets you need to check and the way you need to check them.

[wap32]

Thanks for the heads-up, I'll post back if things don't work out.

Also big thanks to richk for the help, I'd probably still be clueless about this if it weren't for his quick replies.

[wap32]

It seems the fuses really were the problem.

Replacing both the 'P' (F7) and 'F' fuse did the trick and brought back the LED's and backlight.
I've got my ThinkPad back!

Again, big thanks to richk for his invaluable help!

[l8mkb]

It seems the fuses really were the problem.

Replacing both the 'P' (F7) and 'F' fuse did the trick and brought back the LED's and backlight.
I've got my ThinkPad back!

Again, big thanks to richk for his invaluable help!

Hey Wap can you tell me where i could buy the fuses. If possible post some links, it would help me a lot.

Thanks,
L8

[wap32]

I bought the fuses from eBay, 0603 0.5A fuse (replacing the 'F' fuse) and 1206 3A fuse (replacing F7, marked 'P').

The 0603 can be a little tricky to solder and there are other surrounding components, be sure to have a decent soldering iron and tip, thin solder wire, and lots of flux.

[rkawakami]

Littelfuse components can also be found at mouser.com and digikey.com in single unit quantities for about 60 cents each. However shipping is what normally kills that deal .

[richk]

It is easier if you solder the new fuse on top of the old one