240Z goes dark when booting to OS

[solidpro]

HI Everyone

I'm trying to revive a 240Z which had a multitude of issues. The 240Z is quite different to the other 240 series so is worth trying to revive.

The issue I've come across before, but can't resolve is that only after about 30 seconds, the backlight goes out..

I've tested the entire display using 2 other assemblies - ribbon, LCD, inverter and 3 sets all do the same. And these are known-good complete lids. So the issue seems to be confined to the mainboard.

All signal and power comes up one small connector and then splits off. The connector did originally suffer from cracked solder joints (which was apparent with the symptoms of a white display resolved by wiggling the mainboard connector). I flowed this which solved that problem.

When I say i've seen this before, I think it was with T2x machines which have a particular fuse on the mainboard which comes into play when switching to some 'higher power' mode, but perhaps I'm misremembering.

I completely removed the mainboard and went over the board with a toothcomb, maginifying glass and continuity tester and all fuses are working just fine.

Has anyone got any suggestions?

Thanks!

[RealBlackStuff]

The 240 has 2 LCD options, type A and type B.
Could they have some influence?
T2x problem was more the inductors falling off or the BOD.
I'm not aware of T2x fuse problems.

[solidpro]

Hi

Well, there is a SVGA screen for the 240 and 240X and an XGA screen for the 240Z and the entire lid is swappable. However I do have 2 sets of both and tried the other 240Z screen, inverter and ribbon and same thing.

I think the issue with the T2x was if you disconnected the screen without pulling the power lead out of the back it would blow a fuse and cause a blank white screen - I think my memory is fuzzy though because I thought I'd experienced another thinkpad where the backlight would go out after a certain amount of seconds....

[kfzhu1229]

The issue I've come across before, but can't resolve is that only after about 30 seconds, the backlight goes out..

I've tested the entire display using 2 other assemblies - ribbon, LCD, inverter and 3 sets all do the same. And these are known-good complete lids. So the issue seems to be confined to the mainboard.

All signal and power comes up one small connector and then splits off. The connector did originally suffer from cracked solder joints (which was apparent with the symptoms of a white display resolved by wiggling the mainboard connector). I flowed this which solved that problem.

Speaking with a generic knowledge of ThinkPads, the backlight go out when either the inverter or the mainboard feels like some sort of overcurrent or overheating has occured (and won't blow the fuse if it is a minor enough problem).
Test out which pins on the LCD connector you reflowed are the power pins that would probably carry 16V charger power. There should be multiple pins to spread the current load. I imagine if many of those pins have flaky connection or that you have some micro strands of solder shorting it elsewhere, you could have 1-2 pins carrying all the load and could trigger overcurrent protection.

[solidpro]

Good call. I will go over it again. I checked the connector under a microscope and it looked perfect, but that doesn't mean going over it again won't potentially bring about a win.... Thanks

[kfzhu1229]

Good call. I will go over it again. I checked the connector under a microscope and it looked perfect, but that doesn't mean going over it again won't potentially bring about a win.... Thanks

The thing I learn the hard way with the RAM slot legs (which is pretty similar to this of course) is that a solder bridge can still happen completely behind and away from your eyesight. Probe with your multimeter on the adjacent legs of the mains 16V voltage (with power fully cut out of course) and see about continuity

[solidpro]

Hi - yeah it's quite similar to the delicate issue of reflowing RAM connector legs...

I went over the whole thing again with a continuity tester and annoyingly everything checks out fine - nothing is bridged and every joint connects fine between the PCB and the connector.... so I don't think it's the connector. I don't know what it is but I think I won't be able to work this one out if it's not the LCD, inverter, ribbon, connector or fuse!

[axur-delmeria]

Question: if you stay in the BIOS menu for 5-10 minutes, does the backlight stay on?

[kfzhu1229]

Hi - yeah it's quite similar to the delicate issue of reflowing RAM connector legs...

I went over the whole thing again with a continuity tester and annoyingly everything checks out fine - nothing is bridged and every joint connects fine between the PCB and the connector.... so I don't think it's the connector. I don't know what it is but I think I won't be able to work this one out if it's not the LCD, inverter, ribbon, connector or fuse!

I know the one final approach is that you can bridge the mosfet responsible for shutting the backlight down in cases like this. I know on the T43p mobo that I had the unfortunate experience of shorting the inverter by dropping and pinching it onto the magnesium chassis, the fuse blew but the mosfet blew too, so I had to bridge across both and then the machine works basically perfectly fine - except on restart for that few seconds when the screen is supposed to go black and then come back up with ibm logo, the backlight would stay on

[solidpro]

Is the mosfet on the inverter or the mainboard?

cheers

[kfzhu1229]

Is the mosfet on the inverter or the mainboard?

cheers

usually it's on the mainboard. there should be mosfet controlling the on and off of the backlight 16V power rail of that thing.
But I guess if you are as unlucky as I was with my A31 ram slot, it is possible for the cracked joints to lift the 16V main power rail traces and then the insulation material as well and cause a board insulation failure, which would be that power rail shorting with say about 50-100Ohms of resistance, then if the power is not cut, the power rail will work fine but slowly the board can start smoking there!