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Thinkpad 700 Screen

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raoulduke

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Joined
Jan 14, 2015
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New Jersey
I got a Thinkpad 700 today. I could tell that it worked fine before I got it; the only really big issue being the screen - which showed no activity. When I got it home I noticed the backlight flash on power off. I then disassembled the thing entirely except for the system board and the thing that sits on top of them (because without the manual I honestly couldn't figure out how to get to the system board). When I put it back together, I noticed that the backlight now flashes on at power-on and again and power-off, but otherwise no change. I made sure to very carefully and firmly undo and reseat the screen cable and display cards. It came with a 4mb DRAM card, a 120mb HD and the battery - I pulled them all and it made no difference.

There was significant corrosion, but most of it was just on the battery and battery contacts. I've cleaned most of it. I guess I'm looking for any stuff I might not know. To wit, it would seem that I consistently get a system board error code when the CMOS (or PRAM; probably CMOS, right?) battery is pulled - and the machine won't boot at all. But it boots fine (albeit on an external display) when the PRAM battery is plugged in (even though it must be dead at this point). It's not possible the dead battery is preventing the screen from coming on, right? Just looking for some tips before I try to remove and somewhat thoroughly clean the system board. I cleaned the video card very thoroughly earlier, but since the VGA output and output to the screen appear to be through the same card, I don't think it's a video card issue.

I'm happy to post pictures if it'll help.
 
raoulduke said:
It's not possible the dead battery is preventing the screen from coming on, right?
Click to expand...
If you provide me with all of the engineering data (circuit diagrams, software listings, PCB layout, etc.) for the Thinkpad 700, then I should be able to come up with a definitive answer for you.

But in the absence of all engineering data, it pays to keep an open mind on things. For example, following are some unusual symptom/fix issues that have appeared in these forums.

* IBM PS/1: 301 keyboard error - Caused by bad battery
* Unable to boot from floppy - Fixed by turning off the SETUP option of "video cacheable option"
* Compaq Portable 486c: LCD display fades away and gets white. Discovered that symptom does not appear if an external monitor is left connected.
 
I was (maybe shockingly) able to find a 'new' (not clear how yet) graphics card. If that doesn't work I'll try cleaning the system board.

As to the backlight; it seems to go through different wiring, so probably has no bearing on the screen itself. I guess maybe I should replace the CMOS battery but at least per the manual that shouldn't prevent the screen from working. The manual's flowchart, depending on how one reads the somewhat variable symptoms, would suggest replacing either just 'System Board', or LCD - Video Card - System Board, or 'Power Systems Checkout' - Procesor Card - System Board - Video Card - HD - Base Memory Card - Options/Devices - Power Source - Speaker.

I'll confess the manual doesn't make light reading, at least to my taste. However, I haven't so far come across any suggestion that the machine will fully boot without the screen if the CMOS or regular batteries are missing. (The Power Systems Checkout is basically just that - check the AC adapter, battery, backup battery, etc, etc). However, I can replace the backup battery. I definitely cannot replace the regular battery, though there's probably not a lot of harm to sticking it back in for a test - I don't know how in tact its ROM is and I have no doubt it's dead.
 
Don't LCD screens this old use a CCFL (cold cathode flourescent) backlight? If your inverter has gone bad, it might start for a second and then shut down, I've seen LCD screens do that before.

Shine a bright light on the screen with the system on, do you seen an image? If so, the backlight is not working, and it's likely the inverter board. The inverter in most laptops is in the lid next to the screen, a small narrow circuit board.
 
Thankfully (only since I already got the graphics board) I don't see anything. However you may be right about the inverter anyway. I'll keep that in mind as I debug (and yes it is to the right of the LCD inside the screen casing).
 
They sent the wrong card... totally mislabeled in a bag that was labeled correctly. I'm hoping they're cool about return, because I'm pretty annoyed.
 
Sadly the new card doesn't change the symptoms. So that leaves (probably in this order of diagnosis) system board - inverter - LCD. I say in that order because cleaning the system board is the cheapest option, then probably replacing the inverter, then the LCD.** Of course even those don't necessarily guarantee success. I honestly also don't care that much. This has worked out to be a lot more expensive than I wanted it to, particularly given my interest - I mostly collect Macs.

I got the video card replacement from techpartswarehouse. They also stock the system board ($40) and LCD ($299) so that's probably not the way I'm going to go with these other parts.
 
A "trick" used if one suspects the backlight of LCD screens (particularly those with CCFL backlight) being dead is to shine a light onto the LCD screen itself, this way one can often see if it works. Have you tried it?
 
I think, the early ThinkPad displays (700C, 720C, 755C...) don't contain a CCFL. The display contains two tubes 235 x 8mm with two cables at each end:
800px-700c-bruch-ccfl.JPG


The cable on the left (235V) seems to be for the power only and the low-voltage cable on the right i guess is to "unlock" the display-matrix after the lamps have turned on - but it's a theory.
 
If the seller is willing to drop shipping, I might just get a new screen. I still haven't had the opportunity to take it apart completely and check the two boards under everything else - specifically whatever the main board is.
 
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