750c repair

[beshad]

hi all,

i finally received my newly acquired non-working 750c. cosmetically is in like new condition with only one owner since 1993, however it needs fixing. when it's starting up it makes couple beeps and a pause and one more sound and then it hangs there, i can hear the HDD spinning but screen remains blank. i though i'd better start with COMS battery and replaced it with a new one from my 380XD, no result there. then i spotted the standby battery which had lots of bluey sulfation/corrosion around the connection. i cleaned as much i could and then tried to pull it out but it does not come out and i am afraid i'll break the board or the connection if i pull any harder. do you have suggestion how i could remove this standby battery?

below are images of the battery, connection and also a link to a video where i recorded the warning lights and beeps etc:

https://ibb.co/ySLxVHn
https://ibb.co/0cf8bvb

https://youtu.be/IutdaftV64g

https://ibb.co/ZXQR7FD

i also tried to boot the laptop with no HDD, no floppy drive and battery with the same result. previous owner told me it was about 10 years ago when he successfully started this laptop and put it back in storage. any help would be appreciated.

hope everyone is having an awesome holidays.

cheers

[RealBlackStuff]

I take it you have the HMM manual tpvol2.pdf for it?
If not, here it is: http://ps-2.kev009.com/ohlandl/books/tpvol2.pdf

It looks as if you have to disassemble the machine to get to the bottom of the motherboard 66G5036.
Then you need lots of patience, ear cleaners and isopropyl alcohol to soak the corrosion away.
Eventually it will become loose enough to pull it out without wrecking the connector.

[beshad]

I take it you have the HMM manual tpvol2.pdf for it?
If not, here it is: http://ps-2.kev009.com/ohlandl/books/tpvol2.pdf

It looks as if you have to disassemble the machine to get to the bottom of the motherboard 66G5036.
Then you need lots of patience, ear cleaners and isopropyl alcohol to soak the corrosion away.
Eventually it will become loose enough to pull it out without wrecking the connector.

thank you i will try isopropyl alcohol and see if that works before starting to disassemble (prefer to avoid that as much as i can!), yes i have downloaded the HMM manual and its very helpful. would you also have any idea what those beeps mean and why there is nothing on screen? is there a chance it might be related to the battery corrosion?

[beshad]

just finished disassembling the laptop. can't believe that i had to take everything out to get to the bottom of mainboard! the standby-battery socket was completely corroded and full of blue stuffs. cleaned it up (ended up breaking the battery wires while pulling it out but mainboard connection is fine).

https://ibb.co/Jk4Gfyy

unfortunately i am still getting the beeps and a blank screen. while inspecting the LCD i noticed this (invisible unless viewed under direct light):

https://ibb.co/N235VbJ

does this round marking on screen means anything bad? any chance that the LCD needs to be replaced?

googling for replacement parts, should i also search 350, 355, 360 etc as well as 750 and 755? for example here is a for-parts 755CD:

https://www.ebay.com/itm/Rare-IBM-Think ... 4081848304

or here is a LCD for 360:

https://www.ebay.com/itm/Sharp-LCD-for- ... 1986538173

how do i find out if any of these parts are interchangeable with my 750c?


thank you for the help

cheers

[RealBlackStuff]

That screen spot might be a pressure marking from the trackpoint nub.
Beep Symptoms are explained in the HMM.
No idea about parts, someone else?

[Bondi]

First of all to check the parts comatibility I'd refer to the same HMM (https://download.lenovo.com/ibmdl/pub/p ... tpvol2.pdf)
It lists all parts' FRUs for different models. It's just necessary to see if any other model has the same FRU.

[beshad]

i thought first step should be testing the LCD, so getting an external VGA monitor today. HMM says the 'Two short beeps with blank screen.' is the system board error

[beshad]

i just realised there was no credit card memory ram under the floppy drive when i disassembled the laptop. is that external memory card the only ram for this model laptop or there are some built in as well? if that's the case then is it possible the mainboard beeps error relates to non-existence of any ram modules? cheers

[Thinkpad4by3]

Search up IC DRAM. Almost every single laptop from 1992 to 1996 used it. You should be able to pick up a 8-16MB module for pennies on the dollar. Check the MTM because there are 60 and 70ns modules and the 750C should be flexible but just check to see what it can take. Later I can check my 755CDV which should take the same cards.

[Bondi]

i just realised there was no credit card memory ram under the floppy drive when i disassembled the laptop. is that external memory card the only ram for this model laptop or there are some built in as well? if that's the case then is it possible the mainboard beeps error relates to non-existence of any ram modules? cheers

Usually there is also built-in memory. At least my 755CD and 360CE have it. So you can try to boot up without this card. But there is not much hope that this will help.
As the HMM clearly states that two beeps are an indication of failed mobo.

[beshad]

i just realised there was no credit card memory ram under the floppy drive when i disassembled the laptop. is that external memory card the only ram for this model laptop or there are some built in as well? if that's the case then is it possible the mainboard beeps error relates to non-existence of any ram modules? cheers

Usually there is also built-in memory. At least my 755CD and 360CE have it. So you can try to boot up without this card. But there is not much hope that this will help.
As the HMM clearly states that two beeps are an indication of failed mobo.

yes i have come to the same conclusion now that the motherboard is failed. 750c mainboard FRU P/N is 66G5036. is there any way for me to find out what other models of Thinkpads from the same era share compatible model motherboards? i guess it would be quite difficult sourcing an exact 750c replacement mainboard

[Thinkpad4by3]

755C and 370C mobos should be fine....plus they will likely have the faster DX4 75MHz processor instead of a 486SL 33MHz. And its not like you'll find a working battery to notice any battery differences.

How does this look to you?
https://www.ebay.com/itm/85G1493-IBM-Th ... 0558664596

[beshad]

755C and 370C mobos should be fine....plus they will likely have the faster DX4 75MHz processor instead of a 486SL 33MHz. And its not like you'll find a working battery to notice any battery differences.

How does this look to you?
https://www.ebay.com/itm/85G1493-IBM-Th ... 0558664596

cheers for the ebay link (amazing, brand new part for a 25 years old laptop!) , i was also looking at these two listings:

https://www.ebay.com/itm/IBM-85G0993-TH ... 2540628509

https://www.ebay.com/itm/IBM-84G2252-TH ... 2542901671

would it be also wise to publish a wanted ad in the forum marketplace here too? would i be able to get it any cheaper here?

[beshad]

755C and 370C mobos should be fine....plus they will likely have the faster DX4 75MHz processor instead of a 486SL 33MHz. And its not like you'll find a working battery to notice any battery differences.

How does this look to you?
https://www.ebay.com/itm/85G1493-IBM-Th ... 0558664596

also can you pls elaborate on your battery comment? the existing battery (of course not working atm) wouldn't fit anymore? or does it fit but wouldn't work (if it was functional)?

[Thinkpad4by3]

The 486SL is a low power chip and the DX4 is a full power chip running at a higher clockspeed so it will take more power.

And on battery you'd notice this but on an AC adapter it should be fine. Check the HMM but I'm 99% sure it will work.

[beshad]

The 486SL is a low power chip and the DX4 is a full power chip running at a higher clockspeed so it will take more power.

And on battery you'd notice this but on an AC adapter it should be fine. Check the HMM but I'm 99% sure it will work.

thank you. would you know if the 360CSE mainboard is also compatible? from this listing:

https://www.ebay.com/itm/IBM-Laptop-Thi ... 0167.m2940

it is from 360CSE, pretty cheap and also seems to come with the standby battery (mine is broken) and CMOS battery

[Thinkpad4by3]

I'd give you a 95% chance you can drop that Mobo right in and it will fire up. IBM really produced one machine and labeled under the 750/755/360 machines with various processors, RAM, and Displays installed into the same shell.

[beshad]

I'd give you a 95% chance you can drop that Mobo right in and it will fire up. IBM really produced one machine and labeled under the 750/755/360 machines with various processors, RAM, and Displays installed into the same shell.

great. i just bought it. will update the result here when it's arrived and fitted. thanks for all the help

[beshad]

I'd give you a 95% chance you can drop that Mobo right in and it will fire up. IBM really produced one machine and labeled under the 750/755/360 machines with various processors, RAM, and Displays installed into the same shell.

my replacement motherboard arrived yesterday and you were right, its a perfect fit. however i am having issues firing it up. pls see video below:

https://youtu.be/ySp7SW83T7w

any idea why it would behave like this? here is another video i made using the older motherboard for comparison (https://youtu.be/Yg41H5x6ZQ4)

below are also couple photos of the old and the new motherboard (newer one is the one with CMOS battery attached)

https://ibb.co/FJhywbM

here is all the original logic-board components. I was thinking maybe the power management board is faulty but that does give power to the old MB and HDD so that's working, but the newer eBay motherboard does nothing. i am very puzzled!

https://ibb.co/mhTQDR3

cheers

[Thinkpad4by3]

What is the thing you are pushing with tweezers?

[beshad]

It's a red push to reset button at the back. I guess it's supposed to hard reset the laptop.

[fultontech]

I had a 750Cs that the power board that was melted at the adapter connector. I replaced it, but it wouldn't power on so I put it away as a parts unit. I had moved it around the room and into a storage unit where it sat for a couple of years. A few year later I was doing inventory and was checking all the systems I had, and it powered up and booted.
I am guessing that if I had just reseated the cards at the time and not just jostled it around, it would have powered up when I was originally working on it.

Try taking it apart again. The connections just might be oxidized on the part you replaced.

[beshad]

yes did that and still no luck. cleaned all the connections with Isopropyl alcohol and re-set them. the power supply board seems to be working (https://ibb.co/s96Rj79) as it also passes power to battery connectors so MB should be getting its fair of power coming in too. need bit more google around. why technical things don't always work right out of the box as they should lol

[beshad]

i managed to get hold off an external VGA monitor and surprise surprise my old original motherboard (the one with two beeps) booted right up into BIOS i did get errors 161, 163, 301 and 8611. please correct me if I am wrong but the first two are CMOS battery (time/date) related, and the last two are keyboard codes, am i right?

CMOS battery is new however i have not set the time & date and also i booted up the board without keyboard attached to it so my guess is all these 4 codes should clear after i reassembled the case with all the components.

I believe now i have probably pinpointed the problem to display or the graphic card (https://ibb.co/v47w5P7), and the fact that we know the two beeps are motherboard related (motherboard doesn't know if LCD is lighting up or not, does it?!) i am suggesting likely the small graphic card board which mounts on the motherboard and screen flexes connect to it could be faulty? what do you guys think?

how do i find out if the actual LCD is faulty or display module needs replacing?

also what is confusing for me is that i still get the two error beeps WITHOUT display adapter connected, but i do not get any beeps again WITHOUT display adapter but this time with the external monitor connected!

[beshad]

an update on this: using the external monitor i managed to clear all the bios errors and boot successfully into the OS. so that leaves me with a faulty 750c display unit. i have fully disassembled the monitor (the backlit is completely separate part from the LCD itself!!!) now the task of finding a replacement lcd (or backlit) or something else inside the display is faulty, there are several circuits inside there. maybe someone here has a non-working 750c that i can salvage the screen from.

[goldeneagle]

I'm being lazy and not reading the entire thread (though I know I should). OP mentioned the CMOS battery. The battery from a 380ED is NOT compatible. 750/755 and 355/360 machines all used a Vl2020, which is a rechargeable coin cell. You could theoretically do some soldering and spot welding, but Fedco Electronics makes a VL2020 replacement for these machines. I made the same mistake when I was younger. So don't use a non-rechargeable battery - use the correct one.

Also, how many beeps is it making specifically? The HMM lists different beep codes and their meanings.

I recently got lucky and got a 750P for cheap with a bad system board. Also picked up a 750 with a sorta bad LCD. Swapped system boards, and have a working system for my collection. Now I just need to get some Pen-enabled OSes.

Search up IC DRAM. Almost every single laptop from 1992 to 1996 used it. You should be able to pick up a 8-16MB module for pennies on the dollar. Check the MTM because there are 60 and 70ns modules and the 750C should be flexible but just check to see what it can take. Later I can check my 755CDV which should take the same cards.

Sorry to break it to you, but the 16MB cards aren't that cheap. Sure, pennies on the dollar or tenths of a penny on the dollar compared to 1995 prices, but not that cheap. 32MB cards seem to be in the $45-70 range. I've lucked out and got a few super cheap - all my machines have 16MB or (mostly) 32MB.

Keep in mind that there is a difference between the KTM-TP750 (or equiv) and KTM-TP360 (or equiv). The former is parity, the latter is non-parity. 75x class machines will NOT work with non-parity memory. Period.

Usually there is also built-in memory. At least my 755CD and 360CE have it. So you can try to boot up without this card. But there is not much hope that this will help.
As the HMM clearly states that two beeps are an indication of failed mobo.

4MB on all except that 755CE and 755CX, which I believe have 8MB.

[goldeneagle]

an update on this: using the external monitor i managed to clear all the bios errors and boot successfully into the OS. so that leaves me with a faulty 750c display unit. i have fully disassembled the monitor (the backlit is completely separate part from the LCD itself!!!) now the task of finding a replacement lcd (or backlit) or something else inside the display is faulty, there are several circuits inside there. maybe someone here has a non-working 750c that i can salvage the screen from.

I wish you luck. I don't believe the LCD is compatible with the 755C. I got lucky again, and scored a 750C in great condition a few months back, to add to my collection. They're VERY rare on ebay. Heck, 750 in general is rare.

[beshad]

an update on this: using the external monitor i managed to clear all the bios errors and boot successfully into the OS. so that leaves me with a faulty 750c display unit. i have fully disassembled the monitor (the backlit is completely separate part from the LCD itself!!!) now the task of finding a replacement lcd (or backlit) or something else inside the display is faulty, there are several circuits inside there. maybe someone here has a non-working 750c that i can salvage the screen from.

I wish you luck. I don't believe the LCD is compatible with the 755C. I got lucky again, and scored a 750C in great condition a few months back, to add to my collection. They're VERY rare on ebay. Heck, 750 in general is rare.

thanks. i just found out LCD is fine and the backlit is faulty (https://ibb.co/rGywt7T). this 750C was (and is, although fully disassembled and in pieces now!) in mint condition hence me going through all these to rebuild it. having lots of fun doing the troubleshooting and learnt a lot which i am sure comes handy in my my next project. this is the part which i should hunt down now https://ibb.co/bL7n6RK. hopefully there are compatible backlights available as i don't have much hope finding exact 750c replacement part. as you said they are rare.

[beshad]

hi guys,

back on this repair job again after few weeks break. please see the complete test set up below:

https://ibb.co/vwmDpgx (motherboard->display card->inverter board->backlit board (whats this called anyway?!)->the CCFL light)

there is about 5v output at display board and inverter input, then output from inverter is 15v and the CCFL board is also receiving this 15v but when i test the connection right where backlight is connecting to this board there is nothing!

https://ibb.co/FHYvQdx and https://ibb.co/dGb2MpR

does this mean the inverter (one with two sliders) is fine( there are four wires, only the two outer ones, orange and blue show 15v, moving sliders doesn't make any difference in voltage)? when i was wiggling the CCFL connection i managed to get it flicker couple times after each boot, which was quite exciting and i thought it has to be a loose connection but now it is not even flickering anymore. didn't continue cleaning up the connection as i thought whats the point if there is no voltage at it.

any idea which of these three parts is faulty?

cheers

[Thinkpad4by3]

Check it right at the CCFL tube with your multimeter at the maximum voltage(not sure if AC or DC, might be AC) setting. It should be around 800-1200V on the output. Also if you can find another CCFL tube and hook it up to the inverter, see if that works too.