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Unknown capacitor(?) on IBM Thinkpad 770 DCDC board....

S

solidpro

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Joined
May 14, 2020
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45
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UK
Hi Everyone

I was repairing a 770 DC board and there are two electrolytic capacitors on there and for some unknown reason, IBM decided to dab some white paste stuff on them.

So after desoldering the legs, pulling one off pulled apart this small black 'bit' that was underneath it, but I can't tell what it is, to replace it.

It looks exactly like a Tantalum capacitor. There are many others on the board. It's marked S48E which I've googled and does appear to be some kind of solid state capacitor but seems to be any number of values.

This is what we left behind on D33:

U2xHNIB.jpg


And this is what an undamaged one looks like:

xYtZDHA.jpg


I did manage to work out what end was positive end, but have no idea what to replace it with - any ideas?

Thanks
 
My guess is that it is a common 1N5819 SMD diode. Something like these or these. Not sure what the 8E is for - maybe size. Or, I could be completely wrong.
 
Yep. That's a diode. Unless it was knocked off, I'd make sure something else didn't cause it to be "blown away".
 
From what I've heard, the DC boards on the 770 have a bit of a reputation for unreliability going back years. Did this one work before the recapping?
 
From what I've heard, the DC boards on the 770 have a bit of a reputation for unreliability going back years. Did this one work before the recapping?
Click to expand...

Yes, you're right about that. I actually have a box of known-good tested DC boards - but I can't find them! This one had a fuse blown, so I replaced all the capacitors and the fuse and was about to test it when the glue on the very last capacitor pulled the diode off.

I'm still not sure what to replace the diode with... can they be measured?

Thanks
 
When I was big into working on my old Thinkpads I had boxed of parts from spare machines and have no idea what I did with them. One of the best Thinkpad purchases I made was a box of misc. parts that included a ton of OEM stickers for covering screw holes in those old machines.

The 770 series was great, and I have a few, some with working batteries that are probably dead by now.
 
I wouldn't mind having one with the 5:4 screen at some point. You don't find those very often.

Main issue is that they're all covered in rubber that scuffs very easily, and then eventually melts. I'm sure solidpro and yourself have both had to deal with that a lot fixing these.
 
My 701's both have shitty rubber coatings. The 760 and 770's were not bad last I checked. I just clean them off with Windex once in a while.
 
They coated all the 700s in that stuff. It's nasty. One of my main laptops that I use now (dell latitude e7470) is covered in the stuff, and it's just not pleasant. And this one's only 7 years old. I really hope that people have stopped doing that by now. It scratches easy after a few years, by nature of being a coating can wear off, gets oily and dirty very easily, and of course, it melts. I'll actually give IBM credit here - their rubber formula has lasted the test of time far better than the sea of rubber coated products in the 2000s has.
 
The 1280x1024 screen models are options for the 770x and 770z and are hard to find. I tracked a screen down a decade ago but the seller had junked it by the time I seem his advertisement.
 
If it's a different aspect ratio then I've got to be able to spot it in a listing eventually. Just would need to look at a photo of both side by side.
 
13.7" is the 1280x1024 screen while the 1024x768 is 14.1" if that helps (not much of a difference).
 
How many ThinkPads must you have to have
A: box full of 770 DC/DC boards
and
B: not be able to find them?
Click to expand...

This is just the 'work in progress' Thinkpads shelf:

IMG_7068.jpg
 
The rubber that's on 600s, 701s, X20, 530cs, and so on had evolved slightly by the 770, which feels the same but doesn't tend to rot - probably a slightly evolved mixture of chemicals. It does scratch and most of these that appear in the wild now have either been thrown around someone's office or retrieved from a WEEE skip somewhere. The biggest 3 problems with the 770 are related to the solid paint on the palmrest, which wears down to bear metal after years of greasy palms, the hibernation battery under the keyboard kills everything around it and finally the DC board fails.

The biggest cause of the rot, I think is called hydrolysis - "Hydrolysis Reaction. Hydrolysis is a chemical reaction during which polymers (large molecules) break down into monomers (small molecules). During hydrolysis, covalent bonds between monomers break, which allows for the breaking down of polymers. Bonds are broken down using water."

So, basically, humidity. A lot of thinkpads were only available in Asia and Japan in particular suffers terrible humidity for 3 months of the year, causing both a lot of rotting cases and vinegar syndrome LCDs.

Best thing you can do to prevent these earlier models from rotting is don't store them with anything against them and make it's environment dry and mild.
 
solidpro said:
This is just the 'work in progress' Thinkpads shelf:
Click to expand...
Holy cow! That's a lot of ThinkPads. And an L40 SX.

The palmrests are painted on the 770? I thought they were rubber coated too like my 385XD is. That's nice. At least if that wears down, that should in theory be easier to repaint.

My stuff is stored in a basement that according to the dehumidifier is around 50% humidity most of the time. Hopefully that's dry enough! My rubber coatings are are still not sticky in my ThinkPads, for the most part. The 385XD is on its way I think, but still ok for now.
 
Yeah, a few L40SX, a CL57, 3 x 230cs, a 530CS and a lot of unsellable 'perfect' R6x machines!

385XD are plain plastic - no rubber.... I don't think any of the 3xx series was rubber coated. 385XD much rarer than the 380 machines though. Weird, almost-nothing-happened update to the 380 Range.
 
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