Member Avatar for Beavit

Hello there, I have a IBM Thinkpad 390E, when i bought it, it came with an AC Adapter and a battery that was completely dead... So i bought a battery and loved it... Well i bought a car adapter for my laptop, and when i was reading all the diffrent voltage settings and such.. i wanted to make 100% sure that i had everything set up properly, so i wouldn't destroy my laptop....
Well i had the 16 V setting right... but when it came to my Polarity setting... there was the setting on the AC Adapter i could go by, or what was right above my power input to the laptop... Only thing was, is there was no indication of how it was sopose to be. - to + or + to -. depending on how you look at it.. Well in both a haste of wanting to test it and having to guess via the laptop .... I plug the thing in with reversed polarity..... !POP! *i unplug it within 1 second* and it begins to lightly smoke.... i almost crapped myself thinking that i just ruined my laptop.. But it turns out, i turned it on via battery power..... everything seemed to work perfect.... i plug it into the wall with my AC adapter, and it worked perfect....

But now my laptop will no longer charge my battery i spent all that money on...
I did a little reading around on the internet and from what i understand... the laptop has fuses and diods to prevent a complete fry of the entire computer in such a case as reversed polarity....

Now that you know the situation. my very serious question is..... Can this be fixed?
Do i have to replace and solder a diod or fuse?

Any information that i can get on this subject would be a great help to me..

Thank you

- Scott Harrington

If you want to reply directly to me, my email address is

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Good news is the ThinkPad likely sacrificed a tiny part in the power path, not the whole board. Given can run from AC but the battery will not charge, the DC-in rail is alive; the fault is almost certainly in the battery-charge path just downstream of the jack.

Quick, safe checks you can do with a DMM:

  • Verify your adapter polarity first. The correct symbol shows a dot linked to + for center-positive; confirm with a meter before any retest. Polarity symbols
  • Remove AC and battery. Open the base and locate the small DC-DC & BATT board (separate FRU on the 390E). The Hardware Maintenance Manual shows removal and the Power System Checkout. ThinkPad 390E Hardware Maintenance Manual (1999)
  • With battery out and AC plugged in, measure voltage on both sides of the input fuse on that board (often marked F1/F2 or PF). If you see adapter voltage on the jack side but 0 V on the system side, that fuse is open.
  • If the fuse is good but the battery still will not charge, check the clamp/protection device near the jack (often a TVS or diode across the input). With power removed, use diode mode: a short in both directions means it failed short and should be replaced, or it will immediately blow the new fuse.
  • Still no charge? The charging path typically uses MOSFETs to steer and protect current; a shorted or open device here will stop charging while the laptop still runs from AC. At that point, replacing the DC-DC & BATT board is usually faster than component-level repair on this model. The HMM has step-by-step FRU procedures. ThinkPad 390E Hardware Maintenance Manual (1999)

Tips:

  • As and noted, practice soldering first and use a current-limited bench supply for first power-up (set around 16 V, 0.5–1 A) to avoid repeat damage.
  • For car adapters with reversible tips, double-check center-positive before plugging in. Polarity symbols

Recommended Answers

All 9 Replies

Member Avatar for TallCool1

I had the 16 V setting right... but when it came to my Polarity setting... there was the setting on the AC Adapter i could go by, or what was right above my power input to the laptop... Only thing was, is there was no indication of how it was sopose to be. - to + or + to -. depending on how you look at it.. Well in both a haste of wanting to test it and having to guess via the laptop .... I plug the thing in with reversed polarity..... !POP! *i unplug it within 1 second* and it begins to lightly smoke.... i almost crapped myself thinking that i just ruined my laptop.. But it turns out, i turned it on via battery power..... everything seemed to work perfect.... i plug it into the wall with my AC adapter, and it worked perfect....

Most IBM laptops, like most modern laptops, have polarity protection. This consists of a diode across the DC input. Where the power goes in, there's a picofuse, a very small fuse that looks like a liitle green resistor. Downstream from that is the diode. Normally, the diode is reverse-biased, an open circuit. In the event of a polarity reversal, the diode becomes forward-biased, an electrical short. This draws sufficient current to blow the fuse, protecting the rest of the circuit.

What you now have is a blown fuse and, possibly, a shorted diode--thought it's possible that the diode survived, just got hot--hence the smoke. You will need to find the PDF service manual on the IBM site (I can't do all your work for you ;)) for disassembly instructions and replace the picofuse, then check the diode. Even the cheapest (~$10 at a hardware store or Radio Shack) volt-ohm-milliammeter (VOM) will do this just fine. You need to find a full-line electronic supplier in your area, and you will have to buy 5 picofuses to get one (but then you will have spares, right?). If you don't know what I mean by full-line supplier, ask your local TV repaiman where he buys his parts locally, or ask a computer repair shop, or ask either one if he will sell you a picofuse of the correct rating for a price.

Of course this presupposes that you are able and willing to do a little mechanical disassembly and soldering to save a ton of money. :)

Member Avatar for jifiii

Most laptops have polarity protection, you probaly just blew a fuse, and that can jsut be replaced. But I am nto exactly sure on where the fuse would be located at.

Member Avatar for NewEra

I got the same problem with my lap tp Packard bell Easy Note E3228.
any help where I can find the PDF service manual or where I can get help to solve this problem .
Any help is great
Thanks for evry body

Member Avatar for newtocustompc

I am sure if you check there site they normal have all the pdf files, go to your manfactures website and enter the machines sort code or something. Also did the laptop come with a cd with all programs on maybe the manuals are on there also. If not you may have to phone them :(

Member Avatar for kc0arf

Hello,

Be aware that any hardware hack will void your warrenty, and if the machine goes up in smoke because you did a bad solder job, you are looking at new parts. This is not for the casual person.

If you insist on doing the hardware hack, you might wish to use one of those spare diodes and make a practice run on a different piece of circuit board. When you go to the store to get your diodes, ask the clerk for a little 2 x 2 or 2 x 4 circuit board, and some small solder. Practice soldering before you even unscrew your laptop.

Christian

Member Avatar for dcc

If you can recognize the basic components on a pc board, know how to trouble shoot the basic components with a DMM, and have some through hole soldering skills you might want to try fixing this. If you don't, I would recomend that you pay someone with the proper skills to make the repair. If you aren't competent in these areas you could make the problem worse.

Member Avatar for NewEra

I disassemble the laptop and I can't see any strange things there , now I charge the battery externaly by pluging it directly with the charger and it charge Ok and I can start the computer Normal every thing seams to work Normak , but again if I plug the Charger to the laptop won't charge !!!!!!
Battry --Ok
Charger --ok
the computer work -- ok
But I can't Charge the battry throwe the Normal way !!!!!?????????
Any help there Pls.
Thanks

Member Avatar for dcc

If you have a VOM, remove the battery and see if you have any output to the leads.

Member Avatar for jwenting

Seems the charging circuit's fried, not the polarity protection on the mains inlet.
Either the charging circuit has its own protection (likely) or it's a goner and needs replacing (which in a laptop could mean replacing the mobo depending on the design).

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