>I usually do not charge the batteries with the notebook but with a
>better external charger, so I think replacing the batteries with NiMH
>1200  or 1700 mAh-types is possible (for around 1 Euro per cell = 6
>Euro).

Err ... careful with that. 
The "3/4" NiCd cells used in the batteries are about 5 Euro (10 DM) at
www.reichelt.de ... and the battery includes a thermal shut-off sensor. NiCds have a different behaviour after about 90% charge to NiMh, where the charge current rises dramatically after 90% - and the sensor may turn out to be oddly placed and react too slow. NiCds grow hot - NiMhs go low resistance (before getting *really* hot). The U/I characteristics of both differ quite a bit - and the charger circuit is already insuffient for charging NiCd. 

With using NiMh and inappropriate charging curve you may run into quite a bit of real trouble ....
 

Peter said:
The N33 has a main power switch at the rear (above the large bus-connector) - and one on the front. If both are "on" and the machine still don't come up ... well ... then the notorious microfuse in the DC-DC converter is most likely blown.

I have fixed several of these machines already ... the last one reached me in a package ... :-)

The repair itself is not *that* difficult (about level 3 - on a scale from 0
[dead easy] to 6 [almost impossible]) - but the disassembly of the machine and finding the darned thing might take some time. 

Try running it from the mains with the battery out - I have seen several
N33/N51s that appear dead until the battery is taken out. It may indeed
be the fuse problem, as Peter describes.

The N33 portable has some problems with the internal DC/DC converter.
It has two small cylindric fuses. If either one is blown the system either
- run fine on batteries but not on AC and the battery is not charged
- or run fine on AC but not on battery.

To remove and replace these fuses the system needs to be fully disassembled and the fuse(s) unsoldered. This is fairly tricky if you do it the first time. I did it maybe several dozen times back then. The N33 had been part of a portable solution for insurance sales people over here in Germany (along with a portable printer installed in a suitcase) and the machines returned to our repaircenter like boomerangs ... :-)
 

Instructions on replacing the fuse of an IBM N33SX notebook (8533-G15).
Please note that this should only be done if you feel confident in 
a) precise soldering
b) takeing apart electronics and reassembling them without ruining them.

Prerequesites:
a) You have been able to find a replacement fuse (the one(s) we are talking about can be seen when the notebook is opened without the need of taking out the system board - they are two round black plastic fuses about 6 mm in diameter right next to the power supply input (i.e. where you plug in the power supply). The fuse that lets the notebook run on batteries only, but not AC is the one closest to the housing of the notebook, rated 125V, 3,15A. I successfully replaced it with a Wichmann TR3 type fuse, which seems to be the only similar one available around here. I don't know the rating of the other one since I didn't replace that and the labelling can't be read without desoldering it - maybe someone else knows.)
b) You have taken care against static electricity, a grounded wristband
would be best.
c) You have managed to open the housing of the notebook - I leave that
one to you. (If you can't take it apart don't read any further, you might hurt yourself... ;-)
d) You have paper and pencil ready. You need to take notes of what
connectors you opened and where the screws and parts came from. Label
every screw that you take out - there's a big variety of them and if you don't mark them it's going to be hard to put them back to where they came from... Also, take note in what order the screw fixes the parts of the notebook - sometimes the screw will hold a spring, a washer, a metal shield, a metal carried and the PCB at the same time... (You might not do recommendation d) if you are an experienced notebook technician but I prefer to ;-)

Today's ASCII art - this is the notebook seen upside down with the cover
open. It is not an exact scale...
Note:
X means "Screw to be unscrewed" (numbered)
O means "Connector to be unplugged" (Multiple "O"s in a row/line mean a
longer connector...)
! means "look at the notes below"

+-------+--------------------+----------------------------------+
|     4X|       RAM BANK     |                                  |
|  +----+--------------------+                                  |
|  |BIOS|            X3      |          BATTERY      OO         |
|  +----+      +-----------+O|                                  |
|              |           |O|                                  |
| O            |           |O|                                  |
|              |           |O+----------------------------------+
|  O           |           |  X5                              X6|
|  O           |           |                                    |
|  O           |    HDD    |                            +-------+
| O            |           |                            |       |
| O           O|           |                            |       |
|  OO         O|           |                            | FLOPPY|
| FUSES  OO   O|           |                            | CONN  |
|        OOOO2X+-----------+X1                          | PCB   | +-!---------------------+----+                          |       |
|                       | X12|                          |       |
|X11                    |MDM O                          |       |
|       MODEM           |CONNO                          |       |
X10  X9                 | X13|                          | OO  7X|
+-----------------------+----+--X8----------------------+-------+

How to do it:
Unplug connectors whenever possible. It's probably smart to unplug the
connectors to the backup batteries first (the ones of the above
connectors that are two-wire black-and-red connectors - pull these
first...)
Screws 1,2,3 hold the HDD. Unplug the HDD cables too (be careful) and
take out the HDD - nice opportunity to upgrade it, too... ;-)
Screws 4,5,6,12 hold the system board, 5,6 hold the metal lineing of the
battery compartment
Screw 7 hold the floppy disc connector PCB
Screw 8 screws the serial connector assembly to the housing - it's
between the PS/2-Conn and the BUS-Conn
Screws 9,11 hold the modem lining
Screw 10 holds the cover that is in front of the modem space
Screws 12,13 hold the modem connector PCB

Assembly is reverse of Disassembly. There is a "!" in the above drawing.
Take care when reassembling here: At the outer end of this metal there
are two metal noses that hold the system board in place - the board goes
in between the two noses - if you misassemble here, it might damage you
system board.

That should do the trick. Please note that I do not take any responsibility for bad repair attempts. I do not guarantee that the above information is correct.

Have fun repairing your notebook, Norman

PS: If anyone has the possibly and is interested in putting these information on a website, I'd photos too... ;-) PPS: All in all I wish IBM would build an up to date notebook exactly like the N33SX - small, lightweigth and powerful, nice keyboard, nice finish - the Thinkpads et al. are good, but...

-- 
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* Norman Weiss * http://www.normanweiss.de   *
*              * newsreply(at)normanweiss.de *
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