This is an except from newgroups posting regarding an old – but very nice – IBM Notebook. Type N33SX – a 386SX16 notebook which is quite small and still fine for logging (e.g. data logging purposes.)

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.

The manual says that you may not operate the notebook without the battery in place (a dead battery is the same) and that warning is to be taken seriously – one of the fuses has a high risk of blowing if the battery isn’t „ready“.

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. Even if you know what you’re doing, it still takes a lot of time (between one and two hours, unless you are a experienced notebook technician). If you feel confident taking apart the notebook without ruining it and you can solder a fuse (and get a replacement fuse, which might take you some time!), follow the instructions below.

 

Instructions on how to replace the fuse of an IBM N33SX notebook (to be exact a 8533-G15).
Again, Ppease 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 lining of the battery compartment
Screw 7 holds 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