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Bizarre sulfur damaged IBM PC XT


Veteran Member
Feb 16, 2012
New Zealand
I just purchased it for the case, so I'm not fussed about it's dead motherboard, but thought it was interesting.

So the machine came with:
- IBM Model M from 1986, with black XT cable, and no LEDs (so sold with a late model XT).
- Newer BIOS fitted with IBM stickers, that you'd normally see on 256-640KB motherboards.
- Original IBM half height mounting hardware and black half height floppy drive with the "*".
- Bank 0 and Bank 1 are fitted with 41256 chips.

So you'd think it was a 256-640KB XT. But the motherboard says "64-256KB" and it's been marked out with a pen.

Bank 0 chips look like they've been in there a very long time (judging by amount of corrosion compared to other parts) and have 1986 date codes. The chips in Bank 1 are clearly younger, as is the AST card and HDD upgrade (hard drive is dated 1991).

Most likely this is just someone slapping parts together many moons ago. But I wondered if it was possible that by 1986/1987 that IBM Australia had left over 64-256KB boards, and tweaked them to use the newer chips?

I can't power the thing up. It's dead Jim.
Does make a clock signal, but doesn't generate RAS (which is my "should I bother" test).


Space Commander
Jan 27, 2014
I've got some carbon (graphite) contaminated ones... But not sulphur. Sulphur also spues from geysers.

Anonymous Coward

Veteran Member
Aug 11, 2004
Shandong, China
I'm not sure that the 256-640k boards are actually any different than the 64k-256k ones. At least the ones I have seen appear to have a white sticker covering "64-256k". Perhaps they ran out of stickers for yours.


Veteran Member
Feb 16, 2012
New Zealand
Cheers for the feedback guys.

Stone - I didn't know the mod was quite that simple, I'll look it up and see what's on the board.

As for the sulfur damage, (I should've asked the owner) it was originally owned by a company that operated out of an area that was near volcanic activity (you can smell it in the air if you visit). However it was sent to me from a town that was very far away and in a warmer part of the country too - so Orchid supplies seems plausible too.

I was just told it was sulfur damaged and the corrosion certainly is not the normal gig.

I tried to get the 3.5" Kalok drive spinning, had to remove a shorted tantalum, but the motor wasn't even trying. Not a huge loss there though.


10k Member
Nov 3, 2009
South Jersey, USA
[COLOR=#000000]  This document describes how to install 640k of RAM on the system board[/COLOR]in the IBM XT and the IBM portable (which uses the same mother board).
Parts list:
        Quantity    Description
           18       256k by 1 Dynamic RAM chips (any of the following)
                        Manufacturer        Part number
                        Fujitsu             MB81256-15
                        Hitachi             HM50257-15
                        Mitsubishi          M5M4256-15
                        NEC                 uPD41256-15
                        OKI                 MSM41256-15
                        TI                  TMS4256-15
                        Toshiba             TMM41256-15
            1       74LS158 Decoder/multiplexer Integrated circuit
IBM XT Instructions:
1.  Turn off the system unit, and disconnect the power cables, monitor and
    keyboard cables and any cables that may be connected to expansion boards.
    Remove the monitor and the keyboard.  Place the system unit in a convenient
    work area.
2.  Take off the cover from the system unit by removing the 5 screws on the
    back (4 corners and top center), slide the cover forward and tip up to
    remove completely.
3.  Take out any boards installed in the expansion slots by removing the hold
    down screw at the rear of the chassis, and pulling the card straight up.
4.  The floppy disk drives will have to be removed to gain access to portions
    of the mother board.  This is done by removing the screws on the left side
    of the drive(s).  Gently slide the drive(s) out of the unit far enough
    to get at the cables plugged into the back of them.  Making note of where
    each cable goes, remove the data and power cables by gently pulling them
    away from the drive.  When the cable are off, remove the drive(s) from the
    chassis and set them aside.
5.  Refer to figure 1 and locate the jumper block labeled E2 on the mother
    board.  It is located near the edge of the board near the power supply.
6.  A jumper has to be installed between pads 1 and 2 on E2.  This can be
    done without removing the mother board using a short piece of wire.  Hold
    the wire with a pair of needle nose pliers and heat up one of the pads
    with a soldering iron.  When the solder melts, push the wire into the pad
    and remove the soldering iron.  Do the same thing with the other end of
    the wire and pad.
7.  Refer to figure 1 and locate the IC socket labeled U84 on the mother
    board.  This will be an empty socket near the front of the board,
    underneath where the floppies were mounted.  Install the 74LS158 chip in
    this socket making sure pin 1 (marked with a dot or notch) is pointing
    away from the front panel.
8.  Remove the 64k RAM chips in the rows labeled BANK 0 and BANK 1 (9 in each
    bank) on the mother board using an IC puller.  If you currently only have
    128k of memory on the mother board (BANK's 2 and 3 empty) you can move
    these chips to those banks.  Be careful not to damage the pins when
    removing them (you can sell them to a friend whose machine has amnesia).
9.  Install the 256k RAM chips in the now empty sockets of BANK 0 and BANK 1
    making sure they are installed correctly with pin 1 pointing away from
    the front panel.  You should now have 2 banks (0 and 1) of 256k RAM chips,
    and 2 banks (2 and 3) of 64k RAM chips, giving you a total of 640k.
10. Refer to figure 1 and locate the switch block, SW1 on the mother board.
    Make sure that switch positions 3 and 4 are in the OFF position.
11. Re-install the floppy drive(s) by sliding them into the front panel about
    half way and reconnect the data and power cables in the same locations
    they came off of.  Push the drive(s) the rest of the way in and anchor
    them with the screws removed earlier.
12. Re-install the your expansion boards (minus any memory boards that used
    to be in the system) in the reverse order of when you took them out.
13. Put the cover back on, re-connect the cables and install your monitor and
    |                                            |                         |
    |         EXPANSION                          |                         |
    |           SLOTS                            |                         |
    |                                            |                         |
    |                                            |                         |
    |---------------------------------------------                         |
    |                                                                      |
    | ------------------------------------                                 |
    | |             BANK 0               |                            ---- |
    | ------------------------------------                          E |12| |
    | ------------------------------------                          2 |34| |
    | |             BANK 1               |               ---          ---- |
    | ------------------------------------               |S|               |
    | ------------------------------------               |W|               |
    | |             BANK 2               |               |1|               |
    | ------------------------------------               ---               |
    | ------------------------------------                                 |
    | |             BANK 3               |  ---                            |
    | ------------------------------------  |U|                            |
    |                                       |8|                            |
    |                                       |4|                            |
    |                                       ---                            |
                                   Figure 1 [COLOR=#000000]                           (XT/Portable motherboard)[/COLOR]


Veteran Member
Feb 16, 2012
New Zealand
Thanks Stone!

U84 is populated with the 158 IC, and E2 has a removable jumper.
Comparing it to a spare 64-256KB board I have, it also has smaller font sizes, and looks the same as my 256-640KB board in another XT.

I guess the sticker just fell off? I wonder why IBM updated the silk screening, but left 64-256KB on it when (with the jumper) it was really 64KB to 640KB ?

So yeah, would've been a 512KB XT with two banks free.


Veteran Member
Jul 16, 2009
I'm only guessing that IBM had an excess of early boards, The boards i have that have been upgraded to 640k and have the sticker all look like they were done at the factory, They have the removable jumper and the soldering matches the rest of the board. I can see on a couple of boards i have that were upgraded to 640k, Have no sticker, A wire jumper soldered across E2 or a removable jumper and chicken sh&t soldering.