karsten@sunb1.cs.uiuc.edu (03/29/90)
The Mylex 386 Board (Rev 2B) CAN be upgraded from 1MB (using 256K bit chips)
to 4MB (using 1M bit chips) without sending it back to Mylex if you know
how to solder dense printed circuit boards and are willing to assume the
risks of your own mistakes. If not, I suggest you send the board back to
Mylex (along with $100+ and 36 1M chips) and let them do it. In any case,
read through the entire set of directions and make sure you understand them
completely, as you can COMPLETELY screw up your board if you goof up.
(Explanatory information: although the board was obviously supposed to be
designed to allow user upgrading from 256K to 1M chips, the "Changes
and Corrections" sheet in the back of the manual states that "To upgrade
from 1MB to 4MB, the board has to be sent back to vendor.")
Are you still paying attention? Good. You'll need:
Sharp, pointed knife (X-Acto, for instance)
Low wattage, sharp pointed soldering iron
Thin solder (standard rosin core type)
Aluminum foil (recommended)
Thin bare wire (I used 24 gauge)
Chip puller (not necessary, but makes removing old chips easier)
Continuity checker (or Ohmmeter/Multimeter)
Desoldering wick (not for all boards; see step 7)
36 1MBx1 DRAM's, 120ns or better
1. Be careful about static electricity. I suggest you work with the board
on aluminum foil.
2. Remove the motherboard from your system. Carefully label all of the
connections to the board with location and orientation before removing
them so you can easily reconnect them when reassembling. Locate the 36
256K DRAMs (they are in the sockets labelled U96-99, 101-104, 106-109,
112-115, 118-125, 128-131, 133-136, 138-141). Remove them. Unless you
really mangle them they can be used elsewhere/sold. The capacitors
near the RAM sockets stick up beyond the sockets, so you should
place something between the sockets and your table so that when
you are placing pressure on the board (while cutting in step 3)
you are not mechanically stressing them.
3. On the bottom (non-component) side of the motherboard there is a
pc board trace ("wire") between pins 8 and 10 for each of the above
sockets. (If you don't remember, the chip "notch" was away from
the card edge slots.) You need to cut this trace with the knife
so that there is no electrical connection between the two. All of
the chips are set up this way, and these pins are all hooked together
in some inner layer of the board, so you cannot check them until you
have cut them all. BE CAREFUL! Some of the traces are within a
millimeter or two of others that you must not accidently damage.
I found that the best way is to make a "V" into the trace - cutting
in at two different angles, like one would chop wood with an axe.
You don't want to go too deep; this is a multi-layer board. Take your
time here - this is where you can do unrepairable damage.
4. Test between pin 8 and pin 10 of any of the RAM sockets. If there is
continuity, you didn't make a clean break for at least one socket.
Go back to step 3 and fix it. It shouldn't be necessary to test all
of the sockets, and testing them individually won't localize the
error.
5. For each of the above (RAM) sockets, connect pins 19 and 20. I found
that the best way is to melt a solder "blob" on these pins, and then
press the end bit of a wire against these while holding the soldering
iron tip against both pins. When the solder melts (it may not look
like a great connection - you can test it) snip off the extra, and
repeat.
6. Test between pin 19 and 20 (test the socket on the "top" of the board)
for continuity for EACH socket. If there's no continuity, fix it.
7. Locate jumper J6 (it is right next to the RAM chips). It can
apparently take two forms: solder strap or jumper. Observe
the pads on the bottom - one is a square. This is pin 1. The
jumper forms a 13x3 array of pads/jumpers. The numbering system
goes down columns (the column is 3 deep) and then continues to the
next column. From the bottom (non-component) side, it is:
1 4 7 10 13 16 19 22 25 28 31 34 37
2 5 8 11 14 17 20 23 26 29 32 35 38
3 6 9 12 15 18 21 24 27 30 33 36 39
Connections are made down a column. For instance, in the 1-2-3
column, the possible connections are 1-2 and 2-3. You will need
to make the connection exactly the OPPOSITE of what they are.
If there are solder straps, you'll have to unsolder them (solder
wick will help if you've got it). If this is really a set of
jumpers, this is a trivial step. The CORRECT hookups for 4Megs
are: 1-2, 4-5, 7-8, 10-11, 13-14, 16-17, 19-20, 22-23, 25-26,
28-29, 31-32, 35-36, 37-38. There is only one connection per
column of three. Some jumper versions did not have a jumper in
the 28-29-30 column before; if this is so, you'll have to get
a new jumper or solder the board directly. If yours is the solder
strap version you might check the connections with a meter again.
8. Install the 36 1Megx1bit DRAMs in the sockets with pin 1 of the chip
going into pin 2 of the socket. The sockets accept 20 pin chips but
the RAMs have 18 pins, so orient the chips like the originals
(notch/dot AWAY from the card edge slots) and mount them toward
the card edge slot end of the RAM sockets. Helpful (?) picture:
oooooooooo o=socket
||||||||| |=chip pin
########. #=chip body
######### .=chip notch/dot
|||||||||
oooooooooo ------------> away from card edge slots
9. Change the 8-position DIPswitch so that switches 1-4 are OFF
to allocate memory as 000000-09FFFF and 100000-45FFFF (other
configurations are available; see the manual).
10. Place the motherboard back in the case and reassemble the whole mess.
When you turn it on the memory check should go through all 4 Megs.
(Since you probably removed any battery backup you will probably
have to do Setup again). If you have a memory check program you
might want to fire it up. Congratulations! You have successfully
upgraded your motherboard! Now about that 387 chip... ;-)
============================================================================
"Follow the above directions at your own risk. I'm not there holding
your hand, so I can't be responsible for what you mess up with it!"
--karsten kelly (karsten@cs.uiuc.edu)