🚀 go-pugleaf

Hi!  I just got done upgrading a 2-chip ROM Mega 2 ST to TOS 1.4 using a 
six-chip PROM set from Atari.  The source of this upgrade was Lyco in 
Woodbridge, Virginia.  If you are interested in upgrading from a two-chip ROM 
set to a six chip set, here's how I did it.

I'd better add a disclaimer at this point.  The following is merely a 
description of something I did to my computer.  How you use this information is 
up to you.
 
With that said...

    First, I removed the motherboard from my Mega.  I also removed the floppy 
drive and the power supply -- it makes the board much easier to handle.  I was 
careful to observe static electricity minimization, since some of these chips 
contain MOSFET devices.

    I noticed that my Mega had no sockets for the four ROM chips that had not 
been installed; not only that, but the holes were all soldered closed!  The 
first thing I had to do was use a solder-sucker and some desoldering braid to 
carefully open the closed holes for U3, U4, U6, and U7.  It took great care not 
to scratch the ultra-fine traces that run around and between the solder pads.

    Next, I installed the sockets.  I inspected each solder joint with a 
magnifying glass to look for solder bridges and cold solder joints at each pad.

    It was then time to move the two jumpers marked 'w2' and 'w3' on the 
motherboard.  At the front of the Mega's motherboard, right next to the MMU 
chip, there are two small jumpers that look exactly like resistors (or at least 
they did in my Mega).  They are marked with a single black band -- perhaps to 
signify zero ohms?  Whatever the case, they are labeled with small white boxes 
on the board's silk print, with holes 1 and 3 marked on each.  The jumpers were 
installed in holes 2 and 3 in both cases.  These two jumpers had to be 
desoldered and moved to holes 1 and 2.  If I'd been careful, I could have just 
desoldered the wire in hole 3 and moved it to hole 1...  Now that I think about 
it.

    Another jumper, which was not already present, had to be installed in a 
nearby box labeled 'w4'.  This box is clearly marked on the motherboard 
inbetween U12 and U24.  I used a lead clipped from a spare resistor, about 1" 
long for this; any fine bit of wire would have done.  It was stiff and so did 
not need to be insulated from surrounding metal.

    I saved the most difficult part for last.  The final step was to remove an 
integrated circuit entirely from the Mega's motherboard.  This chip is labeled 
U12, and its only purpose is to take the ROM signals that would normally 
activate six separate chips, and funnel them (through the use of a three-input 
AND gate) into the two-chip ROM set.  Since I needed to use all six ROM chips, 
this IC had to be removed.  It was difficult for me because the desoldering 
braid did not remove all of the solder, and my solder-sucker couldn't vacuum 
enough of it out.  It took some severe wheedling to finally get this guy loose.

    Of course, I had to remove the existing pair of ROMs and put the new chips
into the proper sockets.  This took a bit of doing in itself, since the ROMs 
are labeled with the proper numbers for the 520 and 1040 STs, but _not_ for
the Mega ST.  I had to match up the chip designations instead.  This is the
table I came up with:

                520 ST    Designation    Mega ST
                ---------------------------------
                  U2         Hi-2          U3
                  U5         Lo-2          U4

                  U3         Hi-1          U6
                  U6         Lo-1          U7

                  U4         Hi-0          U9
                  U7         Lo-0          U10

    After plugging the ROMs -- which are actually EPROMs with labels stuck over 
their glass lenses -- into my Mega, I plugged 'er in and booted.  The system 
works perfectly, or as close as TOS 1.4 gets (pretty close)!  I am using it and 
TEMPUS II to write this file, and will be using it to upload it as well.  The 
improvement in FAT searching is worth the price, by itself!  Other features 
have also proven handy, including the keyboard cold-start.

    Wether you have TOS 1.4 professionally istalled or choose to do it 
yourself, I can heartily recommend the upgrade.

-- 

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