Successful upgrade of 3B1 from 1MB to 2MB

[andrew.d.hay]

In article <810 at ttrde.UUCP> pfales at ttrde.UUCP (Peter Fales) writes:
>After much trial and tribulation, I finally managed to get my UNIX-PC upgraded
>from 1MB to 2MB on the system board.  I thought others might be
>interested in hearing the procedure.

thanks very much, peter.  a second Mb had been languishing in my
machine unutilized, because i couldn't figure out how to access it.

however, i had to do quite a bit of detective work myself.
my motherboard isn't like peter's: it has no jumpers above JR4 and no
r145.  furthermore the schematics (fortunately [?] i have the tech.
reference manual) are inconsistent, and even inaccurate!

they show the 1Mb board as having JR6 and JR10 -- which mine doesn't
have -- and the 2Mb board doesn't have any of these jumpers, but shows
the same signal configuration as the 1Mb board!

on my board, 25f pin 15 was lifted, and jumpered over to 13b pin 11.
6j pin 3 was jumpered to 6k pin 1.  jr1 was in the e1-e2 position.
by scrutinizing all 3 schematic sets (.5M, 1M, & 2M), i deduced that
these corresponded to jr10, r145 (6k:1 is pulled up), and jr8.

25f:15 is la2, so it was obvious that i had to remove the 6j:3 - 6k:1
jumper and re-establish 25f:13 - 6j:3.  a continuity check showed that
the pad under 25f:15 still went to 6j:3, so i hooked it to its pin
with a short piece of wire-wrap wire, soldered.

i tried booting.  my system still showed 1Mb.  what was wrong?
where i hadn't followed peter's instructions, my board mached the 2Mb
schematics.  scrutinize the schematics again...

the .5M set showed 13b:11 (and 13b:2,5,14) going to ma20 instead of
la2.  my first mod was basically removing a factory hack; was this one
too?  ma20 is available on 25f:8, so i pulled the long jumper off
25f:15 and tacked it to 25f:8.  bingo!! 2Mb on bootup!

so i cast around for a neat way to make it permanent.  ma20 also goes
to 19c:14, which is right alongside the jumper, so i cut the jumper
about 1.5" long, strip the end, and solder it to 19c:14.

it still works!

now i go on to bigger and better things...

-- 
Andrew Hay		+------------------------------------------------------+
Apprentice Polymath	| Yes, the wages of sin ARE death, but after they take |
AT&T-BL Ward Hill MA	| taxes out, it's kind of a tired feeling really       |
mvuxq.att.com!adh	+------------------------------------------------------+