If you are like me, you soon learned that there are several great
programs out there that would run on the PCJr---if it only had more memory.
If you were lucky enough to get a Microsoft Booster before they became extinct,
you can, with about $50 in parts and 3 hours time, soon have a 640K system.
This change ingeniously makes use of some unused circuits on the existing
chips on the board. The board was not designed to address 512K but with minor
modifications, works great. The board rework is broken up into several steps.
This gives you checkpoints to test existing work, or allows you to delay furtherwork to alleviate downtime.
First step is to remove the 74LS136. If you have good desoldering tools
you can probably take it out in one piece, but it's easier for me to just clip
the leads near the chip with a small pair of diagonal cutters and remove the
legs one at a time. Then install a socket and plug the new 74LS136 in. Now
turn power on and make sure the booster still operates. If not, re-check your
soldering on the socket. At this point you may want to put everything back
together and continue later. I did this step before I ordered the 256K RAMS.
B R E A K P O I N T
Next, remove the 64K RAMS noting which way they are orientated (Pin 1 goes
toward the top of the board). Bend up pin 1 on the 256K chips and install them
on the card. Strip about 8 inches of wire and connect all the top pin 1's
together. Repeat for the bottom row. Now solder a wire across the middle
connecting the two rows, forming an H pattern.
Connect one end of the 30 ohm resistor to the cross bar of the "H" and the
other end to pin 11 on the 74LS32. It is important that you connect the circuitas described because of timing considerations in the addressing circuits.
Trim the wires closely and secure the resistor with tape or shrink tubing
to prevent shorting problems. Now install the card on the Jr and it should
still function as a 128K board. If not, recheck all your solder connections.
This again is a stopping point if you want to break the job further.
B R E A K P O I N T
If you have good de-soldering tools remove the switches,
otherwise turn them all off and either tape them, glue them or melt them with
your soldering iron so they can't be turned on. If they are accidentally turnedon CIRCUITS WILL BE DAMAGED.
Remove resistors 9 and 10. Now turn the board over and add the
following wires to the board. I reccommend making a check mark by each entry inthe list as you install the wire, and a different color mark when you (or
preferably someone else) checks the wiring.
pin 1 of 74LS136 to Pin 5 of 74LS08
Pin 5 of 74LS08 to Pin 1 of 74LS32
Pin 12 of 74LS136 to Pin 10 of 74LS08
Pin 10 of 74LS08 to Pin 2 of 74LS32
Remove the 74LS136 and bend up pins 1, 2, 3, 4, 11, and 12. Reinstall it
and make sure none of the bent up pins short to the socket.
Add a wire from the bent up pin 4 of the 74LS136 to pin 3 of the 74LS32.
Install the Booster on the Jr and turn power on. The
power on display should count up to 640K. If it doesn't, check all wiring
installed since the last checkpoint. Software will not run at this time.
On the bottom of the board add the following wires:
Pin 13 of 74LS136 to Pin 4 of 74LS08
Pin 13 of 74LS32 to Pin 8 of 74LS08
Pin 12 of 74LS32 to Pin 6 of 74LS08
The rest of the re-work is on the top of the board. Cut and bend the leads
on the 1K ohm resistor so it will fit between pins 9 and 14 of the 74LS08.
Solder the resistor between these pins being careful not to let the resistor
stick up too high and interfere with the cover.
Add the following wires:
Pin 3 of 74LS136 to Pin 13 of 74LS136
Pin 12 of 74LS136 to Pin 14 of 74LS136
Pin 11 of 74LS136 to Pin 9 of 74LS08 (be careful not to unsolder the resistor)
Pin 2 of 74LS136 to Pin 6 of 4500A
Pin 1 of 74LS136 to Pin 7 of 4500A
Install the Booster on the Jr and power on. It should count up to 640K and
any software you have should work fine. You now can copy disks on 1 pass. I
find it handy to keep two DOS diskettes on hand, one with VDISK configured for
10K and one configured for 360K. The line "DEVICE MEMORY /S:360 /D:64 " in
your CONFIG.SYS file will give you a RAM DISK of 360K and up to 64 file entries.This will let you copy a complete disk to C:, then load and execute at memory
access speeds. By using DOS 3.1 you can assign a second VDISK as drive d: and
really swap files back and forth. I have found that I can run most PC programs
except the direct disk sector addressing copy programs such as Copywrite or
Copiipc.
PARTS LIST:
16 256K RAMS with access timing of 150 ns or faster
