How to Speedup Your PCjr
                          By Tony Cooper
                             12/19/88


Here's the step by step procedure to speedup Jr to 8mhz:

1) Remove the soldered in 8284 and install an 18 pin socket.  It is
located just behind the diskette drive next to the crystal.  I used a
solder removal tool bought from Radio Shack for $6.95. It is a modified
soldering iron with hollow tip and a bulb for easy removal of the solder.

2) I inserted a new 8284 and made sure that everything worked okay.

3) I then built my speed-up: a piggy-backed pair of 8284s in a socket:


           LOWER 8284         UPPER 8284
           ----------         ----------
           Pin Number         Pin number
               1 ---------------- 1
               2                  2 NC
               3 ---------------- 3
               4 ---------------- 4
               5 NC               5 TO SOCKET connection for Pin 5
               6 ---------------- 6
               7 ---------------- 7
               8 NC               8 TO SOCKET connection for Pin 8
               9 ---------------- 9
              10 ---------------- 10
              11 ---------------- 11
(Lower 12     12 -----,           12 NOT CONNECTED
 to upper 14) 13      |           13---\/1Kohm/\/--- (to common pin
              14      `---------- 14                  on toggle
              15 ---------------- 15                  switch)
              16                  16--------||-\/500ohm\/-Pin 9
              17                  17-,      XTL           (GRD)
              18 ---------------- 18 |--5pf-||
                                     |
                                     +-\/500ohm\/--Pin 9 (GND)
Except for pins 5 and 8, lower 8284 was connected to board.

Pins 2, 5, 8, 12, 13, 14, 16 and 17 of upper 8284 were bent out.


4) Connect the toggle switch as follows:

          stacked
          8384's & SOCKET
           +--------+             +---+
    grd    | pin 9  |-----------o-|   |
           |        | 1K ohm      |   |           toggle
   spd swch| pin 13 |/\/\/\/\---o-|   |=======    switch
           |        |             |   |
     +5v   | pin 6  |-----------o-|   |
           +--------+             +---+

5) I then plugged the MOD into the socket on the system board and mounted
the toggle switch in a convenient place.

Norton Utility SI gave 3.1  with 24 mhz crystal and a V20
processor running in upper 512k memory of PCjr. SI reports
.8 for stock 12in the PCjr isn't rated at 8mhz. Thats why I switched
note: the 8088 is NOT rated at 8mhz so thats why I switched
to the V20. Be sure and specify an 8mhz V20 when ordering.


Suggestions to others:

1. Order 24, 22 and 20 mhz crystals; use fastest one which
works in your machine.

2. My memory is all 150ns. 200 ns may be ok too but thats as
slow as I would risk it.

3. I used wire wrap to connect upper pins 5 and 8 to the socket
pins and clipped lower pins 5 and 8.  I then attached this(2 8284'
& SOCKET) to the socket I solderedin the motherboard.


Caveats

Please note that on some undetermined number of jrs, a beep may
periodically occur when entering data in high speed mode only.
The severity of the problem differs with each machine.  I have 3
jrs - 1 no problems, the other 2 have periodic beeps - maybe 3 times per
typed page.

Also you may not switch speeds while running.  Lockup may occur.
It is best to switch then reboot the system in the higher/slower
speed. This is due to bouncing of the switch contacts and may be
corrected with the use of a flip-flop type of circuit to toggle
the speeds.  I found the added complexity not worth the effort.

Have fun JRing ...
                                      Tony Cooper