Modification Turns PCjrs Into Tandy 1000s (Almost)

EDITOR'S NOTE: The modification to PCjrs described in this article has
been tested and we found that it enables a PCjr to run a variety of
programs in 16 colors that would only operate in 4 colors on an unmodified jr.Although our tests and contacts with others who have performed this modification indicate that there are no adverse effects, we feel we should warn jr users who plan to do this that we will not be responsible for any problems that might result. This modification requires some experience with a soldering ironand some skill, although it is basically not difficult. If you want to do it, but feel you do not have enough experience, an electronics repair shop should be able to do it for you inexpensively.
The following software will run in 16 colors on a PCjr with this modification: Battle Chess, Flight Simulator 3, Deluxe Paint2, Deskmate Software (Radio Shack), Tetris,Grand Prix Circuit, Rack'em Billiards, Sorceror Lord Leisure Suit Larry II, King's Quest IV, Police Quest II. Many of these programs will run on an unmodified PCjr in 4 colors, or in 16 colors with a patch (See Disk #56 in the Software Store). But if you want to avoid using patches and enjoy the luxury of simply selecting the Tandy 1000 option when software asks you what computer you are using, then this modification may be for you. In general, software that previously showed horizontal black stripes on your PCjr monitor should operate well after this modification.
Theoretically, there should be SOME differences between the performance of a PCjr with this modification and that of a Tandy 1000 because the Tandy 1000 hasa CGA card installed, whereas the PCjr has its own variety of graphics, but wecould find no problems with it after several months of operation by several different people who performed the modification.
Our thanks to Tony Cooper who provided the original instructions for performing this modification.

                                    ЩЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЛ
                                    К                         К
                                    К       14           8    К
                                    К      кФФФФФФФФФФФФФФП   К  74S86 Piggy
                                    К      УП 74S86 on    Г   К  Backed on top
                                    К      Уй 74LS04 ZM33 Г   К  of 74LS04.
     ЩЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭМ      РФФФФФФФФФФФФФФй   К  Pins 7 & 14
     К                                      1 2 3        7    К  attached. All
     К                                      К К ШЭЭЭЭЭЭЭЭЭЭЭЭЭМ  others lifted.
     К                     ЩЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭЭМ ШЭЭЭЭЭЭЭЭЭЭЛ       Pins 1,2,3 of
     К                     К                             К       74S86 are wired
     К   16            9   К     20                11    К       as shown.
     К  кФФФФФФФФФФФФФФФФП К    кФФФФФФФФФФФФФФФФФФФП    К
     К  УП               Г К    УП                  Г    К
     К  Уй   74LS157 ZM31Г К    Уй      74LS273 ZM30Г    К
     К  РФФФФФФФФФФФФФФФФй К    РФФФФФФФФФФФФФФФФФФФй    К
     К   1 2 3 4 5 6 7 8   К     1 2 3 4 5 6 7 8 9 10    К     Double lines are
     ШЭЭЭЭЭЙ К             К                     ЬЭЭЭЭЭЭЭМ     added wires.
           Г ШЭЭЭЭЭЭЭЭЭЭЭЭЭМ                     Г             Single lines are
           РФФФФФФФФФФФФФФФФФ//ФФФФФФФФФФФФФФФФФФй             existed traces.
                             ^
                             РФ cut this trace here


            40                                     21
           кФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФП
           Г                                         Г
           Г                                         Г
           УП                                        Г
           Уй            8255 Chip  ZM22             Г
           Г                                         Г
           Г                                         Г
           РФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФФй
            1                                      20


* NOTE: WHEN PERFORMING THIS OR ANY OTHER WORK ON YOUR COMPUTER'S MOTHERBOARD 
IT IS ABSOLUTELY ESSENTIAL !! THAT YOU TAKE ALL PRECAUTIONS TO AVOID STATIC
ELECTRICITY DESTRUCTION OF YOUR MOTHERBOARD. IF YOU DON'T DO THIS, YOU RISK
FATALLY DAMAGING YOUR MOTHERBOARD. *

Steps:  1. Cut trace from pin 2 74LS157 (ZM31) to pin 9 74LS273 (ZM30)

        2. Piggy Back a 74S86 onto (ZM33) 74LS04.  Pins 7 & 14 are the only
           2 that need to be soldered to the 74LS04.  All other pins on the
           74S86 should be lifted level so as not to touch the 74LS04.
           BE SURE TO PROPERLY ALIGN THE CHIPS PIN TO PIN...
           THAT IS PIN 1 TO 1, etc...

        3. Add the 3 wires shown in the diagram above:
            a) pin 3 of 74LS157 to pin 1 of 74S86
            b) pin 9 of 74LS273 to pin 2 of 74S86
            c) pin 2 of 74LS157 to pin 3 of 74S86

        4. Hint: When adding the wires, solder to the chip legs, not the board
           traces.  It is much easier to add/remove if this technique is used.
        5. Be careful when replace the disk drive. Wires and cables can easily
        be damaged.
        6. The 74S86 is the correct chip to use. Do not try using another type
        of 7486 chip for this modification.
        7. Use a 32K video buffer.  Specify either Pcjrmem /c or /e. If you
        use Jrconfig, use the v32 switch and s1.TANDY MOD PRECAUTIONS

If you aren't VERY careful when performing the PCjr to Tandy 1000 
Modification, you may ruin your systems board or other 
components. Here are suggested precautions:

*  Make sure to unplug all equipment from wall outlets. ALL!

*  Make sure that the installer is grounded before touching the 
PCjr or the Tandy Mod chip. Use a grounding wrist strap.

*  Lift your feet from the floor and keep them up. Example: Put 
them on the rungs of a chair, or kneel on a chair.

*  Keep your fingers off the 64 pin expansion buss pins on the 
right side of the computer.

*  Remove the internal memory board, disk controller card, disk 
drive and power supply. Touch these internal cards only by the 
EDGES.

*  If you have a socketed 8088 chip, remove it and set it aside. 
Remember to replace it when the mod is complete.

*  Use only a low wattage soldering iron - 20 watts maximum.

*  Be sure that you are working in the correct area of the 
motherboard as per the orientation illustration.

*  Be sure to replace everything you removed.
