Exidy Targ & Spectar
Color Adapter Board Troubleshooting
Differences between Targ & Spectar Color Adapter Boards
The Video PCB used for Targ and Spectar are the same exact board with additional modifications made to the Spectar board to change a few colors (jumpers at 4A & 4C) and some have been changed to add composite blanking (wire patch on bottom of board from pin 2 of J1 at 1B to pin 7 of 74LS151 at 5C, trace cut between pin 7 & 8 at 5C [photo]).
Targ color adapter boards can be used with a Spectar PCB, but the colors will be wrong- most noticeably the maze walls will be yellow instead of green.
Spectar color adapter boards that have the wire patch to add composite blanking will not display blue when used with Targ PCBs, since Targ boards don't send anything to this output and this lack of signal affects the blue output of the 74LS151 at 5C.
Spectar color adapter boards that don't have this patch when hooked up to a Targ PCB display Spectar colors- most noticeably the maze walls will be green instead of yellow.
Summary of how the board works:
The Spectar manual contains an excellent description of each of the game's PCBs. Here's a summary of my understanding of how the color adapter board functions:
1. The video signals enter the PCB from the motherboard via a 14-pin DIP ribbon cable to the socket at 1B.
2. The hex value of the character to be displayed is divided into 4 equal segments in the 74LS74 at 1A.
3. The decoder 74LS139 at 2A merges the composite video signal with the output from 1A.
4. The 74LS148 prioritizes which element is to be displayed at that screen position: Moving Object 1, Moving Object 2, or the character from character RAM.
5. The 3 74LS151 color multiplexers at 5A, 5B, 5C
output each separate RGB color from the color jumpers based on the priority code.
6. The color info goes to the 74LS86 at 6A that allows for output video to be negative true if a jumper at 2C is cut.
7. The sync info
goes to the 74LS86 at 1C that allows for sync video to be negative true if a jumper at 2C is cut.
8. Final video is output to the monitor from the 74LS174 at 6B.
Color is determined by jumpers at 4A (Red), 4B (Green), 4C (Blue):
 |
 |
Targ |
Spectar |
|||||||||
| Jumper Line | Red | Green | Blue |
Resulting Color | Jumper Line | Red | Green | Blue |
Resulting Color | |
| 0 - Background | - | - | Blue | 0 - Background | - | - | Blue | |||
| 1 - Character RAM ($00 - $3F) | - | - | Red | 1 - Character RAM ($00 - $3F) |
- | - | Red | |||
| 2 - Character RAM ($40 - $7F) |
- | Light Blue | 2 - Character RAM ($40 - $7F) |
- | Yellow | |||||
| 3 - Character RAM ($80 - $BF) |
- | Yellow | 3 - Character RAM ($80 - $BF) |
- | - | Green | ||||
| 4 - Character RAM ($C0 - $FF) | - | Light Blue | 4 - Character RAM ($C0 - $FF) |
- | Yellow | |||||
| 5 -? | - | Light Blue | 5 - ? | - | Light Blue | |||||
| 6 - Moving Object 2 (Player's Missile) | White | 6 - Moving Object 2 (Player's Missile) | White | |||||||
| 7 - Moving Object 1 (Player's Ship) | - | - | Green | 7 - Moving Object 1 (Player's Ship) | - | Yellow | ||||
Using the Targ and Spectar Test Roms
The Targ & Spectar Test ROM's video tests send known signals to the color adapter board which can be used to much more easily isloate problems.
LOGIC PROBE TESTS:
Screen 1: Spectar Test ROM 74LS139 (2A) 74LS148 (2B) 74LS151 74LS86 (6A) 74LS174 (6B) |
Screen 2: Spectar Test ROM 74LS139 (2A) 74LS148 (2B) 74LS151 74LS86 (6A) 74LS174 (6B) |
Spectar Test ROM 74LS139 (2A) 74LS148 (2B) 74LS151 74LS86 (6A) 74LS174 (6B) |
Spectar Test ROM 74LS139 (2A) 74LS148 (2B) 74LS151 74LS86 (6A) 74LS174 (6B) |
Targ & Spectar Color Adapter PCB Repair Log:
Spectar Color Adapter PCB: Board not displaying the green playfield. Cycling through the video tests in the Spectar Test ROM showed that Screen 3 displayed nothing except the blue background. With Screen 3 displayed I ran though the tests with a logic probe. I found that the inputs to the 74LS148 at 2B were correct but the all the outputs were HIGH. Pin 7 should be pulsing. The 74LS148 was bad and replacing it fixed the board. |
|
Targ Color Adapter PCB: Colors not displayed properly After cycling through the video tests in the Targ Test ROM I tracked the problem to a bad output on pin 3 of the the 74LS86 at 6A. After examining the bottom of the board, I discovered that pins 3 & 4 were pressed together and shorted. Bending the pins apart fixed the board. |
|
Targ Color Adapter PCB: Sync not correct The jumper for sync video to be negative true at 2C is was not cut. Cutting it fixed the sync issue for my monitor. Another monitor brand might require a positive true input and this jumper to be intact. |
![[photo]](_images/spectar_composite_blanking.jpg){kind=link}