Yugoslavian computer Galaksija

During my debugging I made a quick cheat sheet.

Schema

1. Screen without text and raster
→ 80A, 6116, 2732, 74LS04, 74LS593, 74LS74, 74LS123

2. Raster is present but no text
→ 2716, 74LS04, 74LS74, 74LS156

3. Image is heavily skewed
→ C3, C4, R12, R13

4. Certain characters are not decoded
→ 74LS156, 74LS251

5. Characters are displayed but some are missing, partially drawn, or deformed
→ Z80A, 2732, 6116, 74LS174

6. First and fifth lines display the same characters in parallel
→ 74LS174

7. All characters are displayed but some parts are missing
→ 74LS166

8. Letters are vertically split and lines appear on the right side of the screen (BRK)
→ 74LS174

9. Random characters appear in various parts of the screen
→ Z80A, 74LS32, 74LS156

10. Grainy raster across the entire screen
→ Z80A, 74LS156

11. Variable raster across the entire screen
→ Z80A, 74LS156

12. Diagonal raster with one vertical stripe
→ Z80A, 74LS156

13. Unresponsive text, does not accept commands
→ 74LS38, 74LS74

14. Vertical stripes without text
→ 74LS04, 74LS38

15. Dashes across the entire screen
→ 74LS38

16. Occasional random text appears alongside "READY"
→ 74LS38

17. "READY" appears on the right half of the screen, then characters appear all over
→ Z80A, 74LS74

18. Image moves vertically
→ 74LS04, 74LS123

19. Image is heavily skewed and moves vertically
→ C3, R12, 74LS123

20. Slanted dotted characters
→ 74LS123

21. A few white slanted lines
→ 74LS123

22. Vertically shifted or skewed characters
→ 74LS93, CD4040

23. Vertically moving image with no text
→ 74LS93

24. "PRINT MEM" shows less memory than expected
→ 6116, 74LS156

Debugging tips taken from magazine Računari

• T1, R4, R5, C6 and D1 are part of reset circuit (holds low level for 0,5s)

• left 74LS156 is address decoder for RAM (enables OE on correct RAM or ROM chip)

• 74LS123 with of horizontal and vertical sync (horizontal sync R12 and C3, vertical sync R13 and C4)

• 74LS93 divides oscilator by 15 (6144000 Hz /15)

• CD4040 divides number by 1024  (6144000 Hz /15/1024)

• CD4017 divides by 10 (6144000 Hz /15/1024/10 = 50Hz) (refresh rate), one pin goes via 74LS04 (inverter) to INT on Z80, which triggers M1 and IORQ, which via 74LS74 activates WAIT

• 74LS174 tells the character ROM which line of 13 is currently being displayed. Bad connection on pin 2 and 3 gives a black square on right side

Decimal Binary

------- ------

0 0000

1 0001

2 0010

3 0011

4 0100

5 0101

6 0110

7 0111

8 1000

9 1001

10 1010

11 1011

12 1100

13 1101

• right 74LS156 is keyboard decoder (active low) for columns

• 74LS251  is keyboard decoder (active low) for rows left 74LS156 is chip select for ROM and RAM (active low)