website logo
⌘K
😀RetroSix Wiki
💡Learn Electronics
Byte Swapping (BIOS Files)
Digital Logic Levels
Video Standards
đŸ› ī¸Repair & Schematics
Power Supply Details
Amiga (Various)
Amiga 600
Atari 2600
Atari 7800
Atari Jaguar
Atari Lynx
Atari Lynx II
Commodore 16
Commodore 64
Dreamcast
Game Boy
Game Boy Pocket
Game Boy Color
Game Boy Advance
Game Boy Advance SP
Game Gear
Mattel Intellivision
Neo Geo AES
Panasonic Q
PC Engine
PC Engine Duo-R
PC Engine GT
PlayStation 2
Playstation 5
SNES
Sega Master System
Sega Mega CD
Sega Mega CD 2
Sega Mega Drive
Sega Mega Drive 2
Sega Mega-Tech Arcade
Sega Multi Mega
Sega NAOMI 2
Sega Saturn
Nintendo Switch
Xbox Original
Xbox One S
Xbox Series S
Xbox Series X
ZX Spectrum 16/48k
ZX Spectrum 128
đŸ•šī¸Game Development
đŸ“ĻRetroSix Products
⚡CleanJuice Air USB-C Install (Game Boy Color)
đŸšĨLED Mods
đŸ› ī¸Send In Repair Service
đŸ’¯Prestige Shells & Consoles
🔋CleanJuice Battery Upgrade
🔊CleanAmp Audio Amplifier
🔍Glass Lenses
đŸŦOrdering & Shipping
📐Design Your Own Templates
đŸ•šī¸Game Gear
đŸŽĢStickers & Decals
🎮Printed Shells
🏡Homeware
Docs powered by
Archbee
Repair & Schematics
ZX Spectrum 128

Z80 CPU (ZX Spectrum 128)

5min

After the 40077 Gate Array is checked and you are seeing something on screen (even it if is messed up), the next thing to confirm is operational is the Z80 CPU.

Z80 CPU (ZX Spectrum 128)
Z80 CPU (ZX Spectrum 128)
īģŋ

The Z80 CPU is driven by the 40077 Gate Array IC.

Firstly the 40077 checks to see if there are any expansion port ROM available (such as a Diagnostics ROM) and if so, loads that.

Otherwise, it reads the BIOS ROMs and runs them on the Z80 CPU.

The best way to test the Z80 CPU is to use a diagnostic ROM such as DiagROM. The diagnostics ROMs only need a working 40077 and Z80 CPU (along with all traces between them working).

The BIOS chips, all RAM, Sound Chip, RS232, Keyboard or printer port IC are not needed to test the Z80 is working if using a Diagnostics ROM.

Bad Z80 Or Traces

Presuming you have checked your power supply. the 40077 and basic output of something to screen, and using a diagnostics ROM if it doesn't load properly (outputting test sounds or the screens you would expect) then it is only one of a few things:

  • Bad Z80
  • Bad 40077 Gate Array (unlikely if previous tests worked)
  • Bad traces between Z80 and 4007

Often a bad trace such as D7 being broken between the Z80 and 40077 would result in a screen with vertical stripes, or horizontal lines, or total garbage, with no sound. Often there are flashing blocks too like random cursors.

Broken D7 Trace
Broken D7 Trace
īģŋ
Broken D7 Trace
Broken D7 Trace
īģŋ

Confirming Z80 Is Ok

If using the diagnostics ROM now shows something on screen or the diagnostics beep sounds, then it is pretty certain the Z80 is ok, and you can use the diagnostics ROM to now diagnose further issues with the system.

Updated 20 Aug 2023
Did this page help you?
PREVIOUS
40077 Gate Array (ZX Spectrum 128)
NEXT
Game Development
Docs powered by
Archbee
TABLE OF CONTENTS
Bad Z80 Or Traces
Confirming Z80 Is Ok
Docs powered by
Archbee