Carnival

ByMichael Coates

Carnival on a Gremlin / Sega Vic Dual PCB.

Just had blue bars on screen, although probing the Z80 showed it was trying to run code.

Using a logic probe I discovered that the /WRITE signal of the ram chips was always high. This signal comes from a 74LS10 at U77. Checked this chip and one of the inputs – WG (pin 4) was always low, so the NAND output was always high. WG signal is generated from the 74LS175 at U19 and sure enough, Q and /Q were static.

Replacing this chip returned the write signal to normal, and fixed the game.

Similar Posts

  • Atari Tetris

    Byadmin

    In normal mode, just had a blank screen. Enable test mode and you instead get an error message showing that the ROM test had failed (even though the rom itself checks out OK with correct content) This is because the custom Atari SLAPSTIC chip is not functioning correctly. It is used to page banks of…

  • Make Trax

    Byadmin

    Boot up with garbage, quickly clears screen and then nothing. Used Fluke with Z80 pod – read rom OK, and write ram OK, but timeout reading any Ram location. Check fluke status and ‘wait’ always set. Check pin 24 of Z80– stuck low, trace back through 7H,7C,6C1 to 6B1 pin 2 – always low, although…

  • Hantarex MTC9000/A

    Byadmin

    No picture, Hole blown through LOPT. Had problems tracking down replacement – little tip given to me by Hantarex engineer – you can use the LOPT from an MTC9000 as long as you cut pin 2 off. (If you leave it connected, you will get a direct short to GND)

  • The Invaders – Zaccaria

    Byadmin

    Totally dead, no screen display (blank white screen).  For this I used the Fluke Z80-S2650 adapter to check out the ram and rom, and located two errors. The rom mapping was incorrect, with a large range of locations returning random values. Examining the board (since I have no schematics), all of the chip select lines meet up…

  • Rainbow Island

    Byadmin

    On power up – black screen, no sign of life at all. I removed the 68000 CPU and fitted a socket, so I could use the Fluke on it. This revealed that the main program ram ($10C000-$10FFFF) was totally dead. Replacing the two ram chips IC25 and IC26 solved this problem. Board now started up,…

  • Neo Geo

    Byadmin

    6 Slot – Memory test on power up giving error  Address    Write Read  00008000 5555 55FF Checking with Mame, the memory chips in question were only small, so looking at the board revealed two DIL 5814’s. Chip select signals to these were toggling as expected, so I needed to identify which ram chip was which….