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. Using the test probe I shorted pins 11+12 on one chip, powering on the board now gave read as 51FF so I was looking at the wrong chip. Replacing the other (with a 2018-35) sorted the problem.
No display – When move board, display flickers – go around board pushing each chip in turn, when press on 2E picture appears. Pin 16 rusted, only just in place. replace chip – picture back to normal. No star display. Board had been hacked to play an alternate game, pin 15 on chip 2B had…
Capacitor burnt. (game board), particularly at C37 the capacitor was showing signs of extreme heat. Replaced them all with 22uF 25V axial electrolytics. No Speech / Some SFX missing. (game board) Output from speech chip SC01 was OK, as was output from IO Custom (X3). checked transistors in output stage and 2N4401 was faulty. (Q2)…
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…
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)
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,…
Altered Beast – Colours missing, no reds, other colours looked like diminished palette (like windows in 16 colours) – used test mode to perform ram test, and one ram chip identified as faulty (I10). Replaced this and red was back, but still reduced pallette. Looking at colour circuit, and it uses resistor arrays driven by I11…