Scramble
Fault
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Solution |
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Hangs on cross hatch.
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The two 8255 ICs on the top board (MU-1) had been replaced badly with some of the pins bent. Reseating these ICs with the legs straightened fixed the fault.
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Corrupt background which changed every couple of seconds.
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The background should actually be the stars. It was the star circuit that was causing the fault. The 74LS164 at location 1P on the bottom board (MU-2) was dead. The output of the 74LS30 at location 1N was also stuck low. Replacing these two ICs fixed the problem.
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Screen was complete rubbish. there was no sync and the data that was displayed was rubbish.
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Pin 8 of the 74LS04 at location 8M on the bottom board was stuck low. This caused the hsync circuit to be permanently preset and caused the 256H signal to be stuck low. Replacing this IC fixed the fault.
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No Sound.
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There was a 4.7uF capacitor missing from behind the audio amp. Replacing this capacitor led to the fault below.
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Continuous Tone and no game sounds.
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The AY-3-8910 audio chip at position 3D was dead. Once replaced the sound worked perfectly.
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No Sound.
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There was no chip select being asserted on either of the 2114 ram chips on the sound board (MU-1). The output of the 74LS32 at position 5K, which is part of the circuit that generates CS for the ram, was stuck high. Replacing this IC fixed the fault.
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Space Invaders (Taito 3-Board Set)
Fault
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Solution |
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Completely Dead. No clock.
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The outputs of the 7442 at position 3 on the PROM board were stuck, Replacing this IC fixed the problem.
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Completely Dead. No clock.
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The 74S04 at position 1 on the ROM board was dead. Replacing this IC fixed the fault.
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Garbage on screen.
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The reset signal from the 74161 at location 17 on the IO board was stuck. Replacing the IC fixed the problem.
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Missile & Player hit sound effects not working.
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The output of the noise generator (4006) was stuck low. Replacing this IC fixed the noise generator. The LM3900s at locations 34 & 35 were replaced which fixed the problem.
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UFO Hit sound not working.
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Replaced LM3900 at loaction 30.
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Target hit sound not working.
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Replaced LM3900 at loaction 33.
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Board dead with garbage on screen.
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I removed the first program ROM in order to perform the RAM test as described in the "Standardized test procedure for Midway's Processor Boards" (This is a Taito board but the same principals apply). The RAM test showed vertical bars with horizontal garbage. This indicates an addressing error. One of the address buffer outputs (7408) was found to be faulty. Replacing this IC fixed the fault.
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Game running but screen full of coloured blocks (video is actually at full intensity all over the screen)
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The output of the 74166 (Q5, pin 13) @ position 29 on the ROM board was open circuit. Replacing the 74166 fixed the fault.
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Dead, garbage on screen & constant UFO sound.
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Two of the output pins (pins 10 & 15) of the 75365 at position 10 on the ROM (bottom) board were static. I traced the signals back to the 7442 at position 3 via the 7474 at position 4. Replacing the 7442 at position 3 fixed the fault.
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Lines through invaders at top and bottom.
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Pin 9 of the 74LS153 at location 8 on the top board was stuch high. replacing this IC fixed the fault.
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Game is playing but the background is solid blue (the blue output is constantly 'on').
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The open collector inverter buffer 7416 @ location 36 was found to be faulty on the blue video signal. Replacing the 7416 fixed the fault.
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Dead. Garbage on Screen (with & without roms fitted - no vertical lines with roms removed)
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The ram data bus is latched by 2 74LS174 latches (@ locations 36 & 37). The outputs of the 74LS174 @ location 37 were stuck low. Replacing this IC fixed the fault.
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No Coin Input.
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The output of the 74153 (pin 7) at loaction 4 on the sound board was Bad. Replacing the IC fixed the fault.
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Space Invaders (L-Shaped Set)
Fault
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Solution |
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Dead, Garbage on screen. No Vertical lines when ROM H removed.
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The HOLD and RESET lines of the CPU were both HIGH (they should be LOW). Thw output of the 7404 at F3 on the sound board was open circuit. Replacing the IC fixed the fault.
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Missile sound not working.
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The output of the 74174 at loaction E4 (pin 5) had gone open circuit. Replacing this IC fixed the fault.
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Lines through invaders at top and bottom.
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The 74175 at location B5 on the sound board was faulty. replacing this IC fixed the fault.
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Invader Advance sound (background thump sound) missing.
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The output of the 556 at location H4 on the sound board (pin 5) was static. The outputs of the 7417 at location F5 were all ok. Replacing the 556 fixed the fault.
Note: Half of the same 556 is also used to generate the Bonus Missile Base sound. This sound was also faulty which also indicated that this IC could be the cause. The Missile Base sound was also fixed by replacing the 556.
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Invader hit sound was incorrect.
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The 0.1uF capacitor C16 had incorrectly been replaced with a 0.01uF capacitor. Fitting the correct value capacitor fixed the fault.
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Evenly spaced vertical bars on screen. no game.
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The CPU was found to be faulty. Replacing the CPU fixed the fault.
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Dead, Garbage on screen. Vertical lines only present in 2 vertical columns (out of 4) of the screen when ROM H removed.
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Removing ROM H to perform a simple ram test revealed good vertical lines in 2 vertical sections of the screen and garbage in the remaining 2 vertical sections of the screen. Performing a ram test with a Fluke 9010 indicated that address line A3 was bad. A3 was traced back to the 7408 at location H3 and checking A3 with a scope indicated that it was stuck high. Replacing the 7408 at H3 fixed the fault.
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Space Pilot
Fault
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Solution |
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Sprites are square blocks with a feint image of the sprite data.
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The 74LS194 at position C12 was dead and was replaced which fixed the problem.
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Some of the sprites have been split into two sprites, each one suffering from jailbar effect.
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There are two sprite buffer chips (one for even lines and the other for odd lines). One of the buffer chips was not getting addressed correctly which caused the sprite to be drawn at the wrong place. The address generator IC in question is a 74LS163 at position C5. This IC was found to be bad and replacing it fixed the problem.
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The sprites were upside down and did not respond to the screen flip circuit.
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The screen flip signal is latched into a 74LS74 which then drives the input of a 74LS86 which is used to invert the address. The 74LS74 for the horizontal flip circuit was found to be dead. After the dead chip was replaced the sprites were the right way up.
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The DIP switches and control inputs don't work.
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Pin 3 of the 74LS367 at location A1 on the sound board was at 1.5V. Pin 3 controls the dip switch buffers. Replacing this IC fixed the problem and the game can now be played.
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Star Force
Fault
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Solution |
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Background graphics scroll for 16 pixels and then jump back. The background update ripples up the screen.
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The 2114 SRAM at location H2 on the board labelled 5904C-1 was bad and was replaced. This fixed the fault.
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The picture has no colour, it's just black and white (Literally, no grey levels).
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The 74LS174 at location A4 on the board labelled 5904C-1 was bad. Replacing this IC fixed the fault.
The 74LS174 is a latch which holds the palette ram address. The palette ram comprises 2x2114 SRAM ICs at locations C4 & D4.
NOTE: There is a via under the 74LS174 connected to pin15 that is open circuit between the upper and lower layers of the board. This seems to be intentional because if you connect them the graphics go wrong.
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Foreground graphics, including the main logo, are made up of the wrong characters.The title screen logo is made up of duplicated strips of graphics. During gameplay there is a cross hatch covering the picture.
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One of the address lines (A6) of the forground graphics roms was stuck high. This signal is latched by a 74LS273 at location C8 on the board labelled 5904B. The input to the latch on pin 8 and the clock on pin 11 were both active but the output on pin 9 was stuck high. Replacing the IC fixed the fault.
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Half of Sync Signal stuck low.
(Bootleg)
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The composite sync signal is created by ANDing together the horizontal and vertical syncs. The vertical sync had a 50% duty cycle instead of a normal valid vsync signal. vsync is generated by a 74LS161 counter. Replacing the 74LS161 fixed the fault.
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