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 been cut. Repairing this restored the stars.

No star display. chips 1A,1B,2D and 3C all socketed. sockets corroded, pins not making contact. remove sockets and solder chips direct to board. Stars returned to normal.

No screen display. Check dot clocks (74161’s at 3A,4A,5A and 6A) – nothing on clock pin of 5A and 6A (pin 2). Trace back through 6F, 8L to 7404 at 6E – signal going into pin 5, pin 6 stuck high. replace chip – screen back to normal.

No screen display. Check dot clocks, no activity on 6A. replace this and screen display back to normal.

Mirrored shells. Whenever a shell was dropped, a ‘mirrored’ version appeared at the correct height, but directly above the base. if no shells dropping, an extra shell appeared below the base. Noticed that when I connected the comparator to the 7474 at 5P, the additional shell disappeared. Replacing this IC made no difference, but adding a bias resistor (1k) from pins 4 to 16 on this chip solved the problem.

Sprite Glitches

On a bootleg PCB, the diving triad of ship and galaxians, the right hand galaxian had no eye’s or wings. The sprite ram was OK, and no other galaxians had similar faults. I eventually tracked the problem down as something to do with the 7400 at 2P. When I placed the probe on pin 1 or 5, the eyes reappeared, as did a thin section of the wing. I replaced this chip with a 74LS00, hoping the lower power requirement would help out, but it made little difference. The bootleg board did not match the midway schematics at this point, and instead of the output of the sprite rams being buffered, they were driving the chip directly. There was also a pull up resistor on the output, so I removed this – problem solved! (incidentally, this particular sprite ram had been replaced as it was a different brand to the other 4, so maybe the originals had more drive power)

Constant tone on power up, game looked and played OK, but no start tune. checked output of latch (9J) – all low (usually all high for silence) – check for activity on chip select (pin 11), nothing – trace back to decode chip (74138 at 8M) – no signal being generated. replace chip, sound back to normal.

Game Resets. Game goes through ram tests, sometimes completes sprite check and checkerboard, but then resets and starts all over (even with watchdog disabled) other times does screen ram test then sits on blank screen with stars. Pin 17 of z80 floating, check 7474 at 6F, no output – replace chip, game now runs.

Characters all two coloured, top couple of lines one colour, rest of character another. I examined the output of various chips using the video probe, and when I came to the 74194’s at 2H – 2L, pin 15 showed the characters being output. However, on 2J only the top two lines of each character were being output. Replacing this chip solved the problem.

Wrong colours. Colours looked OK in ram tests when board switched on, but all game characters in same colour, no scrolling of galaxians or base, no missiles or shells. Pin 10 on 5E had the track cut – normally connects to GND, but left floating in this case. (Thanks Phil!) – seems to be a common hack, in this case on an Artic cartridge system board. 

Sprites showing lower half twice (instead of upper half and lower half). Looking at schematics 3B exclusive or’s the original address line for the sprite generation with the horizontal 8 line. Output from this chip was always low. replace chip, sprites back to normal.

Wrong colours, but not all wrong – a lot looked correct, but some things, like the ships that sit on top of the galaxian horde had the colours back to front (red / blue swopped). I removed the colour prom, and powered the board up. I then used the video probe to see what signals were going to A0-A4 on the colour prom. (You get the shapes that would have used that address line for colouring) From this it was apparent that A3 was always on when a character was displayed. Thinking this unlikely, I chased the signal back through several chips until I reached 3P in the video circuit. The signals in showed the full screen (which was the signal I was getting) and the colour bars (which were being ignored). Replacing the 74LS20 returned colours to normal.

Wrong characters

This one passed the memory test on bootup (most of the time), but consistently missed out certain letters on the screen display. Even odder, the missing letters seemed to sometimes appear on the top line of the screen. Phil had already replaced pretty much every chip to do with address decode, buffering and selection, but it still had the same problem. Using the ‘video probe’ (as mentioned in the Galaxian troubleshooting guide) I looked at the various signals used to control the video memory lockout. CMP BLANK is used to control the address selection to the 2114’s, and that looked fine, VBLANK (used in the vram control) also looked normal. The only signal that looked slighty odd was HBLANK – I compared this against a working board and it looked to be enabled for about 8 pixels longer – this reduced the size of the buffer to allow writes to complete before the video started drawing. Following this back to 3D (7474) still had the problem, checking the signals into 3D revealed that one signal was very weak. replacing the chip at 3C (7420) fixed the problem.

Characters on screen all displaying with colour bars – same character drawn in different colour every scanline – random pattern. Scrolling items, such as invader fleet drawn central or in correct place alternating rapidly. Looked like an object ram problem, but ram passed memory test every time. Using video probe I checked out the signals to the object ram, and pins 1 to 4 had correct high signals, but lots of random noise when should be low. Trace back to 74157 at 5E, input signals looked good, but output had noise. Replaced chip and problem solved.

Scrambled Characters

The characters, including sprites, were all ‘scrambled’, each looked like it was made up of the correct parts, but not necessarily in the correct order. Using the video probe, I traced the drawing circuit to see what was being picked up. n.b. to make it easier to follow, remove the colour prom – that way all you can see on the screen is what you are probing. from this, it was apparent that the wrong data was being supplied from the character roms. checking back the address lines revealed that A0 seemed to be inverted compared to the other signals. replacing the 74283 at 5N put everything back to normal.

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