The NEW Build Your Own Arcade Controls
Arcade Collecting => Restorations & repair => Topic started by: mholmes708 on April 11, 2021, 11:01:58 pm
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I am now the proud owner of a 1977 Comotion cocktail table made by Gremlin. It is unfortunately not in working order at this point and I am seeing if anyone might have any suggestions.
I am new to working on arcade machine (this is literally my first) so I'm not really sure where to start. The game powers up fine but on the screen the characters are all jumbled. When a coin is inserted, the buttons and sound work and you can see the game being played but through jumbled characters.
I tested the voltages and everything in the power supply is correct.
This is a video of what comes up: https://photos.app.goo.gl/mE5T3NCMR67Qz7M29
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First check all of your connections and re-seat anything you can unplug. Barring that it looks like your character eeprom is screwed up.... that is if that thing even has roms. 1977 is a super old one and I'm not familiar enough to give more advice than that. Anyway if it does have eproms and they are socketed removing them, cleaning them and re-seating may help. You might have to get a new set burned if that doesn't help.
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It has an Intel 8080A (https://en.wikipedia.org/wiki/Intel_8080) processor and ROMs.
(https://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/KL_Intel_i8080_Black_Background.jpg/440px-KL_Intel_i8080_Black_Background.jpg)
(https://www.arcade-museum.com/images/112/1128016672.jpg)
The manual has schematics and some troubleshooting info.
https://www.gamesdatabase.org/Media/SYSTEM/Arcade//Manual/formated/Comotion_-_1976_-_Gremlin.pdf (https://www.gamesdatabase.org/Media/SYSTEM/Arcade//Manual/formated/Comotion_-_1976_-_Gremlin.pdf)
The end of the video you posted appears to show the first two of every eight pixels stuck "on", creating vertical lines.
A quick look at the system description and block diagram on pages 12-14 leads me to think that the problem could be two stuck data lines, bad character generator memory outputs (#15 on block diagram), a bad shift register (#16), or a bad video combiner. (#17)
Since the character generator memory is where the 8x8 pixel "blocks" are called from (pg 13, paragraph 3), that is the earliest part of the circuit where the stuck pixels could originate and the video combiner is the last part of the circuit where they could originate.
There are 8 outputs from the character generator memory to the shift register (pg 15) so if the outputs for the two pixels on the left of each row in the 8x8 "block" or the associated data lines are stuck, it will cause the solid two-pixel-wide vertical lines seen in the video.
- The other six character generator memory output lines appear to be working fine all the way thru the video combiner to the CRT. :cheers:
Scott