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Galaga - No Boot and Sound Issues


SNESNESCUBE64

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This Galaga board was a two part issue. Hooking up the board, the game booted to a white screen and zeroes. 

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First step whenever you get some garbage like this is to check the signals on the CPU, make sure there are no stuck signals line reset, NMI, or other critical signals. Everything checked out fine, so the next step in this case is to check the ROMs. Verifying the ROMs on my programmer, I found that all were good except 3N, which wouldn't read anything back, completly dead. After programming a new ROM, the game fired right up! Everything worked fine except for the sounds, many of which just did not sound right. 

First thing I did in this instance was just shotgun replace the capacitors in the sound amp circuit. They were old and could benefit from being replaced anyway. Testing after the capacitors were replaced showed that there was no change, meaning the problem was somewhere else.

Looking at the schematic (snippet shown below). It seems that much of the sound in question was generated by a Bipolar PROM whose data is fed into a buffer, through an analog and then sent to the LM324 preamp section. After all that it gets sent to the gameboard's main amplifier and then to the speaker.

Partial Galaga sound generation circuit:

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Understanding the circuit a bit better, I decided the easiest place to start was with the PROM since I could just swap it with a known good one from my other Galaga boardset. Swapping the PROMs ended up fixing the issue, verifying that the PROM in question was indeed bad. This is rather unfortunate for me as it is a type of chip I currently cannot program, so I will have to source a new one. Just for reference, I made a little video showcasing the sound issue.

Well, unfortunately the board is in a state in which it needs some PROMs. It has the bad PROM from the previous repair log that I did on a Galaga boardset. My programmers just are not capible of programming those old Bipolar ROMs, one day I will have one that can, but for now the board goes back on the shelf.

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