galaga sound issue

foose

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Donor 2015, 2024
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hi everyone!

i replaced all the chip sockets on my galaga, and sure enough i got the thing to fire up! there are a few minor issues, like the test mode button isnt currently working, so i havent gotten into that yet, but the game plays well so im not too worried about it for now.

my main issue is that the sound isnt right at all....the explosion sound is the only thing that seems to work flawlessly...every other sound is....how to describe it....its like each sound is missing components of the sound; you only get to hear certain frequencies, everything else is silent, or perhaps represented by a "pop" in the speaker instead of a "bleep"....if that makes any sense at all.

i have read through various threads on this issue, and mostly the advice has been to check for 12 volts, which i have, and it is 12.58, or to re-seat the socketed chips, which i have done, and tested for continuity from each leg to the board, and everything checks out.

i also have an extra board for parts laying around, so on a whim i decided to swap out the sound amp chips, but there was no change in my sound problem after the swap, so that clearly wasnt it.

i also had a few extra caps laying around, so i replaced two of the electrolytics in the sound section, also no change to my sound issue after the fix, so those werent it either.

any ideas? i greatly appreciate your help!

Brian
 
Do you have a logic probe? If so, take a look at the Galaga manual schematics for the CPU (top) board, locate the sound out part and work backwards checking the logic state (high or low) of each of the pins of the corresponding ICs. Looks like the 54xx custom at 6M is involved with the sound circuitry. Floating pins on ICs (dead, not high or low on the logic probe) or stuck pins at either high or low when there should be activity is a tell tell sign, but not always. I am not sure if the pinout of the custom exists, but for anything other that the custom IC, you will likely need to consult with the specific IC's data sheet to make sure it is doing what it is supposed to be doing based on the input/output on each pin.
 
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i actually swapped the LM324 at 5P with the one from my parts board....no change to my sound issue. ive checked all my traces and everything seems fine, so i have ordered a replacement 54xx chip from arcadeshop.com...its not an original 54xx but it should work, and those 54xx chips are rare little buggers these days! i figure ive checked/replaced about everything else, so it certainly cant hurt.

i do have a logic probe, but ill be honest, i have no idea what its telling me when i check the pins. so i opted for the lazy way out and loaded up the parts cannon. ill just keep firing parts into the board until something changes! not my finest moment of troubleshooting, but it will probably eventually work!

thanks to everyone for their input, i really appreciate it!

Brian
 
Good to know....I'll check them out here in a minute...just got off work, so it'll be about 1 or 2 beers before I make it home!

Thanks!
Brian
 
nope....swapped both IC's from my parts board, no change to the sound issue. apparently my new jrok chip will be here tomorrow....i REALLY hope that fixes it, i feel like im almost out of other options! im determined though....ill find it!

Brian
 
Without a logic probe it will be hard to troubleshoot. But you can also check the 7489 ram and any broken resistor packs in the same area.
 
well, crap! my new 54 chip renders the same results...

i actually do have a logic probe, its just that im not sure what its telling me exactly...i mean, yeah, +5 or ground, or an output could be switching, but how would i know if its sending the right information to the next part of the circuit?

as for the resistor packs, i think im correct in assuming they must be unsoldered from the board in order to properly test them, right? ive tested a few in board and theyre good most of the way through, then maybe two legs come up at 600 instead of 2k.....i unsoldered the resistor pack and suddenly everything was 2k like it ought to have been...that resistor pack must have been in parallel with something else because when i resoldered it back in the same 2 legs went back to 600.

thanks again for all your time and advice, i truly apprecciate the schooling!

Brian
 
RM1,2,3 and 4 all check out...i think at this point i should wait for my edge connector terminals to arrive.....i recently discovered that some things like the test button are not even wired into the edge connector, so i figure i should fix that first, from there i can get into test mode and cause the various sounds to happen on command. i figure with this, i should be able to more easily determine if the 7489 ram chips are writing or reading or not, beyond that, i dont have the foggiest what the data sheet is telling me about all the other pins. ( you may or may not have noticed that i dont remember a damned thing from the one semester of solid state i took back in 2002...)

thanks again as always, ill keep everyone up on any changes!

Brian
 
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