Negative Speaker wire has to be connected to ground for sound to work?! Why??

Tighe

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Negative Speaker wire has to be connected to ground for sound to work?! Why??

So this weekend I was making a JAMMA adapter for my Japanese Taito Space Invaders Part II cocktail cabinet. When I got this cabinet it had a Big Kong board in it (converted bootleg Galaxian).

The board in the cabinet plugs into the existing Space Invaders harness (two edge connectors), and has an additional connector for the player 1 and 2 up, down, and jump buttons.

The weird thing was I was getting rolling interference on the monitor with the audio (both the Big Kong board and the JAMMA 60-in-1 connected). I found a wire connected the hinge on the table to the negative terminal on the speaker. When I disconnected this wire the interference went away and it worked great with the 60-in-1 board, but I had now audio in Bg Kong.

I used an alligator clip and connected the negative terminal to the shielding on the power supply. This restored the audio in Big Kong, but the audio on the JAMMA 60-in-1 is quieter when connected.

Additionally, the player 2 side of the controls are weird and I think have a different ground, as they work fine on the Big Kong, but I cannot get them to work on the 60-in-1.

Anyone worked on this kind of cabinet before and can help me out?


 
the 60n1 has a -/+ amp outputs (ie, the - actually goes below ground)

you will need to fix the adapater so it grounds the speaker for for the other board at where you plug it in
 
the 60n1 has a -/+ amp outputs (ie, the - actually goes below ground)

you will need to fix the adapater so it grounds the speaker for for the other board at where you plug it in

Thanks that is helpful. The other board is the "original" board that came with the cabinet and uses the original harness, so I can't modify the adapter to fix that. I think that I will just put in a switch to turn the chassis ground off.

The question is, is that normal for a SI2 cabinet to have speaker negative tied to the ground even though it has its own speaker negative wire?
 
Are you sure the harness hasn't been "altered"? If they have the negative side of the speaker tied to a cabinet hinge, that means they are connecting it to earth ground. That's never correct. Some games tie the speaker's negative to DC ground, but I've never seen it tied to earth (especially, like you described). However the speakers are wired....it should all happen on the motherboard. If a speaker's negative is tied to DC ground, this is done on the motherboard.

Another note, some motherboards do not like the speaker's negative tied to "ground"......one some, it'll even kill stuff in the audio circuit.

Edward
 
Are you sure the harness hasn't been "altered"? If they have the negative side of the speaker tied to a cabinet hinge, that means they are connecting it to earth ground. That's never correct. Some games tie the speaker's negative to DC ground, but I've never seen it tied to earth (especially, like you described). However the speakers are wired....it should all happen on the motherboard. If a speaker's negative is tied to DC ground, this is done on the motherboard.

Another note, some motherboards do not like the speaker's negative tied to "ground"......one some, it'll even kill stuff in the audio circuit.

Edward

I am sure that was done by the op who installed Big Kong (modified Galaxian Bootleg) in the cabinet. It is very obvious where the changes were done, all the wires are of a different gauge. They didn't touch the original harness other than to disconnect the SI fire button wire and run a new wire for the jump button. Everything else was added, like the extra directions/buttons and the ground wire to the speaker. Interestingly enough I get no sound without the wire connected to ground on the Big Kong. :confused:
 
Are you sure the harness hasn't been "altered"? If they have the negative side of the speaker tied to a cabinet hinge, that means they are connecting it to earth ground. That's never correct. Some games tie the speaker's negative to DC ground, but I've never seen it tied to earth (especially, like you described). However the speakers are wired....it should all happen on the motherboard. If a speaker's negative is tied to DC ground, this is done on the motherboard.

Another note, some motherboards do not like the speaker's negative tied to "ground"......one some, it'll even kill stuff in the audio circuit.

Edward

What about the speaker frame, should that be tied to earth ground? Cause it is.
 
I've seen some games where one side of the speaker was tied to ground rather than ran back to the board via the harness. Looking at the schematic for that game, it showed one side tied to ground, so that was the way it worked...
 
I've seen some games where one side of the speaker was tied to ground rather than ran back to the board via the harness. Looking at the schematic for that game, it showed one side tied to ground, so that was the way it worked...

Thanks, which schematic were you looking at? Space Invaders Part II? If so where can I see it?
 
Thanks, which schematic were you looking at? Space Invaders Part II? If so where can I see it?

I've seen it in different manuals that I can't recall, but for your example - SI Part 2 - it's less obvious.

Here is the Audio Schematic for SI Part 2:

SI2Audio.png


Now, we can see that the Speaker- comes back to pins 3/4/5/10/11/12 of a chip labeled as an LM377. Looking up that chip shows those pins are tied to ground:

LM377.png


Therefore, Speaker- is tied to ground. Normally this is tied to the ground on the board, so it comes back to the connector, but I've sometimes seen people just run a wire from Speaker- on the speaker to a nearby ground - and had it work...
 
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