grounding question

stingray2309

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what is the difference between field ground and just a ground? on one of the jamma harnesses one of the grounds was marked field ground the rest were marked ground. now i have a harness that all the grounds are just marked as ground. on the harness is one ground for the ground coming in from the outlet? or is a ground a ground and if so why are there so many on a jamma harness ? do all of them need to be used or can you just use one for the whole cab? i have been using different ones for different areas of the cab just because they were there but i want to make sure im not doing something wrong. for some reason im questioning myself about it so i thought i would ask the pros. thanks for any help
 
'Frame/Field' ground is supposed to be used to safely ground metal parts. Think of it as a last line of defense against electric shock. (it can also be useful for cutting down rf interference, i.e. by grounding a pcb cage, etc)

The regular ground is supposed to be part of active circuits in the cab, i.e. pcb ground, control panel buttons, etc.

Mixing and matching isn't a great idea. Some switchers internally short FG to G but others do not. If you are familiar with the third prong of an ac cable (usually a green wire), that is for the safety/field/frame ground.
 
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A friend of mine bought some parts off line not sure where they came from. His jamma harness does not have all the grounds that a normal harness has and it's not marked so I have to count wires to make sure I have the right wire. How do I know which one will be the fg? If it helps I'm wiring up a 19-1 cab for him.
 
A friend of mine bought some parts off line not sure where they came from. His jamma harness does not have all the grounds that a normal harness has and it's not marked so I have to count wires to make sure I have the right wire. How do I know which one will be the fg? If it helps I'm wiring up a 19-1 cab for him.


There should be no FG on the Jamma harness. The FG should go from all metal components back to the third (ground) prong on the plug. It is not included with the Jamma harness because it is routed differently in every cab.

If you have ever seen early Midway cabs you may remember the fat yellow strap that ran through the cab. The strap is being used as a FG. Other cabs like modern Jamma just use a daisy chained wire to connect the control panel, coin door, monitor frame, and other internal components back to the green wire on the power cord.
 
All the grounds on a JAMMA PCB are common, ie they are all connected together by the PCB. However that doesnt mean you only need one connection back to the PSU. To avoid volt drop between the PSU and the board (ie when you get 5V at the PSU, but only 4.6V at the board) you need enough current carrying capacity between the PSU and the PCB. That means thick enough wire on the 5V and thick enough wire on the ground. Even if you run one thick wire between the PCB and the PSU the weakest point is the finger on the JAMMA connector itself. A side effect of volt drop is heat, using only one wire can cause the wire to get hot, and the finger on the JAMMA connector can get very hot and burn up. If you are wiring up a cabinet you really should run a ground connection to every pin on the edge connector, that way you get enough current carrying capacity and spread the load out on the edge connector.

All the above, and the question of how much is enough depends on the current draw of the board, some old games really chewed power so its wise to run 5v to all four pins and ground to all 8 (or at least as many as the 5V). Its fine to run 12V and -5V to only on of its two pins as the current draw on those power lines is nowhere near as high as the 5V that has to power 99% of the board.
 
Thanks very helpful and I'm a dumbass I was working on the jamma harness at my kitchen table so all I had was the harness but for some reason I kept thinking I remember seeing a fg somewhere and I was right but after reading these posts I went downstairs and open up a game the fg is on the power supply. What a brain fart but again lots of good info thanks again
 
Just make sure it's grounded to earth...

white-trash-repairs-what-i-grounded-it-to-earth.jpg
 
We also call it neutral. :p

Um, no.

While the power company may earth ground the neutral line at the pole, it should never be considered a ground since the earth where you are standing is likely not near the pole; there can be a significant (possibly dangerous) potential difference!
 
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