How would I know if I have the 502 or the 504 board?
There are two white rectangular labels attached to the main MPU circuit board.
If you look at the last three numbers on one of the labels, you will see something like "502" or "504" or something else.
Sometimes there's an audio multimedia lite (MML) piggyback board over half of one label.
Just pull off the MML piggyback audio sound board and you'll be able to see the label stuck to the main MPU board.
Do NOT remove or install any slot machine circuit boards in or out of their sockets while the power switch is ON....always turn OFF the power switch.
hmmm...the "little spark" you saw in the top box is NEVER good.
Somebody was messing around with the top box area.
You should show us some high-resolution photos of up to 1000kb in size.
Maybe a member can spot a problem.
To answer your last question, the hiss sounds you hear are the result of the slot machine now located in a quieter environment of your home or basement rather than a noisy full-blown casino with a zillion machines a'running and people and music playing.
MLG member Lookes has had success in modding a 501 or 502 MPU to have better sounds.
You might want to get in touch with him and see if he has time to help fix your MPU.