MKII - Loud hum from speaker?

Firstly, I'd like to state that I can confirm this is NOT the monitor. My speaker is exhibiting some strange behavior. Most notably, theres a large audible hum/buzz coming from the speaker. Not super noticable during gameplay, but you have no problem hearing it when theres no audio going on.

The only other thing I noticed is that theres a strange, very faint, semi-slow paced Hi/Low ...wheeze? almost, coming out of the speaker when the game first turns on. It sounds like a police siren almost, but it eventually stays Hi and thus the loud hum. Does anybody here have any idea what this could be? I already confirmed the grounds in the cab are OK.

Thanks in advance,
Shmenge

Edit- also, when the machine is first turned on, it makes a loud "DING" sound (from the boards burn in test, which is normal). The one thing I immediately noticed about this sound is that it's completely independent of the games settings. For instance, if I have the volume in-game set to very low/inaudible, that initial sound will still be loud as all hell - and this is the only MK I have that does this (MK1 and UMK3 make the sound at whatever volume level I have set for ingame).

Midway boards have that startup sound and it can't be altered in volume from my xp with my boards.

Maybe someone else knows about the hum.

Agreed on the start up ding...on my NBA Jam this cannot be adjusted, IIRC.

On my MK it's adjustable as it uses a volume pot. The rest of them don't but I can't test the other two without opening them up, which I can't do at the moment.

Maybe it's not that it doesn't adjust with the rest of them, it's moreso that its NOTICABLY louder than the rest of them. That much I can say for sure. After that loud ding, the game plays at whatever volume I selected. Not sure if that has anything to do with the hum, however.

I have another speaker that I can replace the current one with. I don't think that'll solve the problem but it couldn't hurt.

I still say that it sounds like a grounding issue...

The fact that it has a separate audio amp can really exacerbate the situation.

Could also be an interference problem.

my mk2 makes a hiss/hum i think.

also blitz/showtime, mk2, and umk3 all make that startup ding. its annoying but w/e.

not sonic said:

my mk2 makes a hiss/hum i think.

also blitz/showtime, mk2, and umk3 all make that startup ding. its annoying but w/e.

Click to expand...

It's strange, it's the only one out of my collection that has that hiss. My MK1, UMK3 and Blitz/Showtime don't exhibit this problem. I wish I could figure out what it was, hell I may just throw a pot on there myself. Are there any kind of requirements that have to be met when adding volume pots?

Edit- Based on the evidence I've gathered I actually believe that may be the case, the default volume of MKII is through the roof, but is then software adjustable and as such causes the hiss. Adding a volume pot right before the speaker terminals should resolve the problem, I just want to confirm that I can throw any pot on there. If not, I have a spare MK1 board with a pot I'm sure I could use.

Edit 2- Also, how do you wire up a volume pot to a speaker that has a positive and negative? There's only a ground, an in and an out on a pot.

not sonic said:

my mk2 makes a hiss/hum i think.

also blitz/showtime, mk2, and umk3 all make that startup ding. its annoying but w/e.

Click to expand...

Midway anything does this. My Blitz, Hydrothunder and even the pinball machines do the exact noise. Nothing strange at all about it.

To me, sounds like interference in the wires.

So I moved the wires out of the way of the monitor to no avail, and there are ferrite beads around the wires, so I don't think it's interference. It's either A) what I suggested above or from what I can tell B) something on the sound board is going, and needs to be replaced. What that is is beyond me however. I believe it's the board when you consider the faint "police siren" sound it makes when you first turn it on, but this could happen normally on other Midway games and I'm just not hearing it, because the default volume isn't as loud. Anyone ever repair one of these boards?

Maybe try unplugging the marquee light? Perhaps that's causing some interference?

I did try that as well, and the original light and starter were making quite a lot of noise, but after I unplugged it the noise was still there. I've actually confirmed with about 3 other people that MKII does infact hiss. According to a friend, its a DSB2 chip that's the culprit. However I do still believe that wiring a volume pot right before the speaker could resolve the problem. I'll try it and report back.