x 2 with a Grand Lizard.

I've got this with my High Speed as well. I sort of like that it makes the machine sound like an evil robot in attract mode, but that can't be good for the machine.

Ensure all screws on all modules in the head are present and snug. Missing ground screw is most common cause.

I had the same thing in my Grand Lizard. Wicked loud. It ended up being the routing of the cables in the back box. You can still hear it a bit during attract mode as all the lights wave through, but not the across the room noise that sounded like it was ready to fry everything.

Seriously, try it after you check all the ground connections. Have it on, and just reposition the cables in the back box listening for the changes. On mine, it all came down to moving one cable away from the speaker.

If that solved any of my humming machines -- that would be just too easy. It reminds me of dealing with guitar cables, they don't like to cross AC adapter wires for example.
-mof

I know. When it fixed that GL I was ticked it took so long to find.

Had this on my Pinbot. Moved them around and then it was fine. I noticed because the hum went away when I would open the light panel and then come back when I closed it.

My F-14 had a maddening humm/buzz like you guys describe. I accidentally fixed mine when removing and reinstalling the sound board when I took it out to figure out what capacitors were on it. You can't even hear a little hum out of my machine now!

If the loosening and tightening all the screws on all the boards doesn't eliminate the humm, try unplugging and replugging 3J1 on the power supply or wiggling the wires going into it to see if that changes the humm. This is the large rectangular 12 pin Molex connector near the bottom of the power supply board that goes to the transformer. If wiggling the wires on this connector changes the humm, you can try soldering the wires in the plug to make the crimp connection more solid. (NOTE: I'm suggesting you solder the wires to the metal crimp on collectors in the plug, NOT the wires to the pins on the board). Specifically pin 9 and 12 which are the ground connections. You may need to replace the pins on the board and in the plug to make a more reliable connection.

Can you throw up a pic of this? Newbie doesn't quite grasp what you're referring to.

It just means you need to put screws in that might be missing. Need a screw in every hole in every board in the backbox, and tighten them all down. A lot of times, a few screws are missing.

Rather sure he simply means, open the backbox and make sure each circuit board has a screw going into each hole around its border. Sometimes, a specific receptacle is required for it to screw into, and those little parts can be missing. In order to know, simply remove one screw and check to see what it is screwing into... If there's a small part, then you may want/need a few extras of those. You may have enough of them as it is. If you are short on screws, remove 1 and have your local hardware store match em. You want to make sure you get the thread count correct.

-mof

This could be a couple things. You've checked the most common cause.

Here's something to try. "Float" the sound board off it's grounding screws in the back-box. You can do this using a piece of cardboard as a test. Don't put any screws in the sound board and power on the game to see if that reduces the hum (it should)...

That really is a "quick hack type fix" but it does work (long term) a lot of times. If you want to really get down to the cause, follow these steps in order--->

1. Ground screws in all boards and ground braid connected between haed+cabinet?
2. Bad speaker? You can disconnect them one at a time to confirm which one may be at fault. You can also check with a spare working PC speaker. Check for correct ohms resistance on each speaker type in your pin too. Also a great time to check that all the (+) are red wires and all the (-) are black wires on EVERY speaker in the game (so that they are not out of phase).
3. Bad potentiometer in the bottom cabinet? You can test this but I honestly replace them on my games given the chance. Your problem qualifies as a reason I would just go ahead and replace it!
4. Bad caps on main board OR sound board? Hard to test in circuit. Easiest to just replace 25+ year old caps all in one shot.
5. Bad amplifier on main board OR sound board? Most often these fail "open" but they can cause hum if it fails by shorting legs together. Easy enough to test. Not recommended to "shotgun" fix as it's the least likely cause of your problem.

Is it the right ohms speaker? I know some of the older machines took 6 ohms instead of I think 8 ohms. I'm not sure when they switched from 6 to 8 though. I know I've got a couple machines at my work that have 8 ohms in them instead of 6. They are from the early 80s. They are really touchy on the sound either really high or low the pot likes to jump around. Another thing is like yours they hum and it depends on the volume. Also don't replace the pot just spray it with contact cleaner.

Hey Chris,

The fun begins!

FYI, I just took a look on Pinwiki, in the System 11 section. Here's what it says about speaker hum:

4.10.2 System 11 Sound Problems

"System 11 games are known to have some amount of hum present in the sound. To minimize this interference, make sure all boards are secured tightly with all screws installed. This will insure the boards have a proper ground.

Another cause of hum could be an inconsistent +5 volts from the power driver board. An indicator of this being the cause of your hum would be the game occasionally resetting as well. Replacing caps C8 and C10 on the power board may fix this issue."

Hope that's of some help. For what it's worth, I do remember a little hum, but not a crazy amount. You might want to try a different wall outlet, too, just to see if the issue travels from outlet to outlet.

Good luck!

Matthew

If all else fails (like it did on the Jokerz! which had a different Sys11 sound board with significant hum issues after addressing EVERYTHING else); you can do this:

1) Increase the resistance to each speaker
2) Increase the volume to compensate.

This will have the effect of raising the floor level of volume that the speaker will start to reproduce, thus bypassing any lower level hum sound present (so long as the source of the humm is POST amplification, not pre-amp; but it's almost always post). You just then need to turn it up to get past that level and hear anything.

So, in order to increase resistance, you can either replace the speakers with (better) ones with higher resistance , e.g. swap out the 4 or 8 ohm factory ones for 32 ohm after market; or you can just add an inline resistor to do the same.

I have done this to various arcade video games and 2 Sys11 pinballs. Instead of a fixed resistor, you can use a low ohm volume pot as well. This lets you then balance the volume of each speaker as compared to the master volume, giving bass, midrange or treble (if you have 3 speakers) to your preference.

The range of the resistor will need to be around 30 to 60 ohms; so a 200 ohm pot would be about the best you can find; a 100 ohm would be ideal if you can find on.

Remember, adding resistance to the speaker system is safe--you won't blow an amp; that only happens if you lower resistance below it's specs (typically below 4 ohms); basically creating a dead short in those cases. However, by adding resistance you will only lower the maximum volume it can produce and increase the base floor at which it starts making sound. Probably not an issue in home use. In any case, anything above about 140 ohms will reduce the max volume too much and way higher than I've ever seen needed.

Do all the other fixes first -- screw in every board, check wiring, check volume control; apply service bulletins; but if you still can't make it go away, the above will work at very low cost.

I thought I "broke" my HS when I switched it over to LEDs. By reducing the power draw on the switched and GI circuits , I cut the background hum way down. Then, I added about 7 missing screws in the backbox to all the boards and suddenly NO sound was being made in attract mode. I kinda miss that wiping sound in response to the light show now; but don't miss the hum overpowering the engine noise while playing.

Why do you guys hate hummers so much? Just tighten up the MPU screws and make sure they are all there.

--Scott

One thing that has not been really mentioned is potentially "loose" connectors; I "fixed" the hum on a System 11 one time just pulling a Sound Board connector on and off (game was off), and the hum magically went away. If it returned, I was just going to replace that connector.

OP already did this. Stay out of the TECH forum unless you are willing to read all the posts.

Hum is not very often attributed to a connector issue. Very odd for this to happen. OP could detemrine this by simply wiggling connectors with the game on.

I agree, it might not be common, but I have seen this and just trying to get a comprehensive list.

High speed with pinball pro speakers and Rottendog power supply. Horrible speaker hum that pulsated with G. I. Lighting .
Replaced Rottendog board with factory rebuilt board and hum completely went away.

interesting cause I just installed the rottendog board and it hums like crazy.

Yeah. I’ve been told it’s the Mosfets that likely cause the hum.go back to original board and hum should go away.

So I currently have this issue with a Space Shuttle with original boards (recently acquired) and a Sorcerer with a new Rottendog MPU and Kohout Power board. What should I do?

I was working on a Road King, and did everything to reduce the hum or buzz in the audio. Uppon replacing the displays with led (main board and 5 screens) all the sound issues when away, and certainly the screens looked fantastic and all works.

The switching of the displays can absolutely get into the audio. I've actually seen the opposite, LED displays increased the noise. Whatever displays you installed might of had better filtering. I noticed just some extra C on the display driver 5v cleared out a lot of noise in my Dr Dude, even just putting it across the power supply plug where it comes in at.

Hey everyone….I picked up a road kings with HORRIBLE humming during attract/idle mode…there were screws missing in all the boards. I started with the main board, top right screw added. Nothing. Then second screw added on the bottom right. Voila! Humming completely disappeared.!!!

This may have been one of the simplest and easiest fixes ever! All board screws that were missing have been replaced.

Thank you all for this thread! Pinside saves the day again!