S-slot has whine in audio signal

[Bruce G]

Hi all - my first post to NLG.

I have an IGT " Gold Mountain " 5136 S-slot that I'm working on.  After a new battery, bit of fixing and tinkering, I have the machine working.  The audio is working, and I have the machine sounds happening when they should.  But there is a distinct buzz / whine in the audio output at the speaker.  The noise is definitely from the CPU crystal / oscillator.  The noise level is unaffected by the volume setting. 
I suspected a dried out electrolytic on the supply rail for the audio output, but all of the electrolytic capacitors test good.  I actually pulled them all off the board to check them.  I re-soldered the header pins for the harness plugs hoping to cure a bad ground, but no change.

If I touch the outside of the 10uF coupling cap for for the LM388 output, the noise level increases.  Grounding the outside of the can to the chassis makes the noise increase.   It actually seems like it's RF radiating into the components of the output stage.   

Has anyone experienced this before on one of these machines or similar?  If so, did you find a cause and solution?

Thanks! 

[knagl]

Do you have a 16MHz board in there by chance? The S+ 16MHz boards are known to be noisy. In a loud casino it was never noticed, but in a quiet home environment it's pretty obvious.

If you're not sure what kind of MPU board you have, the following link shows the two kinds:

http://newlifegames.com/nlg/index.php?topic=1095.0

[Bruce G]

Thanks for your response.  This is an earlier game using the S board - 3 harness plugs - two at the top - and one down on the side.  As for the noise being inherent to the design, that did cross my mind.  The 10kHz crystal is located about 1/2" from the output chip and it's associated components.  In a loud Casino, with the volume cranked up - it wouldn't be very noticeable, if at all.  But in someones home - it's pretty annoying.  There is no component shielding to filter RF from the audio stages.  I have a taken a video of the problem but the file is too big to attach here. 

[jay]

The difference between the two boards (10mhz and 16mhz) is most evident by the presence of a physical volume control vs changing via software.
The 16mhz board suffers from that continual hum, I have not heard of the issue on the 10mhz board. I own 3 of these and have 5 boards to play with and don't have the issue.

If the MPU board is removed does the humm continue - I am thinking a possible short.

[Bruce G]

Thanks for trying to help Jay.  I'm attaching ( hopefully) photos of the board. 

The problem is unaffected by the board being mounted or not.  If you look at the last photo, my finger is pointing at C43 Notice the crystal is right next door to these components.  While the processor is powered up, the closer my finger gets to C43, the louder the noise becomes.  You don't even need to touch the case of the capacitor to increase the noise.  That tells me the noise is  propagating / transferring via RF.  If I connect a ground between the chassis and the top of the can ( shell ) the noise increases dramatically.  The wad in the cap is not connected to the can it's in - so it's capacitance / EMF / RF that appears to be getting in the signal.  In other words, the capacitor seems to be acting like an antenna.  Interestingly, the schematics and parts lists I have don't have C43 in the build.  I'm not sure C43 is collecting the noise, or if it's radiating the noise.  I'm starting to think the capacitor is radiating the noise to components around it.  I'm going to trace back the circuit to try determining what circuit is is attached to.  I may try moving the component location away from the audio section - if possible

[jay]

Yours is a 16mhz board "with the humm" ...

The 10mhz board..... has a blue physical volume control on the edge of the board as per this pic....while the 16mhz board adjusts its sound level through software.
The 10mhz doesn't suffer from the Humm.

I am not an electronics whiz but you might be able to isolate that cap or use a larger one to reduce the noise but that would be an upgrade as all the 16mhz boards have that humm....

From a home perspective the 10mhz boards have a lot more games available to them and you can get features like winning music rather than the classic Ding, Ding, Ding if you use older game chips like SP731. Newer game chips had these features removed to make room for more accounting. To wit: there are no older 16mhz game chips with the music features etc.

A 10mhz MPU is a simple drop in replacement - you will need a different game chip but your reel chip will be fine.

[Bruce G]

Thanks Jay - This board does have a control pot, but it is earlier than the S+ board.  I'm actually unsure if this is just an earlier format.  It doesn't plug into the interface like the later boards.  This has 3 plugs - one down on the side right next to the volume pot.  The crystal is marked 10.00 which I'm assuming is 10kHz.  So what board is this? 

Anyway after obsessing over this thing, I have fixed the problem.  I had to experiment a bit to determine how to even try reducing the noise.  But I was able to use two small capacitors to all but barely eliminate the whine / high pitched buzzing.  I used a 330 pF across pins 1 & 2 and a 224 pF across pins 6 & 8  The noise is barely audible with the door open and ear close to the speaker box.  With door closed, you can't hear it.  The audio is loud and clear, and the chip temperature is the same as before. 

[Tilt]

Yes, that is a S Slot MPU, not an S+.  The S slot MPUs didn't come in two different MPU speeds like Jay was referring to. 

I have one and about all I can tell you is the background noise you experienced isn't normal as mine is quiet when not playing sounds.  Glad you found a fix to the problem and thanks for posting what worked for you.  It may help others down the road.

[Bruce G]

Thanks Tilt- I appreciate hearing about your machine.  I couldn't imagine that IGT would make something with that much noise. 

[knagl]

Bruce,

Agreed with what Tilt said -- you do indeed have an S (non-plus). Glad you found a solution to the noise issue, and thank you for sharing it here.

For your future knowledge, if you ever need to post a video of your machine, we encourage folks to create a YouTube account if they don't already have one (free), upload the video there, and then just post a link to the video here.