Eliminate Backbox Hum and Buzz

(Compliments of Mike Schudel)

First of all, any modifications you do to your game and CPU/Sound board, you do at your own risk.  Performing this procedure requires removing the CPU/Sound board and some basic soldering.  If you don’t feel comfortable doing this yourself, either leave it alone or have a qualified electronics technician perform the fix for you.  I take no responsibility for damaged boards or personal injury.

Now with that out of the way!

When I first received my NIB Monopoly and finally got it all set up and turned on, one of the first things I immediately heard was a quite obvious humming noise coming from the backbox.  The humming is a low frequency hum which also accompanied a higher buzzing noise that increased and decreased in pattern with the playfield lighting in attract mode.  At first I thought I had a bad transformer, or possibly a faulty ballast for the fluorescent light in the backbox.  Well, everything checked out ok.  It wasn’t until I happened to lean close to the backbox that I realized the noise was coming from the speakers.  I followed the wiring from the speakers to the backbox and saw connector CN4 on the left side of the CPU/Sound board.  I disconnected the connector and the game fell silent.  So I knew there was something in the sound circuit that was causing the humming and buzzing noise.

I called Stern and talked to Chas whom is their technical support person there and he explained to me what is causing the noise and how to fix it.  I have made the fix to my own pin and since it worked like he said, I thought I would pass it on to anyone interested in fixing their machine.  I know some people are purists about their machines and would never modify it in any way.  If I had to listen to that hum for one more day I would go crazy.

 Chas stated that there are two portions that make up the amplification of the sound for the game.  The first is the digital portion which is the level you can adjust via the three buttons on the back of the coin door.  The second portion is the analog amplification and this is fixed from the factory.  Since these pins are usually set in noisy arcades, the analog sound level is cranked way up to compete with all the ambient noise.  Since most of you who are reading this have them in your homes, the sound does not have to be up quite so loud.  Straight from the box, I had the digital volume set on level 1.  When I set the level to 2, it was too loud for other people in the Family Room.  There was no fine adjustment in between these two levels, so I was stuck.

Basically what this repair does is turn down the amplification of the analog circuit which will both eliminate the buzzing and give finer volume adjustments using the digital volume controls.

In your Monopoly manual on page 125 there is a schematic of the sound amplifier circuit.  The right side of the page shows Speaker Outputs CN4 pins 1-7.  Following pin 3 back is a 22K Ohm resistor R106.  This controls the gain of the amp for backbox speakers.  Similarly, back from pin 6 there is another 22K Ohm resistor R110 which controls the amp gain for the cabinet speaker. What this fix will do is reduce the resistance value of these two resistors.  The lower the value, the lower the amplification.  But the next question is what combination of resistor values works?  Well, this is where I have done the experimenting for you.  As it turns out a value between 3.8K Ohms and 4.7K Ohms works perfectly.

Now, there are two ways to change this resistance.  The first is to remove both R106 and R110 an solder in a ¼ watt, 5%, resistor between 3.8K Ohms and 4.7K Ohms.  But this seemed like too much work and way too much room to screw something up on the CPU/Sound board.  Not to mention that you would have to bend the resistor leads push them through the solder holes, providing you could clear the old solder out first.  If you feel comfortable doing this, it will still work, but I have an easier method.

The second and easier method involves adding resistance in parallel to the existing 22K Ohm resistor effectively reducing the combined total resistance.  The formula for determining parallel resistance, that I use, is 1/R1 + 1/R2 = 1/Rt.  R1 is the 22K Ohm Resistor, R2 is the new resistor value that we need to add, and Rt is the total combined resistance of between 3.8K Ohms and 4.7K Ohms.  Without getting into all the algebra, Rt vales fall between 4.7K Ohms and 6K Ohms respectively.  So if we buy a ¼ watt, 5% tolerance resistor between 4.7K Ohms and 6K Ohms and solder it in parallel to the existing resistors, we will accomplish the same thing as removing the existing resistor and only soldering in one.  Remember, the lower the resistance the less amplification.  So the 4.7K Ohm resistor will reduce the output volume more than the 6K Ohm will.  I had originally soldered in a 1K Ohm resistor and had to crank the digital volume to max to get it loud enough.

In my machine I ended up using a 4.7K Ohm, ¼ watt, 5% resistor I bought from Radio Shack which came in a 5 pack for .69 cents.  We will be using only two resistors, 1 to solder in parallel across R106 and 1 to solder in parallel across R110

Basically what you need to do is unplug the pin, remove the fluorescent lamp and remove the CPU/sound board from the backbox by removing all the connectors from around the edge of the board.  Be sure you have dissipated any static in you body by touching the ground braid that runs all over the pin or some of the metal chassis of the game.  Loosen the screws and lift to remove the board.  Find R106 and R110 located near the left center of the board.  Flip the board over and solder 1 of the 4.7K Ohm across existing resistor R106 and another 4.7K Ohm resistor across existing resistor R110 on the solder side of the board.  There is no polarity to a resistor so you don’t need to worry which way the resistor goes in.  Be sure and then make sure again you are soldering across the correct resistor!!!  Basically you should end up with a resistor in the same spot as R106 and R110 but on the solder side of the board.  Be careful soldering and trim the excess leads before installing the board.   I was surprised to see other small components soldered to this side of the board as well from the factory!

Install the board, tighten the mounting screws and reconnect all the connectors around the edges of the board.  Remember to reinstall the fluorescent lamp and plug the game back in.  It actually took me a half hour to perform this fix and all I had was some basic soldering skills.

When you turn the game on the buzzing is totally eliminated.  You will have to turn up the digital volume of the game by using the left and then the left and center buttons behind the coin door to increase the digital volume. After the modification there is more flexibility in volume, I have it on 6 right now and I can increase and decrease in much smaller increments.

And that’s it...total peace and quiet!

Personally, I think Stern should change this resistor value on their pin games.  Even with the analog amplifier turned down, you can still crank it up loud enough using the digital controls that it could be heard in an arcade.

Enjoy your Monopoly pin, and I hoped this helped some of you eliminate that annoying humming.

 Mike Schudel

Monopoly #161072 (11-28-2001)

Photo Compliments of Todd George