Tuesday June 18, 2024

Stern Pinball SB-100 Sound Board

Diagnosing - How Dead Is Your SB-100?
The very first thing to check if you're having sound problems are the solder points where the wire harnesses/connectors plug into the sound board.  So turn off the machine, remove the harnesses & remove the sound card and look at the back of the sound board at the solder points at these connectors.  If any of them look cracked or if in doubt, reflowing the solder points may fix any lockups/sound issues occuring.  If not, then choose your path below..

Is you MPU board locking up when you connect the SB-100?  Skip to the "Diagnosing - Sound Board Locking Up MPU?" below.

Is there a loud THUD when you turn the machine on, followed by what sounds like MANY sound effects playing at once?  Skip to the "Diagnosing - Miscellaneous Troubleshooting" section.

If you don't have any sounds, when you turn the potentiometer to both extremes do you hear a buzz from the speaker?  That means the LM380 power amplifiers should be working and you can eliminate them as the problem.

Do you have some sounds, but are missing other sounds?  Check voltages at the multivibrator test points in the "Diagnosing - Voltage Test Points" section below.

Is there constant white noise that increases/decreases in intensity when you turn the volume potentiometer?  Skip to the "Diagnosing - Miscellaneous Troubleshooting" section.

Diagnosing - Sound Board Locking Up MPU?
If your MPU board is locking up it may be the SB-100 causing the issues.  Think of the SB-100 as another peripheral IC that the CPU is talking to, except instead of being a single chip the "sound chip" is composed of various logic ICs, amplifier ICs, etc. There are data and address lines going from the CPU into the sound card that are shared by other stuff like RAM, ROM, PIA chips, etc.  If there is a short on the board, a bad IC, cold/broken solder joints that allow an address or data line to float, etc... the SB-100 can cause the CPU to lock up or do weird things.

So your MPU is locking up..  first step is to turn off the machine, try disconnecting the wire harness at the sound board's J1 connector and turn the machine back on and off several times.  If the MPU boots, once again connect the sound board harnesses to verify the MPU locks up.  If the sound card is indeed causing the MPU to lock up.. make sure you've checked all voltages as above!  If the the board is missing 5v at TP1 or TP6 then some of the logic chips aren't being powered & can cause logic levels to "float" and cause lock-ups.  If voltages look good, double-check to make sure the harness isn't connected backwards (ie pin1 on sound board goes to pin1 on mpu board).  Try wiggling the connectors while powering on/off the machine to see if that makes a difference.  If not, then you'll need to turn the machine off and pull the SB-100 sound board to examine for cracked/cold solder joints or obvious issues.  It's usually a good idea to reflow any header pins for the 32-pin header on the SB-100 that look suspect.


Diagnosing - Voltage Test Points
The following test points should be checked with a multimeter first before trying anything else.  Some of the ICs on this board are hard to come by or expensive, so why start cutting and hacking when there may be a more systematic approach to finding where the fault is, especially if its in the initial voltages to the boards.

If any or all of the voltages at TP1, TP7, TP8, TP9 are not present you will likely not have any sound.

TP1 - 5v (for ICs u1 thru u5)
TP2 - around 2.6v (if multivibrator is working)
TP3 - around 2.6v (if multivibrator is working)
TP5 - around 2.6v (if multivibrator is working)
TP6 - 5v (for all other 5v logic chips, ICs)
TP7 - 11.5v (for ICs u20, u21)
TP8 - 6.2v (for ICs u10 thru u13)
TP9 - 10v (for ICs u14)

*All voltages will be D/C

Missing Voltages?
If any of these voltages measure low or are non-existent, check for 12v and GND directly at the wires connected to J2 on the SB-100.  There should be around 12v at J2 pins 5 and 6.  The 12v is used to supply power to everything except u1 thru u5 (which are powered via pin 30 on the ribbon cable from the MPU board that hooks to J1 on the SB-100).

CONSIDER SHORTS!  If no voltage is present at TP8, but voltages are present at TP7 and TP9.. then CR2 may be shorted.  This was the case with one of my SB-100 cards and after removing CR2 (a 1n763a diode) it read 0 ohms between both pins.  After replacing CR2 the voltage re-appeared at TP8.

Voltages Present
If voltages are present at TP1, TP6, TP7, TP8 (if board has chimes circuitry) and TP9 then you can now check for voltages at the multivibrators.  Test points TP2, TP3 and TP5 should be roughly 2.6v DC.. this is because the multivibrator output at these test points is a 50% duty cycle.  If your multimeter can measure frequency (Hz) you could also check the frequency here, but you would need to have an idea of what frequency to expect and they will vary depending on the game so easier to measure for 2.6v.  If the test points measure very low (say 100mv or something far less than 2.6v) then it's likely the LM324 IC or 4013 for that particular multivibrator circuit.  These test points should always read around 2.6v as long as the sound board is receiving power, regardless of whether you've started a game or not.

Diagnosing - Checking Frequencies at the 7408 IC (U7)
As long as the sound board's not locking up the MPU, you can continue to pinpoint the issue.  If you have a logic probe and have electronic sounds enabled on the pinball machine (ie. not using the CHIMES circuits) you can quickly eliminate 90% of the components on the board from being the problem if you check for pulses at the U7 IC.  The frequencies from all of the multivibrator circuits feed to the U7 IC.

Using the logic probe, test the output of the 7408 (U7) at pin 3, 6, 8, 11 while the game is in sound test mode or during game mode and hitting switches on the machine.  Again you will most likely need someone else to help press these switches while you hold the logic probe on the pins.  You should see pulses (HIGH/LOW) at these pins depending on what sound was being generated for the switch pressed.

You can also use a multimeter -- it won't show pulsing but you should see the output from the U7 go from 0v to 1-2v for a split second.  Try hitting switches a few times and seeing if the voltage jumps from near 0v to 1-2v.

Pulses / Voltage at U7 Output
If you are seeing voltage or pulses at the U7 output pins corresponding to the tone being played when you press a switch, but still don't have sound, then you have just eliminated a large amount of components as the issue.  Check that U6 pin 6 is LOW since HIGH here would mute the LM380 power amps.  If it is LOW, then check for frequency (if you have Hz on your multimeter) at LM324 Voltage Follower (U18) pin 10 which is the sound input and also pin 8 which is the output.  When a sound is played, the frequency should jump to something like 90-400 Hz for a split second.  If you're hitting switches or the game is in sound test mode and you don't see the frequency changing on pin 8 or pin 10 of u18, but you had pulsing at the U7's output, then the LM324 at U18 may be bad.

No Pulses / Voltage at the U7 Output
If you're not seeing any pulses, then check the input pins for U7.  There are two inputs, one that enables the sound and the other with the frequency.  Check that one of the input pins measures around 2.6v (input from multivibrator) and that there is pulsing on the other input pin when a switch is pressed (or if using the multimeter you should see voltage go from 0v to 1-2v for a split second).  You will need to find the correct pins for the tone being played for the switch you are pressing if you're manually hitting a switch and not in sound test mode.  If both input pins are working as expected but the output pin does not pulse or jump from 0 to 1-2v when a tone is played, then suspect the 7408 and check the U7 pin with the machine off using the diode setting on your multimeter.

Test Point 2
Picture: Measuring Voltage at TP2 shows 2.5v DC (multivibrator shoud be functioning)

Diagnosing - Miscellaneous Troubleshooting / Some Other Helpful Tips
If your SB-100 makes a *very loud* HUM / static noise constantly, but the game sounds are there, the 5 electrolytic capacitors on the board may need to be replaced.  Take note that C47 is a bi-polar 220uf electrolytic capacitor.  The main ones seem to be the 1000uf capacitors but if you'll be replacing those you might as well replace all of the electrolytic caps.

If you wiggle the 32-pin harness and sounds fire, stop working, or it locks the MPU, then most likely you have cold or broken solder joints on the sound board or MPU board.  Pull the boards to examine this.

If you hear a loud *THUD* when you turn the pinball machine on and then what sounds like a whole bunch of sounds all at once, it could be cold/cracked solder joints at the 32-pin connector.  Try wiggling the connector or putting a little tension up or down on it while turning the machine off/on to see if that makes a difference.  If so, pull the board & reflow any cracked header pins.

At power-up U6 pin 6 should very quickly blip HIGH then LOW.  If it stays high then it would cause muting on the LM380 power amps.  You may need someone else to turn the machine on/off while you hold the logic probe on the U6 pin.

If you've selected electronic sounds on the MPU's DIP switches then U2 pin 8 should be HIGH to enable the 7475 ICs that correspond to electronic sounds (U3, U4).  If you've selected chime sounds then U2 pin 6 should be HIGH.  If these don't check out depending on the sound you've selected, then suspect the U2 or U1 ICs and trace the signal back.

You may want to pull the board and with no power going into the board, check all of the U1 through U6 ICs with a multimeter in diode test.  Put the red lead on the GND pin for that IC & check the other pins with the black lead.  Most of the pins should read 0.5-0.6v if the internal circuits are working in the IC.  This isn't always 100% but you may be able to diagnose the issue quicker this way.  This is especially necessary if the MPU board is locking up and not booting at all when the sound board is connected.

To check whether the CPU is even sending any sound data through the wire harness at J1 (or to see if you have a wire harness issue) you can test the input pins on U3, U4, U5.   You could also check inputs to U1, U2 and U6 in this manner.  It may just be a bad connector pin or wire causing an issue.

If you hear speaker "white noise" when the machine is on, then the LM380 power amps are probably fine.  You may hear a click if you try testing voltage at r65 (resistor directly before the LM380 power amps) which would also tell you the LM380 amps should be fine.

If sound is *very* faint and the volume knob does not make a difference when turned in either direction, check the wiring for the potentiomer.  With the machine off use the continuity test between the connector's pins and where the wire connects to the potentiometer.  Also make sure the two wires from the pot are hooked to pins 1 and 3 on the J3 connector.

If all voltages test points on the board check out, you're getting pulsing at the 7808 output pins, the LM324 (U18) has frequency at its output pin 8 when a tone is played, but there are no sounds or "white noise" coming from the speaker and no clicking noise if you test voltage at r65, then the problem should be between U18 and the speaker itself (the very right-hand side of the circuit).

Potentiometer / Frequency Values

As mentioned previously, the potentiometers at r2, r6 & r13 adjust the frequency of the tones.  These appear to have been set at the factory.

Picture: Back of SB-100 Board (r2, r6 and r13 potentiometers)


While I'm not very sure if Stern had set any of these potentiometer values different per game, or if some may just be different because people have messed with them -- these are some values from machines that have passed through.

Nugent (unconfirmed)
r2   - 87.7 ohm
r6   - 2.99k ohm
r13 - 1.99k ohm

Hot Hand (unconfirmed)
r2   - 18.45k ohm
r6   -   2.86k ohm
r13 - 14.09k ohm
TP2 - 307Hz
TP3 - 95Hz
TP5 - 206Hz

I've fixed quite a few of these boards now and they are much less intimidating than they once were.  I'll try and keep this page up to date as I repair the boards.

What I found very interesting is the on-board potentiometers being used to set the frequency of the tones, so even if you have two SB-100s that have the exact same components on them, the tones could be completely different if the sound board potentiometers have been played with by a previous owner.  It actually threw me for a bit as I was repairing an SB-100 board and it didn't sound the same as the previous board I was using but now it makes a lot more sense.  This is also my first time working with a sound board so it was a large learning curve and I really didn't want to take a shot-gun approach to replacing components.  There's almost always a way to break apart a problem into chunks to help narrow down the issue.

Thanks for reading & if you have any comments or if I've mis-stated anything or completely missed the point of a circuit, just comment or let me know via email!

Other Resources

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Stern Nugent Sound Board needed
Posted 05/03/12 1:53PM by nowears
Please let me know when / if you have a stern sb-100 sound board available. My Nugent does not have one.
Re: Anonymous Techdoser
Posted 12/31/11 2:41PM by AceBHound
Hi Bob, contact me via email on the "Contact Us" page on the left-hand navigation menu on this website. We can discuss details further.
Anonymous Techdoser
Posted 12/26/11 4:04PM by Anonymous Techdoser
Hi, Do you have an SB-100 for sale and how much? Would you give a discount for my non-working board? If not, how much to repair mine? Thanks, Bob
Re: Great Info
Posted 12/05/11 6:38PM by AceBHound
I may have an extra SB-100 available soon -- I'll email you. Also for anyone else interested, I will likely have more for sale down the road & entertain repairing some boards for people as time permits if they aren't severely hacked. Repair costs would depend on how "dead" your board is. Email is on my Contact page on this website.
Great Info
Posted 11/03/11 4:43PM by bobcav
This is great info. I'm currently looking for an SB 100 sound board as my Nugent machine has never had a board since I've owned it. Any thoughts on how to acquire one, either in working order or not?
Stern SB 100 sound board
Posted 03/10/11 8:24AM by Anonymous Techdoser
what a great help!
The test point voltages certainly gave me the confidence to make a repair to a Stern "Magic".
I had replaced the chips in their entirety, capacitors, transistors, zenner diode and a couple of blown resistors. Until I found your voltage listing I could not have any confidence in the work I had done. I soon traced the problem to a cracked lead on the circuit board, in the voltage splitter prior to the amp chips.