Stern Pinball SB-100 Sound Board

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..

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.

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.

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.

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.

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 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.

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).

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.