Welcome to Pinside

First up, what game do you have and what's the actual sound problem you're experiencing?

When you adjust the volume control in the cabinet, do you hear any noise/hum from the speaker?

Your Test Point voltage levels look ok.
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Yes, High/Low means the signal is alternating (pulsing) between high and low.
If the game is trying to activate the sound you're probing, the logic probe should show both High and Low LEDs on/flickering.

U18 (a LM324 Operational-Amp) operates as a mixer/preamp on this sound board. A LM324 has 4 internal op-omps, and the fourth op-amp on pins 12, 13 and 14 are not used on this circuit board.
U18 pin 8 should be connected to pin 9 and also to resistor R88 where it then goes to U14 which is a volume attenuator (electronic volume).

If you're able, post clear pictures front and back of the sound board.

Note, in order for stability and to dissipate heat away from the two LM380 amplifiers, they need to be soldered direct to the board (not socketed). I'd also suggest trying to make mock up copper heatsinks (cut/flatten some pieces of copper piping if you have any).
You can probably hold off on restoring the amplifier setup until you get the sound working.

I've got to head out, but in the mean time there's some useful diagnosis in this thread that might help until I get back later to continue:
https://pinside.com/pinball/forum/topic/tech-hot-hand-sb-100-no-sound

Nice, those copper fins should work fine.

The pins on chips are numbered in circle - see below: Do you know how to identify pin 1?
You can see pins 12, 13 and 14 are not connected which is ok and pin 8 connected to pin 9 as per the schematic.
You might want to resolder pin 1 to make sure the solder isn't shorting to the trace next to it.

When you get the board put back together and in the machine can you do the following:
Using the logic probe can you tell me the readings at the following points to see if the sound board is receiving communication from the MPU board properly:
U2 pins 2, 9, 10, 11
U1 pins 2, 6, 11, 12

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SB-100_U18a.jpg

Great find, well done! and nice work on the extension cables

BTW, I see a lot of classic Sterns on YouTube with poorly tuned SB-100 sound boards. The rev C-1 SB-100 boards have the tones keyed low so the melodies are in tune. You can check and adjust if you like.

Open this online piano keyboard here:
https://web.archive.org/web/20191109111921/http://piano-player.info/

The following keys closely match the tones that the SB100 Rev-C-1 board was tuned to.

Keep hitting a 10 point score and adjust the R2 pot on the sound board until the frequency matches the Black piano key 15 (note G#3) [208Hz]

Keep hitting a 100 point score and adjust the R13 pot on the sound board until the frequency matches the White piano key 20 (note F3) [175Hz]

Keep hitting a 1000 point score and adjust the R6 pot on the sound board until the frequency matches the Black piano key 12 (note C#3) [139Hz]

Nice. That will help endurance/stability of the LM380 amp chips and reduce thermals to minimise internal distortion.
You should be good to go

What issues are you guys having with the Alltek MPU and the Stern sound boards?

If TP2, TP3 and TP5 are measuring mid voltage and you hear faint sounds, then you need to look downstream at where the sounds are mixed (U18 LM324), the volume attenuator (U14 MC3340) or the amplifier circuit (U20/U21 LM380's).

Are you measuring 10V power at pin 8 of U14 and 12V power at pin 14 of the LM380 chips?

Check that the mute signal (pin 1) at the LM380 amplifiers is around 7.3 volts.

You can try bypassing the U14 volume attenuator (MC3340) if you don't have spares by removing the chip and installing a 10k ohm resistor between U14 PCB pin pads 1 to 7. The 10k resistor should give you loud enough volume as a test. Note removing U14 disconnects the cabinet volume control, so adjusting the volume will have no effect..

Could also be one of the LM380 amp chips is dead. What DC voltage do you measure on output pins 8 of U20 and U21 when the game is in attract (quiet) mode? They should probably be somewhere near 6 volts.

Ok, with volume turned up, start a game.
This time measure the AC voltage at the LM380 amp chips on output pins 8 of U20 and U21 to get an idea of the output amplitude of each amplifier. Need to do this while the game should be playing sounds i.e. keep pressing a score switch.

Is your multi-meter one of the below?

Can you try the measurement again but this time swap the leads around.

MultiMeter_Yellow.jpg

That's better, on the presumption the amp chips aren't receiving enough audio input.
Are you still getting very faint tones when the game should be playing sound?

If you have a logic probe set it to TTL and pulse mode, and confirm the tone generators are being properly selected.
In Solenoid Test mode when sounds are being activated at test number:
20: U7 pin 3 should change from Low to Low/High/Pulsing for a quarter of a second (10 point sound)
21: U7 pin 6 should change from Low to Low/High/Pulsing for a quarter of a second (100 point sound)
22: U7 pin 8 should change from Low to Low/High/Pulsing for a quarter of a second (1,000 point sound)
23: U7 pin 11 should change from Low to Low/High/Pulsing for a quarter of a second (10,000 point sound)

If your board is exhibiting this behavior with a logic probe, then it's time to bypass the U14 volume attenuator as mentioned above in post #22.

I wonder if you have connector issues with the ribbon cable.

Another thing to try when you get a chance, grab a pointy screw driver and hold it by the metal part. If you touch the pointy end on the input pin 2 of the U20 amp, do you get any noticable hum from the speaker? This will partly indicate if the amps are working.
If pin 2 of U20 is hard to get to, look for resistor R65 to the left of U21, and touch the screw driver on either of the resistors legs.

The logic probe indications I listed in post #35 were incorrect and they've been corrected.
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The volume attenuator MC3340 chip (U14) is a common failure point.
If you have a spare 10k ohm resistor, try bypassing the volume attenuator. Unsolder pin 7 of U14 so that the pin floats in the middle of the PCB hole (make sure the pin is disconnected from the solder pad). Solder one side of the 10k ohm resistor to pin 1 of U14 and the other side of the resistor to the top pad of resistor R19 (R19 is located bottom right of the board).

Do you now get any sounds?

What test tools have you got? i.e. multimeter, logic probe, oscilloscope?

Have you measured voltages at all the test points on the SB100 sound board and if yes can you list them?

Any chance you can post a picture of your multimeter or a web link showing which it is? Just makes it easier to tell you how to set it.

The sound board has a number of test points marked TP1 through to TP9 scattered around the board.
The first step in diagnosis is to measure the voltages at the test points. Set your multimeter to DC voltage. If your meter is not auto-ranging set it to the 20V scale.

Connect the black meter lead to test point TP4. This is the ground test point on the board. Use the red meter lead to measure the voltages on all other test points on the sound board. Report what you find.

For reference, below is the clearest scan of the sound board schematic I've come across on the web.

Stern_SB-100.png

No voltage at TP1.. This is meant to have 5V power for the logic chips on the sound board.
Most likely that purple multi-wire connector is hooked up incorrectly in one side. Pin 1 of the connector on the MPU board is on the right, pin 32 is on the left. From memory pin 1 on that sound board is also on its right side and pin 32 on the left. So make sure that cable is connected so the matching pin numbers mate.

What exactly has changed between you having 5V at TP1 and now not having any voltage at TP1?

Remove the sound board and have a good look at the soldering on the pins of the J1 connector on the back of the board. Some of those pins might have fractured soldering which you'll need to resolder.
If you can, post a clear high res picture showing the soldering on that connector before you resolder any pins for our curiosity. Also a picture showing the actual pins in case they're badly tarnished/corroded which might be giving you poor connectivity. Check the 32 pin J5 connector on the MPU board too for tarnished/corroded pins (though your MPU board does otherwise look in decent condition).

You mean the dark spot above the middle of the long connector? I can't tell if the trace is burnt, or whether it's just solder rosin. Can you check and comment?

Are you measuring 5 volts at the TP1 test point on the sound board now?

BTW, I can't quite tell from your pictures but the J5 pin header on the MPU board doesn't look in good shape (tarnished).

Cool. If the tin plating fell off and exposed the copper/brass on some pins from the wire brush, then longer term the problem might come back at which point you'll probably have to replace that J5 pin header on the MPU board. If you're not equipped to do it, just leave it and see how it goes.

You've confirmed the speaker is ok? Does it measure 8 ohms?

What test tools do you have? Multi-meter, logic-probe, oscilloscope?

An oscilloscope is your friend with analog audio circuits!

But it will get tricky if you don't have the game with you.

barakandl , do you remember what happens if the volume pot is open circuit/disconnected? Does the sound board go to maximum volume or dead quiet?

Indeed, it's across the 12V supply rail from the SDB. It will eventually blow up like that but it's not the problem with the sound board being silent.