barakandl might know.

you could power the board and see that the square wave generator circuits work and you could introduce some noise through the amplifiers, but the way the way the sound board is right on the MPU's CPU address/data bus you cant really ground an a pin and get a sound a like you can with an early WMS board. So I think it is best to test the sb100 and sb300 sound board attached to a mpu. typically that means mounted in a game.

The LM324 + 4013 circuits seem to cause the most problems in the sb100. test points 2, 3, and 5 should adjust the square wave/frequency/average voltage reading when you twizzle the trim pots.

cheating the 7th flash might not be good enough. I am not sure if the software will run enough to get to solenoid/sound test without the zero cross detector working. The game has to see a zero cross transition to interrupt the CPU into check and perform sound/solenoid stuff.

It works fine with cheating the 7th flash. I've bench tested SB-100 sound boards with no problem.

You'd have to modify the ROMs to do that. The sound board is addressed differently to driving solenoid based chimes.

What's wrong with this SB-100 sound board?

Board #1 - If you have 2.5V at pin 3 of U8, then U8 is suspect, otherwise U17 is suspect.

Board #2 - Check for 10V at TP9. Check for 7V at pin 1 of either U20 or U21. If all ok, then U14 (Audio attenuator), U18 (audio mixer) or U20/U21 (final output amps) are suspect.

They're both easy repairers, but look at board #1 first.

If you get the second one working, I'd buy it off you.
been looking for a working one for a while.

Before doing the mod, try and adjust the tones down to less piercing frequencies. I've previously posted on SB100 repair posts on Pinside about using an online piano as a frequency reference for tuning these boards. I'll have to hunt through my old posts if you want it.

The frequencies in that link were to reset the tones to high pitch for the original Rev A SB-100 boards used in Lectronamo and Wild Fyre. Those frequencies might get a bit grating/annoying after a while, hence the reason why Stern lowered the frequencies on later games and the service bulletin/mod.

This link is how they are tuned on my Hot Hand which has the Rev C-1 board - the frequencies are about the same as the mod on the pinwiki link and are what Magic and probably Trident are tuned to:
https://pinside.com/pinball/forum/topic/sb-100-no-sound#post-3966433

If you can't get the frequencies this low then just scale them down as far as you can go - essentially they are musically keyed.

CR1, R91 and C52 are missing because rev B boards don't have the chime simulation section.

I don't have a board handy to check but I presume the extra resistor and capacitor might be sitting on the reset line to effectively delay the release of the output amplifiers mute pin so you don't get any noises/squelches on power-up.

The reset line from the MPU board starts active low on powerup then goes high once 12V and 5V have reached approx nominal levels.
The sound board uses this reset line via U6 to mute the two amplifier chips on power up to prevent any abnormal noises.

The release of the reset line is extremely quick, and probably deactivates the mute too quickly on the Rev A boards. I'm presuming the resistor/capacitor is a simple RC network to delay the release of the reset line at the U6 74107 chip so it effectively holds the mute active for longer. But I'm guessing - would need to see more pictures front and back of the board to confirm how these components are wired in order to add it to the schematics.

In the mean time with the board back in the machine and in a locked up state what logic level are you seeing on the reset line? (measure at pin 40 of the CPU should suffice).
If you manually short pin 39 to pin 40 of the CPU (this reactivates /RESET) does the game then boot?

Great stuff!
I've only had a very quick look at your pics, but I'll modify the SB100 schematic later with the mods and post it here. In case there's still a piece to the puzzle missing I'll come back to you.

BTW in the middle picture, the solder joint to the left of your red circle looks a little fractured - you might want to resolder it.

Cheers.

You can easily bypass the U14 volume attenuator by disconnecting and lifting the leg of resistor R19 that's connected to pin 7 of U14 (output pin). Connect a spare resistor 10k - 22k ohms between the lifted leg of R19 and pin 1 of U14 (input pin). If you get audio, U14 is suspect, or connection to the volume control is suspect.

A simple basic test of the output amplifiers: grab a metal screwdriver. Hold the metal part with your hand and place the tip of the screwdriver on the input to the final amplifiers which is the trace running between C54, R65, C39 and pin 2 of U20. You should get noise/hum. If you don't, the output amps are suspect.

Can you verify the value of the extra tantalum capacitor on the reset line? In post #19 you mention 4.7uF, but in post #25 you mention 3.3uF

Secondly, when I was repairing one of these boards a while ago, I found the schematic is missing a link connecting 5V to the trace between C18, R24 and the emitter pin of transistor Q1. Unfortunately I didn't note if the trace was connected to 5V from VDD (TP6) or VCC (TP1). Can you verify which 5V source it's connected to?

Thanks, can you do me one more favor and check the continuity from pin 14 of U19 to TP1 and TP6 and tell me which TP it's connected to?

Looking at the pic below from another thread (click to zoom in), the lower solder joint of R24 connects to the "VCC" trace running just above it. If you still have your board out, can you confirm? Looks to me like the other the replica transistor circuits for Q2, Q3, Q4 and Q5 used for the chime simulation (to the left of Q1) also have that respective point connected to VCC - these links are also missing from the schematic. I mention "VCC" here only because the schems say pin 14 of U19 is VCC, but I'm finding it hard to trust the schematics. It could be VDD.

Oh another error on the schematic, U6 is shown with two pin 13's - the Q output of U6 is really pin 3, not 13.
SB100_Q1x200_mod.jpg