I'm aware that I'm getting older, but earmuffs should not be required equipment to work on a pinball machine. My Apollo 13 volume is way to loud. In the settings I can adjust the volume, 0 = no sound, 1 = super loud and it does get louder. I can adjust the volume numerically higher and it gets louder but it acts like the base volume is just starting way to high.

Board 520-5136-00 Rev. D - So no need to update OS
Originally the sound had a lot of static so I replaced U26 (TDA1543) which cleared that up.
I replaced the U30 (LM833) with no change.
I replaced R110 and R106 with 33k and 10k resistors respectively.
Checked R14 & R9 (both 22K)
Checked R21 & R22 (both 100K)
Checked R16 & R17 (both 2K)

So with that I've about exhausted what I know to do. I have got some Caps coming but I don't think that's it. Is there supposed to be a Pot somewhere by chance that I'm missing?

Swapped out a different CPU/Sound board and got the same results, volume extremely loud.

So I don't see any Pots on the schematic, so there goes that idea. Speakers look to be original and checked at 3 Ohm (at least the backbox) Not sure what the they should be but I doubt they've been swapped. 3 seem a bit low to be honest so I'll look into that some more.

Voltage is fed from VR1 and VR2 which both look good, -5.4VDC and +5.4VDC.

I decided to check the Decibel reading just to get a frame of reference: 80 Db at ear level while on volume 1. Granted it is just through one of those free apps, but still pretty loud.

I decided to check continuity from CN4 to each of the speakers and found my Left speaker was not getting any signal, eventually found a severely corroded connector where the Backbox speakers connect into the CN4 wiring harness. Repinned and put on a new Molex connector. Now it's louder... awesome.

While that doesn't solve the issue, at least it solved a problem I found.

Since the wiring is all checked out as good, I'm back to the board. The only common factor now is the I/O but the only thing that comes from it as it relates to sound is the voltages.

Did a Factory Reset, no change unfortunately.

On the up side, I've read and learned more about Bias Currents, Op Amps and Bias Current Gain then I'd ever anticipated.

So after a few days of working on other projects I revisited the subject.

Going back to the speakers, I ohmed out the 3 speakers ( two in the backbox, 1 in the cab) while they where disconnected from CN4.

3.1 ohms - Left Data East 031-5004-00
3.2 ohms - Right Data East 031-5004-00
3.6 ohms - Cab No label

From what I can tell these should be 4 ohm speakers, and possibly 8 ohm in the cab? At any rate, I don't know if a 1 ohm drop in resistance will equate to the volume levels I'm hearing, but it is almost 25% drop so it probably isn't helping. I'll probably change them out and see where that gets me.

supermoot The struggle is real! Misery loves company right

So, as expected the speaker change out didn't affect anything in a tangible way. I didn't bother changing out the cabinet speaker as I was lazy and I didn't actually purchase one

Left speaker 3.9 ohm after the change
Right speaker 3.8 ohm after the change

The Db reading went from 80 to 76 but that could be any number of factors so it is of no real importance but was fun to mess with anyway.

It is possible that both CPU/Sound boards have the same issue, and since I've rang out everything on the cabinet that I know of I"ll go back to the board.
I read the datasheet for the TDA1543 a gagillion times and tried to understand what the following sentence means.

"The output current of the DAC is a sink current. The current Iref at the Vref output is adjusted by a resistor or a current source. The current Iref is amplified with gain AIbias to the bias currents (IBL; IBR) which are added to the output currents"

So if say there was an issue with R16, R17, C48 or C50 that would then be added to the current coming from the TDA1543 and going to the Op amp, thus making it louder. R16, and R17 checked out good earlier so that leaves C48 or C50 as a possible problem. But the odds of both of those caps being bad bad on two separate boards seems suspect.

So, maybe C33 which affects the Vref line directly is the problem since it affects the input to both channels? R7 checks out so that's not the problem.

We will be changing out some more Caps next it looks like. I give it 60/40 odds of failing so feel free to chime in with ideas once this fails.

Problem Solved!

The Short:
Faulty TDA1543 - Yes, the new one I just put in to solve the static noise

The Long:
Board 1 - Original TDA1543
Board 2 - Replacement TDA 1543 - The one I've been focusing on

I took all the Sound/Game ROMS/BSMT/RAM from Board 2 into Board 1 and ran Speaker Test to verify that the same issue on both boards and get a Db reading. During Speaker test it the Sound roms would fluctuate between very loud and normal volume. Test U21 - Loud, test U17 - Normal, back to U21 - Normal, and back and forth with the loud then normal volume. I power cycled the machine and retested 4 more times with all tests being at normal volume on Speaker Test.

I then put all the chips back onto Board 2 and took the TDA1543 from Board 1 and sound worked without issue. My assumption is that the Loud then Normal volume level during tests is possibly a failing TDA1543?? Either way that's the issue.

I purchased 3 TDA1543s from Marcos at different orders but I'm sure they are all part of the same batch. All 3 caused super loud volume in whichever board I put it in. Edit: They all have the same batch/date code.

With that, I'm off to troubleshooting a U7 issue on Board 1, it's like it's out of tune, but for now I can play me some pinball... without earmuffs.