I have this strange issue happening on my HS. Once I switch the power on, everything seems fine in attract mode (with a low hum that's not too unusual). But, I then hear some sort of power surge ramping up at exactly 1 minute and 30 seconds. Don't know how important that is, but it is consistently at that time period. The low hum through the speaker suddenly (and rapidly) surges up at that exact time.

If you can turn your volume up, I think you can probably hear it in this video of the final 10 seconds; for obvious reasons, I shut it off before it gets too far:

I'm also attaching a picture of my power supply board. I've already tested the +12, +5 and ground going to the MPU, which all checked out fine.

I don't know how common this is, but I can't find any other posts that mention a surge at this specific timeframe. I'm rather new at this, but would love any feedback where to start troubleshooting this problem. Thanks so much.

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That would certainly make sense about the failing caps, as I've been told to replace the ones on the power supply already. I'm attaching a new photo of my sound card, and it looks like the only two on the board are the 35v 100uf cap (black in the middle) and a 25v 10uf (grey on the left). Assuming these are the ones we're talking about, right? Will get all the replacement caps ordered tonight.

I did check/secure the screws on my boards to minimize ground loop, @ThePinballCo-op, and have the speaker wire isolated from any others. Oddly, it seemed to make the hum louder, but that could just be me... And, yes, @slochar, it was functioning normally earlier. I haven't run the tests recently but will also trouble shoot those areas you recommended.

Thanks for the suggestions, everyone...I'll update as soon as I get the caps replaced.

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Forgot to add this part (rather important)...I'm working with a Pin-Bot MPU (11a) board in the High Speed (11) game. When I put the old acid-damaged board back in, the hum "surge" went away. However, that board has other problems (target lights don't work; stop light doesn't work).

So...I'd like to use the 11a board but am trying to determine if the "surge" is coming from bad caps or if it has something to do with my set up. I didn't make any modifications, but just plugged everything back in where it would typically be (and this Pin-Bot board clearly 1J15 already)...I have speakers @ 1J15; volume control on the sound card, and 1J16 connected back up to the sound card. I'm getting ready to re-cap, but wanted to see if I just missed something.

Unplugged and moved the volume control from 11J2 to 1J16, but still have the "surge" sound.

When trying to diagnose this problem, another popped up (go figure). All of the controlled playfield lamps just aren't lighting. None are lighting at all (aside from a couple in the backbox). I've checked the fuse, but see nothing burnt out. I've tried the test mode, and "All Lamps" doesn't light any of the controlled lamps. "Single Lamps" test is no better, just displaying "Game Over" on the display. So, any idea where to look first when all of the controlled lamps aren't working?

I've swapped the board several times and triple-checked the connections. Nothing behind the board.

Completely disconnected the "sound board" from the system and am still having the problem.

So...I think we've isolated the "humming surge" problem to the 11a MPU, as you've noted @pintec. After ruling out the audio board, I plugged the old MPU back into the HS, ran few music/audio tests, and the surge went away. So, I've ordered new caps and amplifier for the 11a board and will wait.

In regards to the controlled lamps, it must have been a loose connection somewhere, because after I went in and re-sat everything, they all seemed to come back (go figure).

But, in the meantime, I've a High Speed machine back up and running! I'll come back and update this post once I get the new amp/caps in and tested. Thanks everyone for your help with this one!

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I may have taken a victory lap a bit too soon...After about 2 days, the music suddenly went out. My understanding is that the background music comes from the sound card (U4 B/G Sound ROM), through the 20-pin ribbon cable, to the MPU. Is this correct? Or does it come through the 1J16? Because it worked before, I'm trying to determine where the glitch might be coming from...MPU starts with a single ping. So, leaving the HS in MUSIC TEST mode:

- I've switched the ribbon cable (verifying the #1 pins alighted), but still no music. (However...I've had this strange situation where after unplugging it one of the times, some remaining music briefly played or played distorted, while unplugged).

- I also noticed that the 20-pin connection on the sound card seems to not be seated all the way down. When I took the board off, and looked at the other side, this seems to be original and not something that has moved over time (again, it was working for quite some time before). Still...Seems like a logical place to start, right?

Ruling out the ribbon cable, I'm trying to determine if the music issue is a sound card issue or an MPU issue.

Any guidance of suggestions would be hugely helpful. As always, thanks for your input.

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So, theoretically, the music could be coming through fine via the 4-pin, but could potentially be halted/stopped via the ribbon cable communication to the MPU. Correct? If so, I would imagine a process of elimination first. (Again, forgive my newness to this, but I still have some basic questions)...Using my DMM, could you please tell me how to test the four pin against the schematics uploaded to verify that the music coming to the MPU is not the issue? Then, same question via the ribbon cable. Appreciate any input or advice. Thanks!

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Perhaps "ruled out" might be a bit overly confirmed. I switched the ribbon cable with another and had the same results; I suppose there could potentially be two defunct ribbon cables. So, is the gas tight seal only needed on the 20-pin connection (and not the 4-pin)? Is there any way to test it prior to removing/replacing the 20-pin connectors on the sound card (and possibly the MPU)? Also, looking at the picture, do you think that gap under the 20-pin on the sound board could potentially be causing the problem? Thanks again!

If the gap is not a problem, I'm confused where the gas tight seal is actually needed (at the 20-pin to the ribbon cable?).

Also, if I understand your post earlier regarding the path of audio, the method to "look for analog audio with a scope being spit out of the converters" is on the MPU and not the sound card (upstream). Correct?

(Quick background: This MPU has recently come back from a reputable technician who repaired some tracing issues and told me everything was tested and working...and I do show a 0 at U46 and single ping at startup. My concern is that this was not a problem prior to sending the MPU out for repair...So, I'm trying to see if it's the sound card or the MPU)

Regarding the oscilloscope, I'm still learning the DMM, so it's really a question of how frequently I'll use the scope (not to mention the price). Can you possibly recommend an inexpensive one for beginners?

Excellent! I'll certainly be reading about gas-tight Molex connections today (thanks for sending the link). And, yes, I have all other sounds/speech, but am simply missing all of the music. Interestingly, I had it working for about 24-hours on this machine and MPU, then it suddenly stopped (nothing else was changed in that timeframe).

With the oscilloscope, the JYE Tech DSO150 seems to be a real basic digital one for about $30-$35. Should this do the job for basic audio troubleshooting?
ebay.com link: itm

So...I'm somewhat following, but am clearly getting lost. I've uploaded a higher-res photo of the schematics (highlighted areas mentioned). Here's what I understand: Music comes in to the Peripheral Interface Adapter (PIA) U9 to the Digital to Analog Converter (DAC) U2...I think it follows out IO4 from U2 and heads to the Sound/Speed Mixer. From there, I'm not exactly tracking how it then moves out IJ16 pin 1 (volume control?) to the sound board then back to the MPU (through IJ16 pin 2?), until it leaves the speaker at IJ15. Am I anywhere close to correct?

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That was it! The power surge problem on the 11a board came down to faulty capacitors. Changed them and that specific board (and machine) are working smoothly. Still trying to figure out the missing background music issue on the original High Speed (11) board, and hoping the oscilloscope coming at the end of the week will put me on the right path. As always, any advice is gratefully accepted.