Hi guys. I'm hopeful you will find the following information useful to fix the distorted audio problems. I have repaired 2 DCS boards (both from TOM machines) with the same symptoms: Crackling and distortion.

Specifically, what I discovered on both of them was a severe imbalance in the +/- voltage rails. You can check the DC voltages at C18 (+) and C19 (-) and the magnitudes should be relatively close to each other (within 0.5v). Note that when measuring the negative supply rail you will be measuring between ground and the negative side of the capacitor as the positive side of the capacitor is tied to ground. Typically what I have found with a misbehaving card is a significant difference in the rail voltages. As you can see in the image below, the positive rail voltage is ~13.5v with the negative rail at ~-5.6v. They should be balanced.

IMG_1887 (resized).jpeg

These rail voltages supply the power amplifier ICs directly. This can cause one half of the waveform to "clip" prematurely (in this case, the negative side) which results in very distorted audio on loud passages. However, the rail voltages are also regulated to +/- 5v at U25 and U26 to supply the TL084 op amps. The 78L05 (+) and 79L05 (-) regulators don't do well when the input voltages get close to the regulated value (+/- 5v in this case). In fact, these regulators will drop out altogether when the input voltages get at or below +/- 5v (which is not uncommon in these boards from my experience). An imbalanced supply to the op amps can cause significant distortion.

In the case of the board in this example, I was powering it via a test setup, using 17v AC from a transformer I had laying around.

IMG_1886 (resized).jpeg

The fix, fortunately, is a simple one. Adding a 470 ohm, 1/2 watt bleeder resistor to the terminals across C20 and C21 (the large filter capacitors), provides a continuous load (~40mA) to keep the rails balanced.

IMG_1888 (resized).jpeg

As you can see in the image below, the magnitude of the rail voltages after the modification are within 0.4v. This modification removed the distortion in both cases.

IMG_1889 (resized).jpeg

One other thing to check while you have the board out. I have seen on one occasion, where electrolyte from a leaky capacitor had somehow worked it's way to the bottom side of the board and started arcing between an adjacent PC board trace and the terminal of C19 (circled in the picture below in red). This caused a lot of popping and cracking as the arcing occurred, and extremely erratic rail supply voltages (even after the resistor modification). Simply cleaning the electrolyte and the area impacted by the arcing fixed the remaining problem on that board.

closeup (resized).jpg

I suspect that there are a number of components (capacitors, resistors, op amps, etc.) that, over time, drift slightly from their nominal values and eventually magnify the impact of the imbalance. I have had excellent success with this simple modification.

I sincerely hope this information is useful and you are able to fix your sound board and get back to a fully working machine.

Best regards,

Jeff

That is great info JeffHecht ... but what is the root cause of the imbalance? Is there a way to fix it by replacing other components?

(I found this thread because I have a Shadow with somewhat sub-par sound... Hard to explain but it does not sound as clear as my other DCS games. I have not done any diagnosing yet and will try this for sure.)

Therein lies the mystery. On the first one of these that I encountered, I replaced a ridiculous number of components (capacitors, amplifier ICs, op amps, regulators, etc.). NOTHING made a difference and the original parts all worked perfectly with the modification. Troubleshooting should be deliberate and logical. If we think about this in that frame, there is likely an unbalanced load on the power supply rails, which manifests itself as an imbalance in the voltages. I would love to be able to go back in time and see what the rails looked like in a brand new board. The nice thing is that with this simple modification, it doesn't put a significant increase in load, but it forces the voltages to stay balanced now, and in the future.

Please let us know how your sound rail voltages look in Shadow and, if you decide to do the modification, if you notice a difference in the quality of sound.

I experimented with this today. I measured the voltages on JD and they are the same on both rails. Pics from DM (fully working) for comparison.

I will try the modification anyway and report back.

IMG20211027200939 (resized).jpgIMG20211027201000 (resized).jpgIMG20211027201424 (resized).jpgIMG20211027201510 (resized).jpg

No change after installing the bleeder resistors, although the voltages are good so I'm not sure what I expected to happen.

I removed the two caps as described on Pinwiki in my Congo.
This removed distortion that was present even after changing all other caps.

Screenshot_20211027-135503_Chrome (resized).jpg

arolden I wouldn't have expected to see any difference considering that your +/- rails are balanced. I would remove the bleeder resistors if you haven't already. Also, as Zigzagzag has mentioned, that modification is more likely to have a positive impact.

I already removed those capacitors as a first step. Unfortunately, no effect.

The odd thing is it seems to be an inconsistent problem. Sound seems to improve after the game is left on for a while. Maybe this is a problem with some of the caps after all. I will replace a few others and see if that makes a difference.

Yes, I have made some progress on this. I realised that the sound improved after the game was on for 10 minutes or so. That led me to believe that the problem was caused by the cold, and as the components heated up, the sound improved. I tested this with some freeze spray today. I sprayed most of the board components and found that sound quality worsened a lot when spraying U20 (AD1851 converter). When I used a heat gun to heat U20 up, the sound improved.

So I am assuming at this point that U20 has somehow failed. I'm going to try reflowing some solder and replacing the caps associated with U20 and if that doesn't work I'll order a replacement. Will keep his thread updated.

Small update. I can't find a copy of WPC-DCS schematics anywhere, so I had to trace the board circuits manually. John's thread here was a great help: https://pinside.com/pinball/forum/topic/repair-log-ngg-wpc-95-av-board

I found that the 79L05 regulator was only putting out -4.8v instead of -5v. I cut the output leg of the regulator and confirmed that the voltage was still low when it was out of circuit. I confirmed the voltage coming into the board was good, so I figured I had a bad -5v regulator. I replaced it, and I now have just above 5v and -5v coming out of each regulator respectively, but the issue still remains.

At this point I am still suspecting the AD1851 so I replaced the small ceramic caps associated with it at C10-13, but this made no difference. I have ordered another AD1851 but it will take a while to get here. Will update again then.

arolden I tracked down a clicking / popping noise on my TZ several years ago and it turned out to be a voltage issue caused by a bad cap that only tested poorly under load. I know this isn't a DCS board, but concepts can still apply. I've got it written up here https://www.aaarpinball.com/TwilightZone/TwilightZone.htm under "Clicking / Popping Sound Problem".

Maybe it’s interesting? Best of luck with your continued search for the root problem!

Hi altan,

Interesting article. Unfortunately I have already replaced all of the electrolytic caps on this board, so the issue lies elsewhere. But thanks for the suggestion!

Currently awaiting some replacement AD1851s to see if they make a difference.

Looks like this issue is solved. The new 1851s arrived yesterday and I just installed one. Sound seems to be all OK now!

IMG20220604193337 (resized).jpg

Hi @arolden,

I'm facing the same trouble with my World Cup Soccer since few months. I have already replaced all electrolytic caps and resoldered all connectors on the audio board.
From what I understand reading quickly, I have to replace the 1851 to solve the problem. Could you confirm?
Here it is a video of the trouble.

Thanks

Yes that's right. Let us know if that fixes it!

Hi
New AD1851 received and replaced but the problem is still here

Bad luck for me
20220729_140315 (resized).jpg

Bu I see on your photo that you alse repalced caps for the AD1851 voltage supply.
Before or after fixing the problem ?

I replaced the caps before I installed the 1851. The caps weren't the problem.

OK thanks for the feedback.
I must investigate further to solve the problem.
Does anybody have an idea?

The only other issue I had was that one of my regulators was starting to fail. Check the voltages being output by the 79L05 and 78L05. Should be -5 and +5 volts respectively.

Your issue seems a little different in that you can still hear the callouts and sound effects. There is just some static in the background.

Double check the connections on the large capacitors if those were replaced. Very common that the through hole plating gets pulled out causing an open there. I've had to rework quite a few boards with that issue.

Yes, I replaced the big capacitor but after the sound issue appeared. I will check connections anyway. Thanks for the tip.

I have double checked big cap soldering. It seems to be ok.
I have pluged an extenal speaker using line out and the sound trouble is also on the speaker. So my problem is not related to the amplification stage.
Surprisingly, the noise sometimes disappears when pushing the volume up. Maybe a cpu volume control matter ?

I had a problem that sounded 100% identical to what arolden was experiencing. Really bad when cold, but after 10-15 minutes got noticeably better. From a warm state (with sound like 80% ok), I tried using freeze spray on U20 (with the machine powered off), and then powered it back on. And the sound was REALLY distorted. But after a couple of minutes it went back to being 80% ok. And I also felt like I could impact the level of distortion when touching U20 with my finger.

So I pulled the board, and reflowed U20. And after putting the board back in the machine, the sound was clean and crisp. YAY!!

Hopefully my problem is fixed now, but if it returns I will update this post.

Thank you to OP and arolden for your descriptions and write-up's. Without those, I would definitely still be debugging my board.

For reference, here is what my problem sounded like:

Good to hear! Glad the information was useful.

Well, it seems I was a little too fast... The problem is still there, although not nearly as severe when cold as before.
Perhaps reflowing heated up something inside the chip itself that made it better, but not perfect. So I am guessing a U20 replacement could still be a good idea?

arolden I don't know if you tested this... But after replacing your U20, dit you try hitting it with freeze spray again, to see if it would affect the sound with the new chip? If you did, and it did not affect the sound, that would be a good pointer for me to also replace my U20.

I didn't try freezing it again after I installed the new one. I'll try and test that on the weekend for you.

If that's possible, it would be amazing. Thank you!