(Topic ID: 291174)

Black Knight speaker hum (SOLVED), lots of tricks tried, no cure found yet

By Coyohtay

6 months ago


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Topic Stats

  • 15 posts
  • 9 Pinsiders participating
  • Latest reply 6 months ago by Coyohtay
  • Topic is favorited by 1 Pinsider

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#1 6 months ago

I am working on bringing a Black Knight back to life. I ended up replacing the sound board with one from Swemmer Electronics

" WSM147-4-20, Williams System 6-7 Sound w/Speech Module, D-8224 w/ROMs from Swemmer Electronics "

I have the game completely working but I am chasing an annoying speaker hum. This sound card is fed AC voltage directly from the transformer (18.7 VAC according to the BK manual). I tried every solution that I could dig up from the internet, floated the sound board, disconnected all the other components leaving only the sound card powered, new volume potentiometer, new speaker, new wires and connectors for the pot and speakers, etc. It still hums

I even pulled the sound card and powered it up on the bench with a new 18 VAC transformer to completely isolate it from the machine.  Even on the bench, it hums. I tried moving all the components far away from each other to make sure that wasn't the issue and no dice.

Does anyone out there have some technical insight on this issue or experience with the Swemmer board?  Is the board maybe defective? Are there any other solutions out there that anyone knows of that I can try?

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#2 6 months ago

First of all, even thought the board takes AC power, the components in the amplifier run on DC. There is onboard rectification taking place. Check the DC power for proper level and AC ripple. There's a good chance filtering components are faulty, such as those electrolytics. Recapping the board will solve that, if that is the cause. Other things could be at fault such as the final amp components or DC blocking caps, but start with the power supply onboard.

#3 6 months ago

I'd say the power regulation capacitors are shot, which must be common since a number of places sell the replacements kits. Arcade's posting says it is 10 capacitors. Have a look at TP1,2,3 & 4. Use TP3 as your 'ground' and then measure to TP1 should be +12vdc, TP2 should be -12vdc and TP4 should be -+5vdc. If you have a scope it should be clean on each of the three TPs.

A smaller kit is available at KSarcade, but here's the link to the 'full' kit at Arcade:

https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-kits/williams-system-6-7-sound-speech-pcb-cap-kit/

Description:

Williams System 6 & 7 Sound / Speech PCB 105C 100% Axial Cap Kit. This Williams sound and speech board cap kit works on Pinball games such as ALGAR, ALIEN POKER, BARRACORA, BLACK KNIGHT, BLACKOUT, COSMIC GUNFIGHT, DEFENDER Pinball, FIREPOWER, FIREPOWER 2, GORGAR, HYPERBALL, JOUST (Williams), JUNGLE LORD, LASER BALL, LASERCUE, PHARAOH, SCORPION, SOLAR FIRE, STAR LIGHT, TIME FANTASY, TIME WARP, TRI ZONE, VARKON, WARLOK, and any other williams system 6 and 7 pinball games with Type 2 sound and speech pcb that use the D-8224 sound and C8228 speech pcb (1C-2001-137-4). “Mega Deluxe Cap Kit” All capacitors are 105c Axial in this kit. All of our Cap Kits / Get Well Kits include a sticker so you can place it on the PCB to help you keep track of the work you did and when it was done. 10 Capacitors total included in kit with great instructions. Our kits have some of the BEST quality, low ESR, and longest life capacitors in them. NOTE: If your sound PCB has a 4700uf capacitor at C27 than use our D-8224-3006 Kit and if your C27 has a 12,000uf capacitor and its a type 2 1C-2001-137-4 PCB than use this kit. We also have the Type 1 kit which only uses a single PCB and no separate speech pcb like the type 2.

#4 6 months ago

My BK had quite a bit of hum when I got it. I did replace all the sound board caps and am running a new xpin power supply board, but was never able to totally get rid of it until I went with an external amp and bypassed the sound board one...

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#5 6 months ago

If this is a new board you should contact Swemmer directly.

#6 6 months ago
Quoted from algum123:

My BK had quite a bit of hum when I got it. I did replace all the sound board caps and am running a new xpin power supply board, but was never able to totally get rid of it until I went with an external amp and bypassed the sound board one...
[quoted image]

Those are great little audio amps for the price. I used one on my RFM when I converted it to a standard PC.

#7 6 months ago

II finally cured it! I noticed the test points were all voltages that can be got from a PC power supply (+12vdc, -12vdc, +5vdc and Gnd. I grabbed a ps I had laying around and it worked like a champ, essentially I bypassed the Ac/Dc conversion portion and fed DC right into it. A 7 pin IDC connector with alternating pins removed fit right in and a quick solder job. Wired it all up and its dead silent! Had to add a switch 120Vac plug so it would turn on with the switch and removed all the excess wires from the PS.

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#8 6 months ago

So was this the normal amount of hum you'd hear in any BK or similar Wms? Or only an issue with Swemmer's boards where it was way worse than that, or only this particular Swemmer board had a problem? On my original BK there is a slight background hum if you're standing right near the machine but in attract I honestly can't tell any diff in it between vol off and vol at max.

#9 6 months ago

No idea what this machine had before, nothing much was working when I grabbed it from my friend. After installing the Swemmer board it was immediately loud to the point it would give me a headache when working around it. Noise didn't change whether in atttract or in a game. The distributor I bought it from says they fully test them before shipping so I believe my original transformer was very, very noisy and their filtering isn't strong enough to knock it out. Hence, the creative work around to fix it. Its now crystal clear now that I'm not using the AC leg. It sounds great!

An electronics wizard friend gave me a bunch of checks and measurements to make to ensure this mod was working right, the powersupply was appropriate and didn't create any unintended consequences that might overheat components and cause fires. Voltages are consistent at machine idle and when in audio test mode with the volume cranked.

Quoted from frenchmarky:

So was this the normal amount of hum you'd hear in any BK or similar Wms? Or only an issue with Swemmer's boards where it was way worse than that, or only this particular Swemmer board had a problem? On my original BK there is a slight background hum if you're standing right near the machine but in attract I honestly can't tell any diff in it between vol off and vol at max.

#10 6 months ago

So the hum is ac ripple not being rectified?
Trying to get my mind wrapped around this for future fixes.
-Mike

#11 6 months ago

I suspect it is. I don't have an oscope to verify

Quoted from Grizlyrig:

So the hum is ac ripple not being rectified?
Trying to get my mind wrapped around this for future fixes.
-Mike

#12 6 months ago
Quoted from Coyohtay:

I suspect it is. I don't have an oscope to verify

The hum would be rectified AC that is not being properly smoothed, most likely due to bad filter component(s) as I said in my earlier post. You don't need an oscilloscope to verify, just switch a DMM/voltmeter to the AC setting and measure on the DC power rail. A generic guess at most audio circuits is that you want to see something less than 1V of AC on your DC power, preferably something around a few hundred millivolts. If you see 2VAC or more you are definitely going to hear noise.

#13 6 months ago

^^ what wayout said. You were getting the noise from rectified AC induced into an audio trace.
Plus if the board designer ran one or more of his audio traces too close to noisy power traces without guard tracks - you will get noise induced from trace to trace. I suspect this is why removing the FWB rectifier out of the active circuit and using a separate DC source cleaned things up.

#14 6 months ago
Quoted from G-P-E:

^^ what wayout said. You were getting the noise from rectified AC induced into an audio trace.
Plus if the board designer ran one or more of his audio traces too close to noisy power traces without guard tracks - you will get noise induced from trace to trace. I suspect this is why removing the FWB rectifier out of the active circuit and using a separate DC source cleaned things up.

The first prototype version of both the rectangle and square wms sound board I did had a nasty hum. Just changing the routing cleaned it up almost totally unless you get your ear right next to the speaker. Mostly with the ground fill around the amp seamed to help the most, but added extra caps and isolated the audio signals best possible.

The audio signal on this sound board takes quite a trip. From dac all the way over to speech board to get mixed in with speech then it comes all the way back to the amp.

-12v filter cap could probably be bumped up in value too.

#15 6 months ago

Update: consulted an electronics guru buddy of mine to make sure I wasn't doing anything that would cause an issue. He recommended removing the 7508 5 volt regulator at IC8. He said they don't like being backfed current so taking it out would be recommended if I keep the PC power supply workaround.

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