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?

20210402_130722 (resized).jpg

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.

20210403_135448 (resized).jpg

20210403_143051 (resized).jpg

20210403_143137 (resized).jpg

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.

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

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.

20210410_165426 (resized).jpg