Just installed this kit from Flipper Fidelity (with the optional coindoor relocation cable) in Monster Bash last night.

**UPDATE** Yes, I've removed all traces of the kit to test. The hum persists.

http://flipperfidelity.com/hardware/williams-bally-speaker-systems/williams-bally-complete-replacement-speaker-system-for-wpc95-machines.html

Everything sounded great for about 5 to 10 minutes. Was adjusting volume as indicated in their instructions. Their new volume knob was at about 25% (1/4 turn) and the game's volume was at 27 or 28 (full volume is 31).

Was just sitting there in a sound test listening to the sound and suddenly the sound quality dropped. Initially I thought I'd blown a speaker; but that doesn't appear to be the case. It was a loud hum introduced in the signal, which completely robbed the bass from the audio signal and made the treble sound tinny.

I disconnected the new speaker harness from the sound board and disconnected the woofer from its leads so that nothing was wired into the boards. The hum is still there.

*EDIT* It's a constant loud hum that DOES get louder or softer during attract mode when the lights flash. It appears as soon as the game is switched on (before any of the software tests complete).

For a first step, I checked for the obvious things (any loose ground braids or grounding wires that weren't re-connected during install). Then I thought maybe a screw had rattled loose during testing (which may still be the case) but all the boards appeared to be screwed in tightly. Then I floated the sound board (removed it from its screws completely) and let it hang. Hum was still there.

Thoughts as to what I should check next?

I thought it was significant that the hum wasn't there at all for about 5 to 10 minutes and then it just started. The sound changed immediately--not gradually.

While the cabinet speakers were disconnected from the audio board I suppose they were still mounted on the speaker panel--which means they were still connected to the grounding wires--if that's significant. The woofer (without the leads connected) didn't have any ground connected to it.

Thanks for any ideas
Chad

Here's what the game sounds like with the audio harness disconnected at the board (all speakers)

Here's what it sounds like with the woofer only disconnected. Notice the hum still present

Here's what the game sounded like 5 minutes before (sounded great)

Removed the entire harness (FF) and disconnected everything. Hum still present.

Left all audio harnesses out of the game.

Disconnected each of the power connectors to the A/V board. Only the J605 (high power 18VAC/80VAC/100VAC) connector seemed to make a difference, but it was a small difference. The hum was lower in volume but still present.

Keep in mind I'm really a noob at electronics (coming from software)... but

I tested for continuity between pin 1 and ground on the J504 connector and there was none. There was continuity between pin 4 (negative) and ground. (J504 is the backbox speaker connector).

On J505 (cabinet speaker connector) there was continuity between all pins and ground.

Not sure what that means, but J504 and J505 are inconsistent in that Pin 1 (positive) on J504 has no continuity with ground and J505 Pin 1 does.

...

Decided to pull the board out and look at the back of it in case there were shorted traces. (shown upside down) The section with the transistors looks concerning to me; but again, the hum wasn't there at all and appeared suddenly during a music test. Seems like more of a mechanical failure than a board failure to me.

That said, I can try to swap in the board from Dirty Harry... (if they're compatible... I assume they are).

IMG_0928 (resized).JPG
...

Anyone else have any other ideas? Thank you for any help (it's Friday and everybody's tired... or just tired of me )

Thank you Chris. That at least removes one variable from the equation.

Looks like Rottendog WAV095 and the PinSound board (keeping the current board for DMD) are replacement options... but I don't want to try replacement just yet.

Kinda stuck on what to do next. Short of pulling the playfield and all the PCBs out (extreme) not sure what to try first.
A general set of steps would be appreciated if anyone wants to lay that out.

Recap:

• Speakers completely disconnected, including from screws touching ground.
• FF backbox audio harness completely removed
• A/V board completely disconnected (all power/data cables)
• When A/V board "floated," hum remains
• Data cables disconnected. Hum remains. Due to the intensity of the sound, I doubt it's coming from a data (low voltage) channel
• Hum does intensify during GI and lamp flashes
• Hum likely isn't leaking through FF (provided) audio harness. It was removed
• Hum likely isn't leaking through cabinet speaker harness (came with game) because it shows up even when that harness is disconnected at both ends

When the a/v board is completely disconnected, the hum is much quieter... but it is still noticeable.

Logically this means that the hum is coming from elsewhere than the A/V board, and that it is leaking into the audio circuit.

Logically, if something that filters out hum on the A/V board exists... it isn't working.

Logically if there isn't such a thing, it would still be impossible for it to leak into the backbox speakers when the harness is completely pulled out.

I still hear the hum in the backbox speakers when the backbox speakers are connected and the woofer is disconnected. So probably no hum leaking into the cab speaker harness and into the board that way.

Summary: The hum must be coming through one of the power channels into the A/V board and then it gets picked up in the audio circuit. Possibility of hum leaking into a harness has been eliminated. If something on the A/V board normally filters out hum it has failed. If not, a "leak" of some type has begun to get transferred into the A/V board via one of the power connectors.

I'm leaning toward it coming from the H/V connector on the A/V board... but I don't know what to do there. I can't just strip off the molex connector and start reconnecting each wire can I? Should I do something with any of the ground pins in that connector?

Anything I should test (with multi-meter) on the H/V connector?

Any advice please? Thank you!

The only thing that for sure eliminates hum from the audio circuit when everything else is connected is disconnecting J605 (from transformer). But doing that also prevents DMD and audio showing up at all, of course. The rest of the game seems to respond (lights; start button; etc).

J605 comes from the transformer secondary... meaning no PCB in the way. Advice?

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Looks like I'm flying solo from here on out.

I did float the A/V board semi-permanently by using nylon 8-32 1/2" screws and washers from Lowe's. I didn't expect that to fix the hum (since I'd eliminated that possibility by letting the board hang before) but now there's no disputing it... the hum is entering the board via one of the main power connectors--not from ground connections to the board.

Looked up the schematics. Seems on connector J605 the lower 2/3 is dedicated to the audio circuit. After looking at the traces at the back of the board it looked like pin 5 & 6 went to ground and sure enough, that is on the schematic.

We're left with pins 7 & 8 and 10 & 11 (pin 9 KEY).

Measured voltages:

Pin 5: ~
Pin 6: ~
Pin 7: 13.72 VAC
Pin 8: 13.72 VAC
Pin 9: ~
Pin 10: 13.75 VAC
Pin 11: 13.75 VAC

schem_03 (resized).jpgschem_01 (resized).jpg

Now according to this page, the schematic is wrong (it claims 18V when it should be closer to 12V).

I don't have an oscilloscope to hook up to any of the pins (to see the sine wave).

I could do some kind of temporary connection for three of the four pins at a time to the board.

How is that typically done... temporary connections from the plug to the board so you can do the process of elimination? I figure connect up pins 7 & 8 and then either 10 or 11. Then 10 & 11 and either 7 or 8. Maybe the ground pins could be eliminated?

(Keep in mind I'm a software guy... no EE degree!)

Could I provide a temporary 18V (~13V) source from another power supply to any/all of the pins to eliminate the transformer as a source? If so, would any old 18V power source do?

Hi Chris,

I haven't done any electronic component replacement(s) as of yet. The 10000μf caps don't seem to be bulging or anything. (I'm willing to do just about anything though).

Would you recommend doing that before moving forward? For my own understanding, do the large capacitors on these A/V boards typically condition the incoming power to eliminate existing hum from the audio circuit?

(I assume you're talking about C36 and C37 on the A/V board)

IMG_0944 (resized).JPG

As a sanity check, I "reinstalled" the old woofer in the game and connected the leads and turned on the game. Hum was coming out of the original woofer as well.

Not sure if I'm testing this correctly but it looks pretty close on C36

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