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WPC Speaker Hum What Causes It?

hmmmmmmmmmmmmmmmmmm

DC voltages getting through to the speakers because the filter caps on the sound board have failed (you can measure this with a meter or scope) - common on DCS boards

OR

The usual grounding issues.
 
don't question the guru of fixing stuff , Lukes always right even though he is wrong saying its DCS.
Sort out the filter caps on your WPC board.
 
I was just saying it's common on DCS boards so may apply to other WMS sound boards too ;)

Stick a multimeter across the speaker on DC volts and see what it reads
 
I have a similar problem with a borrowed F14 I have at the moment. What happens is the speakers start humming but the hum gets louder and louder until it turns into a horible distortion. First few times it happened I actually turned the machine off quickly as it sounded like damage was imminent. Anyway I've found that unpluggling the cabinet speaker stops it happening. I've tested the speaker and it seems to be fine, 4 ohms and nothing to earth.

Is it fair to assume it has a similar sound board problem? Caps maybe? I'm guessing the added load of the 3rd speaker is causing a weakness in the soundboard to show itself. How does that sound gurus? I can't see any problem with the wiring or connectors leading to the speaker.

Is this a know problem with system 11 machines?
 
I know through google searches and Luke ect speaker hum is a common fault in wpc and wpc95 so wouldn't surprise me if they all follow suit
 
RudeDogg1 said:
I know through google searches and Luke ect speaker hum is a common fault in wpc and wpc95 so wouldn't surprise me if they all follow suit
Click to expand...
Yeah but this is a bit different, not just a hum which they all seem to have. This gets louder and louder until it's unbearable and then starts crackling/distorting badly like it's about to explode or something :D
 
Hum is typically from failing capacitors on the sound board power supply. Check with a multimeter the AC component on what should be DC. You should see very low AC reading. If not the smoothing capacitors are failing. This puts AC on the amplifier rails, and causes the hum. As the capacitance gets worse with temperature the AC increases and the hum gets worse.
 
I'm curious. I understand that if the capacitors on the sound board are getting a bit long in the tooth then they won't be doing their job properly and any ripple on the supply will end up being amplified to an unacceptable level. But at the same time wouldn't it also be worth checking the power supply itself to ensure it's creating a desirable smooth regulated supply? Garbage in, garbage out and all that.

And +1 on checking the grounding, but due to the nature of pinball machine design (a big wooden box full of lots of ground wires and unshielded interconnected boards) it's probably impossible to eliminate internal ground loop hum.
 
The supply voltages for the amplifiers are generated onboard the sound board. So checking the capacitors is the same as checking the power supply.
 
Probably. They're the ones that cause major hum when bad. Could be other culprits
 
Because i became fed up with my old B/W pins always having problems.
Can't beat new and shiny. I don't mind waiting 20 years before my caps start drying up :)
 
Arnt new sterns equally as reliable as 20+ year old b/w brand new targets and toys breaking in the first few games ect
 
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