System 11a Audio Hum

[Game On]

Hi all. I have a problem with a Millionaire I am restoring for a friend. It is Williams system 11a and following a full cabinet strip and re-decal everything is back in the cabinet and it all fundamentally works however I have a problem with the audio hum. I simply do not know if it was preaent before I stripped it out but it is there now. It is worse when the lamps flash and cycle in demonstration mode at the end oif the demo cycle there is a brief period of total inactivity when the hum disappears. so it appears to be something to do with the switched lamps etc. Ihave done / checked the following...... 1) All boards are tightly secured to the backbox and the earth braid is continuity good the the mains plug. 2) I have tried moving all the cables to and from the Audio board and have checked all screens on the cabling nothing is one legged. 3) The Power Supply is a new Pinsound Board and scoping the 5 & 12 Volts DC shows very little ripple or hum and is stable . 4) I have changed all the electrolytic capacitors on the audio board. 5) Swapped out the audio to CPU ribbon cable. The only other thing I can think of is it is the capacitors on the CPU but cannot see how it would affect the audio in this way? The annoying thing is that I do not know if it was present before buit I really want to get rid of it for the customer. I understand System 11 does suffer from Hum issues but other games I have worked on have been fine. Any Ideas people??

 

[Game On]

Solved! By Isolating the Audio Board with Nylon washers and screws the problem has gone! Literally zero hum clearly somehwere in the machine is a floating earth. A quick and cheap fix.

 

[Spandangler]

Do these games rely on metal screws making contact with the metal backing plate for grounding? My whirlwind has a bad hum and this sounds ideal, however I don’t want to jeopardise it by not having it correctly grounded. Just a thought

 

[Game On]

Do these games rely on metal screws making contact with the metal backing plate for grounding? My whirlwind has a bad hum and this sounds ideal, however I don’t want to jeopardise it by not having it correctly grounded. Just a thought

The audio board is still grounded but the Ground on the rest of the machine has residual hum. Almost certainly somewhere on one of the boards there is a leaky component probably a capacitor. that is causing hum to be apparent of the machine ground. The only part affected is the audio By Isolating the audio ground is that it is now floating and isolated from the ground with the hum on it. Background hum and hum causd by the controlled lamps is down 95% It cannot damage the audio board I have discovered that sysytem 11 is notorious for audio hum and data crosstalk In a perfect world one would locate the leaky component but even with a scope it would take many may hours to locate This is a simple work around and in my case has worked after everything else has failed,