Audio Hum - Mappy cocktail

musicman282

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I've got a Mappy cocktail that has a hum in the audio and was investigating the cause. This machine has a switcher installed and I've noticed that the AC lines are powered on the secondary side of the monitor isolation transformer. Do you think this could be causing my hum issue? The switcher is wired into the old midway linear. When I unplug the connectors to the old linear power supply the hum goes away, but so does the audio itself. Anyone have an idea on what could be causing this? I was thinking about just wiring the switcher into the primary side of the iso transformer and pulling the fuses for the other transformer. I was reading the schematic for the game and can't figure out the audio circuit. I appears that it is using the power supply for amplification??
 
HuMm..

I had a spy hunter set up the same way.. Switcher was for the +5 and the old ps was still being used for the +12 (audio).. But I didn't have any noise issues..

Could be the old ps is in need of a rebuild, since a switcher is hacked in.. Or the game is really getting the noise from the switcher, which is a typical problem when u use a switcher in a MCR game..

Rebuild the original ps... Bob Roberts sells the kit
Buy a rebuilt one..

Or if you just want it fixed and done with.. I've got an ATX MCR adapter and ATX ps kit.. No hum and it would be new :)
 
I'm not sure if that would cause the hum, but I would move the AC taps for the power supply off of the secondary side of the isolation transformer and to the primary side. Basically you wouldn't have an isolated 120 volts AC to your monitor if it was left that way. I would agree with what local said for dealing with the hum. My Galaga, which shares the same wiring as Mappy, has a switcher kit running in it and it doesn't need to have the original Midway supply for anything. I would consider installing a new plug n' play switcher kit or rebuilding the original Midway supply so at least you are running one of the other and not one for +5 and the other for power.
 
I had a spy hunter set up the same way.. Switcher was for the +5 and the old ps was still being used for the +12 (audio).. But I didn't have any noise issues..

Could be the old ps is in need of a rebuild, since a switcher is hacked in.. Or the game is really getting the noise from the switcher, which is a typical problem when u use a switcher in a MCR game..

Rebuild the original ps... Bob Roberts sells the kit
Buy a rebuilt one..

Or if you just want it fixed and done with.. I've got an ATX MCR adapter and ATX ps kit.. No hum and it would be new :)

Where is the rebuild kit? I don't see PS rebuild kits.
 
Ooops

Mappy isn't an MCR game, I made a mistake.. It's got the same ps as galaga.. Might wanna look for a galaga ps rebuild kit, if there is one.. Or just get an adapter and use a switcher.. Had my galaga on a switcher for 8-9 years worked great!
 
I've got a Mappy cocktail that has a hum in the audio and was investigating the cause. This machine has a switcher installed and I've noticed that the AC lines are powered on the secondary side of the monitor isolation transformer. Do you think this could be causing my hum issue? The switcher is wired into the old midway linear. When I unplug the connectors to the old linear power supply the hum goes away, but so does the audio itself. Anyone have an idea on what could be causing this? I was thinking about just wiring the switcher into the primary side of the iso transformer and pulling the fuses for the other transformer. I was reading the schematic for the game and can't figure out the audio circuit. I appears that it is using the power supply for amplification??

First of all, a big thumbs up for looking at the schematics.

Regarding the isolation tranformer. Powering the switcher from its output isn't a problem in and of itself. (In fact, powering switching power supplys through an isolation transformer is a good idea if you even intend to probe or measure inside of one with it powered.) The only concern I'd have is perhaps you're pulling more power through it than it was designed for. It was originally intended for only the monitor's power, but now it has power for the switcher going thru it as well. I kinda doubt it's the source of your hum, but I'd personally re-wire it so the switcher is NOT being powered thru the iso xformer.

Not sure which sheet of the schems you're looking at there, but on the diagram I looked at, the audio amp is powered by +12VDC. This is a pretty common situation. Now, I can only look at the original wiring shown in the schematics... I don't know what's going on in your game--there are as many different ways to hack in a switcher are the are guys wielding wire cutters. But it sounds like maybe they left the original power supply there to supply the +12V. Take a look as the switcher's outputs and see if the 12V output is used (or if it even exists). If not, the original circuitry to 12V power must still be used. In that case, you options are to fix the existing linear power supply (probably needs a new filter capacitor) or use a 12V source from a switcher (either the existing one, or a new one if it lacks 12V).
 
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