help with Russian Big Muff PI mode

[metamp]

I have a black Russian Big Muff PI
This box does not have a 9V DC input jack and runs on 9V battery only
When I connected a 9V adapter on the battery pins the pedal works but it hums
I was told to solder a 1000uf 16V electrolytic capacitor on the battery pins "+" to "+" and "-" to  "-" to make the hum go away but it didn't help at all

Can someone help with this
thx

[theehman]

Are you positive the 9v adapter is putting out DC and not AC?

[metamp]

that's what it says on it output 9V DC 300 mA

[theehman]

Make sure the positive lead of the capacitor is going to the positive pad on the board and that the negative lead is actually going to ground.

[MetalGod]

make sure you use a regulated DC supply - unregulated ones will hum like crazy.