strange problem with mixing guitar and keyboard signals

[Wounded Paw]

So I built a mini mixer from GGG and added an LPB to one channel.  That channel is used for guitar while the other 3 are for keyboard inputs.  The person I built it for reports some very strange problems when using this with a 9v adaptor.  Having a guitar and a keyboard works fine off of battery but as soon as you plug in the adaptor the light goes off and the sound dies.  Is there some problem with the ground from the keyboard patch cord and the guitar patch cord and ground from the adaptor all being connected together?  Something else I'm missing.

here's the schematics of the mini mixer and the lpb, the output pot of the lpb becomes the input pot of one channel on the mixer.  And the led is always on if there's power, there's no switches anywhere:

http://www.generalguitargadgets.com/diagrams/mixer_sc.gif

http://www.generalguitargadgets.com/pdf/ggg_cb1_sc.pdf

[Processaurus]

Either the wiring to the DC jack is bad, the power supply is bad, the power supply plug is the wrong polarity, or (unlikely) it is being daisy chained with a positive ground device.