DOD Ice Box

[Transmogrifox]

Anybody have a schematic to this?  I have one and I find that the chorus itself is not a bad sound, and the pedal is useful, but it has an awful hum/background noise to it. Some is power supply related, but it also has the hum with a battery, so there is something noisy in the circuit.

I disassembled it (put it in the state of all of my commercial pedals so far) and noticed it has a bunch of 4558 opamps in it (BA4558, looked up the datasheet, they are equivalent to JRC4558).  Would these add that much noise?  Would it be worth replacing them with low-noise IC's?

A schematic would be nice, but suggestions, hints and other comments about the Ice Box are also very welcome.