Modding Boss DSD-2 for 18v

[Max999]

Greatings to all,

I am having some unforseen trouble with modding a Boss DSD-2 ( which is quite similar to a DD-2) for 18v operation. Initially I thought a few upgrades and some small things would do the trick, only to find out that the circuit acts really different then what I expected.
All the digital parts in the pedal work off a 5v regulator, which is doing it's job fine with 18v input. All the IC's fed with the full voltage, except for the BA634 flip flop, can take more then 18v.

What have I done to it:
- taken out the overvoltage/ polarity protecting diode ( 11v) at power supply input
- upgraded the 16v electrolytics in the power supply section for 50v
- taken out the flip flop ic BA634, because it is fed by the full voltage and is rated for only 16v
- connected the source to drain on the fet that the BA634 switches

When I use the pedal at 9v it works flawless. At 12v and 18v the pedal becomes very noisy. The effected part of the sound becomes a lot louder, and the feedback knob becomes so sensitive that even having it barely open makes the repeats go on forever and oscillate. It is unusable on 12v and 18v.

Something in the pedal is producing gain and noise when I up the voltage.
What can this be?

I thought about the compander ( NE570), because it is the only part that does not use a voltage divider for the Vref. The strange thing is that when I make the calculations for the NE570's bias, Boss did not set this to 50% in the first place ( when working on 9v).

Do you guys and girls think that setting the biases of the NE570 compander to 50% will solve my problem?

Any other ideas why the pedal is doing this? Service manual for schematics can be found at http://www.synfo.nl/pages/servicemanuals.html

Thank you, kind regards