A previous owner installed a 9 volt DC jack in a Schaller Fuzz. That is understandable, as it only ran on 9 volt battery, and can be inconvenient for pedalboard users who do not wish to unplug the cable every time, or else it would drain the battery.
Many DIY modifications like these mean that the 9 volt battery is not usable anymore as people use the wires of the 9 volt connector to the 9 volt DC jack.
Note that the lug for 9 volt battery is not occupied and there no longer is a 9 volt battery snap. So it can currently only be used on external power.
Unfortunately, these pedals however do not have any DC filter installed, as this is not originally needed when used on battery. If you hook them up to the mains, the electrical noise associated with this will be heard through the pedal. Therefore, when considering to mod a 'battery only' pedal, apart from installing the obvious 9 volt jack, to also include a simple DC filter in your mod. The most basic filter would be a 100uF electrolytic capacitor in parallel with a 47nF ceramic capacitor and a 100 ohm resistor in series. This will filter out at least some of the largest ripples when using a cheap power supply. Best practise would be to add a diode (cathode to the positive lug) to prevent reverse polarity damage.
Simple, reasonably effective, and a few cents in parts. To prevent short circuits or breaking of components, hot glue provides excellent insulation and fixation, and is easily. removable if needed.
Something to consider if you wish to mod your battery-only pedals.
If you do not require DC filtering (e.g. high quality power supply), and are fed up with batteries, we also sell a convenient clip-on cable to fit to the 9 volt battery snap.
No need to drill into enclosures then, and the pedal remains original!