I took a (minor) chance on this Bruel & Kjaer bandpass filter. It was super cheap. Turns out the previous owner had left the batteries in it, and they were a corroded mess. The plastic battery compartment was also broken.

It does have a connection for a power supply on the back, so one option would be to find one of those. There was some corrosion on the componetry inside, though, so it may not be worth the effort.

Sad face. ☹️

#musicStudio #ebay #sadFace @synths