Recently, I found a Seiko DM-20 digital metronome in a nearby second-hand electronics shop. I really like its compact, thin design and retro appearance, and thought it would be useful during band rehearsals where I often have to bring a much larger metronome.

The only problem was its 2.5mm headphone socket. Most modern headphones and audio cables use a 3.5mm connector, and although an adapter would have worked, it would also have added another small component that could be lost or fail during a rehearsal. I decided to replace the socket so the metronome could connect directly to the equipment I already use.

After opening the case, I “carefully” enlarged the original socket cavity enough to fit a 3.5mm jack with a screwdriver and sandpaper. Space inside the DM-20 was limited, so the main challenge was removing enough material for the larger connector without damaging the case or interfering with the existing electronics. I then soldered the new jack into place and reassembled the metronome.

The modification worked, leaving me with a practical rehearsal metronome that keeps the original compact form while connecting directly to standard headphones.