For some time, I'd been having a crosstalk issue with the C4 pad on my ZX. Raising the noise floor helped, but I didn't like the decrease in sensitivity. Plugged it into ZenEdit and tried to tweak the trigger with Rob Anderson's help, but no difference. I recently noticed that the pad was a little wobbly and wouldn't tighten up just by pressing it down, so I yanked on it from the inside and snugged it up. Also applied a little bit of glue to hold it in. When I examined the piezo under the cap, the wire attached to the center popped off. I re-soldered it, reassembled and tried it out. All OK now. My problem was either a cold solder joint or a loose pad (or both!). Both were easy fixes.
Reminder: If you have the internal battery modification and your Zendrum goes dead, even when hard-wired through the battery box, check the wire connections to the battery holder!