Vintage Simmons Drums (UK) Users Group

hello everyone. i dusted off my sds400 and sds8 and placed them along my other sds8 and my sds800. when i raised the volume on the channel on my mixing desk certain channels from each of the units emitted a tone or noise depending on what was selected. i haven't even hit my drum kat yet. is there a particular solder join or component that is causing this bleed? i thought i should ask the knowledge base rather than going in blind. it does not happen on every channel.

any help would be greatly appreciated.