That's called diagnosis - you or someone else needs to look at the gear, get an idea where there may be an issue, and experiment. If you look inside some cases it's a nightmare of cables running everywhere - think old school Japanese; and on the inside of some gear considered well of there may be a single circuit board, with very short paths to external connections - the NAD I'm currently using is of this design style, and even in raw form does many things well.
And don't get a hangup on a particular substance to use - plenty of times I just use an everyday hot glue gun to "tie" things together - something cheap, easy to use, easy to undo - experiment, experiment, experiment ...