It will be a bad connection - if you can take the covers off and gently tap away in selected areas - with the handle of a wooden spoon perhaps -you might be able to narrow down just where the bad connection is.
Without drawings - test equipment and experience fault finding could be a long drawn-out affair. Careful examination of the soldered joints might be useful if you know what to look for.
Use Google
Select Images
Enter Dry Joint
Good luck...
February 2015
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