Generally speaking (using scope)
1) If scope DSO place it on infinite persistence and look at power supply rails
to evaluate noise. If scope analog, but storage, run a very slow sweep looking
at rails with controls set to long persistence.
2) Any parts in sockets lift the part at each end slightly and push back into socket.
3) Dirt/dust/grime buildup clean PCB.
4) Ground input jack for a channel and using scope proceed to look, starting at
input, and proceed along the signal path all the way to output. Repeat with a
signal source, like simple tone. Here you can get free signal generator from your
PC/Laptop via sound card.
The following for people w/o scopes, but the software also have sound generators -
If you don't have a scope, but have a PC (limited, will not be able to see high speed
transients, but still somewhat useful) -
You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.
https://www.zeitnitz.eu/scope_enhttp://www.zelscope.com/http://www.ledametrix.com/oscope/http://www.virtins.com/downloads.shtmlBut first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".
For example
http://makezine.com/projects/sound-card-oscilloscope/Sound card impedance bridge -
http://www.marucchi.it/ZRLC_web/ZRLC/Steber_An_LMS_Impedance_Bridge.pdfhttp://www.sillanumsoft.org/ZRLC.htmRegards, Dana.