This post: http://adadepot.com/index.php?topic=374.0;topicseen (http://adadepot.com/index.php?topic=374.0;topicseen)
is why I always suggest replacing and/or scooping-up/cleaning as needed.
Only now will you be sure the circuit behaves as the manufacturer intended.
Electronics do age, not just the mechanical parts (like potmeters and connectors).
Replace:
* all electrolytic capacitors, not just those in the power-supply section
* all small diodes (like 4001) and rectifier diodes/bridges in the PSU section
* high-ohm resistors to metal-oxide film dittos in high voltage/plate circuits
* potentiometers
* jack sockets with build-in on/off switch
Clean:
* all IC sockets and IC legs
* mains fuse holder and fuse
Generally, I see all too many replacing mostly the larger electrolytic capacitors in the PSU section.
The other components are quite inexpensive and small in comparison, but mostly aren't swapped.
Sure, potmeters and jacks sockets do have a cost, but..