Volume failure or if you need to wait for 10, 20 minutes to your HD300 show some sound or effect, the problem is in the pre-amplifier of the guitar input. First thing to do is remove the metallic shield that cover the connector and the circuit of the preamplifier. So, you must remove the four electrolitic capacitors (pictures atached), and replace-it by new ones. Chean the place with alchool. You probably will see some leaking or rust signals there. Dry the place before replace the capacitors. As old this device is, more corrosion you will see.
