I have a National RAO-7 military receiver that I recapped about 5 years ago.
I also replaced a few out-of-tolerance resistors. The receiver performed
well beyond original specifications at that time. It was unused for about
the last 2 years and now when I listen to it, I get distortion in the audio
output at about 1/3 to 1/2 volume. The more I increase the volume, the
worse the distortion becomes.

What might this be a symptom of?

Thanks in advance for your advise and help.

Cal in TX