[jts1957]

Can't answer the speaker tapping portion of question, but on something that old, intermittant switches, controls and bad solder joints are notorious for causing problems.

[jw24raleighnc]

I suspect I have a problem in the right channel of a '79 Nikko Alpha II power amp (100 watt rms/ch), but I'm more interested in the explanation as to why tapping a speaker restores the sound. Sorry for the long-winded narrative that follows. The right channel sound will intermittently cut down on the bass output a lot while leaving some treble. The left channel output remains fine. There is also some distortion and occasional "broken-wire" hash and spitting during the cut-down. Tapping on the woofer cone normally restores the full sound. I am actually running A + B speaker pairs, but the cut-down occurs on both right channel outputs anyway. I originally thought the problem was in just one of the right channel speakers, but that was incorrect. I swapped speakers as a trouble-shooting procedure and the problem is still in the right channel, on both A and B speakers. I checked the wiring, the connectors, and played the CD player versus my tuner during a cut-down. The cut-down continued regardless of the sound source. The amp has VU meters that indicate normal measurements even during the cut-down, for what that's worth.
Here are the questions-- If the problem is not in the speaker itself, why would tapping the woofer cone restore the sound? Moreover, why would tapping a cone from either the A output or the B output restore the sound for both outputs? Is there some sort of impedance loading or feedback thing involved in the output circuits? I'm trying to understand the mechanism.
If more info is needed, please let me know. Thank you!