Edge connectors. My 33 went into pops and bangs when tapping the case too. Do what it says in the service manual, and gently push the connector fingers in a bit with a suitable tool. Apparently the vast majority of 33's that were returned to Quad for fixing had that problem. They later changed the edge connectors for gold plated ones to overcome the problem. Except they still relax over time.
The other thing that went on mine (which was also a pops and bangs when tapping the case event) was the on/off switch on the back of the volume control - it had burned clean through one contact. Cured that by buying a NOS single gang control on eBay that had the same AB brand switch. Removed it and put it on the old control. Sorted.
The other thing that went on mine (which was also a pops and bangs when tapping the case event) was the on/off switch on the back of the volume control - it had burned clean through one contact. Cured that by buying a NOS single gang control on eBay that had the same AB brand switch. Removed it and put it on the old control. Sorted.
