Bob the Tech Audio recently had a pair of McIntosh MC2152 amplifiers in for repairs. A beautiful new all tube design, 70th anniversary edition. The one unit had a couple of open resistors, but the 2nd unit was a more interesting problem. In the 2nd unit, one channel’s output tubes were arcing, resulting in a shutdown. Swapping tubes didn’t resolve the problem, It was an issue in the unit. The usual tube amp bias issues were not evident, so we looked elsewhere.
A problem with load impedance can cause tubes to arc. We evaluated the output transformers to ensure there were no shorts between the primaries to ground or to each other. No issues there, so focus shifted to the output windings. This is where things got interesting. The output windings In one channel measured as expected, with slight impedance increases from ground to 2 Ohm, to 4 Ohm, and to 8 Ohm taps. In the other channel, there was no increase step by step. We removed the screws and dropped the transformer an inch or two to look “under the hood.” We found one of the output winding leads pinched between both mains and output transformer housings and chassis (photo on the left).
It was clear that pinching the lead cut the insulation, shorting one of the leads to ground (center). Bob the Tech removed the damaged section and repaired it with a pair of Faston terminals (right). With the repairs in place and transformers reinstalled, the unit runs great.
If you have a McIntosh MC2152, or any vintage or modern McIntosh gear, in need of repairs contact Bob the Tech Audio for service.
