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When I flipped that chassis to look at the point-to-point wiring underneath I saw that each input had an axial cap in series, a 250uF/15v electrolytic. Testing showed these caps were far out of spec, with both of them around 330uF. One had an ESR in excess of 2 ohms.
I’ve never seen caps installed in series on inputs before now. Nelson’s schematic for the MF12 showed a more traditional arrangement with a cap and resistor in parallel with signal and ground.
What is the purpose of these caps?
Forgetting about the MF12 project for now, if the amp goes back into service in stock condition, should those caps be replaced, or can they be eliminated in favor of a more traditional arrangement?