In this case, I have two Magnavox Micromatic changers both with original P156 (EV150 series) carts that are presumed OK. As I service the one, I used the other to test the amp performance and was quite surprised by what I heard.
The tonearm wiring on these consists of two thin shielded cables, braided outer conductor is grounded. The polarity seems foolproof from RCA jacks to cartridge, but I suspect the two inner terminals on this cartridge are NOT both negative. The two were joined by a wire assuming both are negative. This would explain the cancellation of material common to both channels on a stereo record.
Both Pfanstiehl P226 (Varco TN4) and P192 (BSR SC8) are a larger size but can be made to fit the black tonearm with single screw mount. The Tetrad (EV5632) series carts are smaller but require a plug which has polarity marked. Normally, this is my go-to repair for these changers.
