JoeE SP9 and I have same thought. perhaps your cartridge has opened up from too much current from output of preamp. I would suggest using a digital voltmeter with numbers, not an analog meter with a needle. Analog meters tend to shoot a fair amount of current through the cartridge while measuring it, putting fine coil wires at risk. Cartridge may have opened up from too much output from preamp, but no way to know unless you measure. Specified cartridge resistance may be listed in cartridge data base. Try looking it up. But, in a moving coil cartridge, there should be continuity between the outer shield of RCA plug and the center pin of same RCA plug. Chances are if it's infinite resistance, it's either an open coil or a break in wiring between RCA plug and the cartridge. To determine if it'a wiring or cartridge, take cartridge clips off the cart and measure between the right signal and right ground pin, and then measure between the left signal and left gound pin. Colors are red to green, and white to blue. If you get continuity through cartridge your problem is a wiring failure, which can be found by measuring between red wire and center pin of right RCA plug, Green wire and outer shield of right RCA plug,white wire and center pin of left RCA plug, and blue wire and outer shield of left RCA plug. Wires should measure 1 ohm or less.
And good luck from that colorful, yet digital, old guy,