The 68k feedback resistor must be measured out of circuit. if you look at the schematic, one end ties to the top of the 1.5k cathode resistor, which itself is tied to ground. On the other end, Of the 68k, it is tied to the output transformer, which is also tied to ground. The output transformer will show almost no resistance when measured with the meter. So if you read it in the circuit, it would only show you the 1.5k next to it. I would guess that the 68k resistor is fine, you can double check if you want, but you must at least remove one of the leads.