turntable hum and reduction of output

Is the hum the same in both channels?
Are both channels low in volume?
Have you checked continuity of the phono cables from the plug tip through to the cartridge connection? And the earths as well?
Is the sound normal, apart from being very quiet? For example, are the low frequencies missing?

The normal best wiring arrangement is:
Cartridge left channel hot and return connected through to hot (core) and return (screen) of left phono plug, with no other connection
Ditto for cartridge right channel
Turntable chassis earthed to earth terminal on back of amp/preamp
Tonearm metal earthed to turntable chassis.

You can also measure the resistance across the cartridge outputs, separately for left and right, and tell us the make and model of the cartridge. The cartridge's DC resistance, for each channel, should be only a few ohms if it's a moving coil cartridge, and probably less than 1k for a moving magnet cartridge (I'm not sure).

 

Sounds like the earth connection has become damaged even if it looks ok (. The (usually) very thin earth wire may have a break in it, or could have been pulled loose from the PCB inside, perhaps if the cable has been pulled/yanked or trod on. Time to get out your continuity meter.

If it walks like an earth cable problem, and it talks like an earth cable problem...

 

Thanks but note that this post is over 5 years old.