Mark Levinson nr 26 vintage

[Pierre1984]

Hi, there,
I own since quite some time a beautifully sounding ML 26 preamp with MC/MM phono inputs and a 20.5 amp.
For a while I did not use the phono input, having switched to CD’s as main source. As I recently re-discovered a pile of vinyl’s in my attic, I wanted to play them with my new ProJect turntable and my good old Koetsu cartridge.

The right channel performs beautifully, yet to my dismay the left channel provides much less gain and makes a wobbling sound at about 5 to 10 Hz (the frequency seems more or less affected by the sound level….). In idle condition, when no signal is applied e.g. the phono needle rests on the arm support there is no sound nor noise at all and everything is quiet (as it should be).

Could anyone kindly advise me how to solve this problem ?
Pierre