Problems with Quad 909 (606)

I have a problem with a quad 909, which fails a channel. Initially, the pin 6 of IC1 had an increasing voltage from -100mV to -1,5V. I've temporarily replaced by a TL071 obtaining at the outlet -70mV. But now only amplifies the positive wave. I have checked all the transistors and diodes and all are correct.
Any ideas?
Thank you.