Cgs synth

hi, i've recently built the steiner vcf; but it hasn't been working
properly. when i first tested it i connected the power backwards, and
now when i use it it constantly oscillates even with resonance turned
completely down. the sound is like a standard feedbacked signal, loud,
and high-pitched. when i turn the cutoff i can heare some filtering
through the screaming feedback though.
The tl071 is still fine, and i've checked+reflowed the solder joints;
my question is, any of you smart people know where the problem most
likely would be/ where i should start troubleshooting?
should i replace (test?) transistors, caps?

any help is welcome

cheers, marcel