Vintage Synth Repair

I found this on the net about the 4075 filter "This was ARP's own
24dB/octave filter, and appeared in many of their later instruments.
It offered low noise and distortion, but a design fault (easily
rectified) limited its maximum cutoff frequency to just 12kHz or
thereabouts"

So how do you fix it? I cant find any info.

Also, my arp quadra makes a kind of low frequency thump on the attack
of the string sound..to me it seems wrong but I guess it could be
another quirk?

Any ideas about this?

Thanks, Phil.