dbx 160x - no gain reduction

[sircletus]

Greetings, all. I'm in the process of reducing gear clutter in my studio and decided to part with my dbx 160x pair. One is fine. The other passes signal, but does not compress. Well, it sort of compresses: under very high signal levels with threshold all the way down it'll compress just a few dB.

1. All PS voltages (±24V and ±12V) measure good.
2. With no signal present, both compressors measure all the same DC voltages at all test points.

Things get weird when signal is present:

1. DC voltages around the RMS detector on the stereo link (and the opamp immediately after it) go bananas. Specifically, the second half of U104 that contains the threshold control in its feedback loop hangs out at a constant -9.3 Vdc regardless of threshold control.
2. Threshold pot is fine. 20k across outer leads, wiper sweeps the entire ±12V range.
3. When stereo linked with a TRS cable, the bad one compresses regardless of which is master and which is follower and both compress to the same extent.

Weird. Any thoughts? Anyone have a similar experience? I'm ready to pull my hair out.
View attachment 160x Schematic.jpg