Mesa Rectifier FX Loop Switching Issue

[67SLP]

I’ve got a 1994 Mesa Revision G Triple Rectifier with a factory serial fx loop (this is an oddball as most Rev G Rectifiers had parallel loops with send and mix pots). The fx loop works fine when activated with the back panel rotary dial selector: ie on, off, or bypassed. But the external footswitchable fx loop selector jack on the back panel is not functional (just under the footswitchable channel selector jack, which is functional). I’ve replaced the jack itself, resoldered all suspect joints, and swapped out the V4 tube which is the preamp tube that controls the fx loop signal routing and mixing.

Here’s a schematic of what I have:



and another:



What should I try next? I somewhat suspect a bad LDR? Any advice appreciated.