Ok...now for my next noob mistake...I posted this last night when it was late and I was tired and I neglected to make sure I was including the most recent and up to date version of the schematic. Very, very sorry for the confusion. I'm attaching here for clarification rather than to edit the beginning of the post. Not sure if this is the best etiquette but seems better than changing the attachments after several people have already commented.
Basically, I added two 10k resistors, R51 and R52, at the recommendation of floobydust, to get rid of the 2.5VDC offset at the non-inverting inputs to the gain stage. R52 is actually a THT resistor that is jumped over the board...it's not connected by a trace to ground as the pcb indicates (but it is grounded in the same place). I did that because it was the easiest way to show the added resistor without reworking the rest of the traces. I hope to get this board working but if I have to do a redesign this will be updated properly.
The other change is that the op amps are not TL072, but OP1656. I didn't change the schematic because KiCad doesn't have the same style op amp symbol in it's library and I was too lazy to create my own. The pin outs are the same so seemed like an harmless fudge. Obviously, that's not the case and it led to my cardinal sin of posting an inaccurate and outdated schematic. This will be #348 on the list of mistakes I won't ever make again!
Assuming I haven't thoroughly alienated everyone with my oversight, here's the rest of my post with questions.
bdunham7...I agree it's most odd to see different readings in opposite directions. I think it's most likely human error even though I've tried several different angles to get on the pads and I keep getting the same result. In either case, the measured value does seem to point to an open in the resistor so that's easy enough to replace.
Regarding the placing of R52... the first time I added it I used a 1206 smd because I had it and it fit nicely between pin 3 of U4 and the ground pad on R11. Having fixed the DC offset problem I now focused on the oscillation and I assumed I had created some sort of inductive loop situation by grounding R52 to the ground pad on R11. So I reworked R52 using a THT 10k resistor so I could easily ground it elsewhere. I picked a power rail ground as I had for R51 thinking that it would not interfere with the signal since it hadn't had that effect on the other channel. My question here is, is there any reason why I shouldn't put R52 with an smd and ground it to the ground pad on R11? That would make the addition of the 10k resistor a lot more tidy on the board.
fcb...
The TL072 has a very high input impedance, and you have a polarised cap on the non-inverting pin ONLY. This is likely to slowly float-about or rise and pin a rail - you really should have a small load (perhaps 1M) on that to 0V to keep it centred on 0V.
If you create an HPF (high-pass filter) with the 33uF and say a 1M, you'll have unnesaarily low cut-off, so you could make the cap much smaller. Also the load is very small, do you really need a TL072 buffer between the output of the digipot and the gain TL072?
I know the size of the cap, and that it's not a film cap, is less common. I used this set up because I was following an app note from Maxim that has these values in their sample circuit for how to create an offset for use with Dpots that won't tolerate negative voltages. Someone else commented on the buffer, U3, as perhaps being superfluous, and I had the same thought I but I put it there because it was in the app note, and because another person on this blog recommended I keep it. I'm sure the fine folks at Maxim assumed that anyone using their circuit would know enough to account for the issue you mention. In my case, I didn't and so here I am with a Frankenstein circuit I'm hoping to make work. To that end, there is currently a 10k resistor to ground, R52, on the channel that is acting up. It was put there to address the DC offset problem and is working well on the other channel. Is this what you are referring to?
fc would be about 482 mH. For sure that's very low but I thought the filtering effect here was an unavoidable side effect of using a decoupling cap and not necessarily something that was needed to filter out unwanted frequencies. Is there a downside to having such a low cut off?