TS-940 Monitor Feedback

So, since I have my TS-940S apart, I decided to try and fix an irritating issue I’ve had ever since I first got the rig working.

When I am using the audio output via the front phones jack or the rear external speaker jack, and I have mic input enabled, and I am using the monitor feature to listen to my signals, I’ve been getting feedback. A loud squeal any time I key the mic. I can lower any one of three controls to work around the issue. I can lower the AF level to a point where I can barely hear myself. I can lower the monitor level to a point where I can barely hear myself, or I can lower the mic gain until I have to yell to be heard.

This persists even when I use a hand mic and headphones, so it’s not crosstalk in the headset cable.

So, my assumption is that there’s crosstalk in the cable loom itself. Here’s what the wiring loom looks like:

This is after I rerouted the wires in question, but you can see what a thick bundle it is. The mic and speaker lines were tied together before beginning the bundle, so it’s not unreasonable to expect they were closely aligned inside. Further, the audio routes to the Phones jack then, if there’s no plug inserted, the switch routes it back along the same path to the external speaker connector on the back. It potentially goes through the bundle twice.

It took about fifteen minutes to cut each tie-wrap, extract the speaker wires, then rewrap at that point. I started at the front, without disconnecting it, and I got to the bend toward the back of the rig where it became tangled up enough that I couldn’t extract it without disconnecting. I then unplugged the wires from the board and worked back to the bend I had stopped at.

Once it was extracted, I routed it as far away from the main loom as possible, and am currently holding it in place with electrical tape. I expect this to fail soon, and next time I’m inside I’ll want to replace the tape with something that will last a bit longer, but I want to make sure this fixes the issue first. I’ve highlighted the new route here:

It does go close to the filters, and I’m concerned about what it can mess up over there, but since it’s taped to the lip, there’s about a centimetre between the wires and anything else. Hopefully this fixes the longstanding issue for me.