DSL100 FX loop problem

[LarBal]

Got a pesky issue I can't get to the bottom of. This was a beat up DSL100 I restored a while back. No problems with the amp until the owner starting putting things in the loop.
I can confirm that even with just a patch cable jumping the send and return, there is a drop in volume, and a lack of bass. If I inplug it, the amp returns to normal.
I replaced both fx jacks, and issue remains.
Even after replacing the jacks, I could wiggle the jacks, or even the board, and could affect a positive change back to full signal.
The dB switch next to it doesn't seem to do anything, either.
My next step is to replace that, but I've got to find it and order it first. All I can assume is that it's broken, and halfway stuck in the -10dB or whatever position, but I can't imagine that would suck all the bass out.
Anyone experienced this?

[The Dude]

......The dB switch next to it doesn't seem to do anything, either......

If that switch makes no difference, there's likely a problem with the switch itself or solder connection to the switch. There should be a noticeable difference between -10 and +4.

[LarBal]

If that switch makes no difference, there's likely a problem with the switch itself or solder connection to the switch. There should be a noticeable difference between -10 and +4.

That's what I figured , too, but then I tested it for continuity, and it checks out fine, and is switching properly, at least by testing the solder pads underneath the board

[drewl]

I don't think there's a change in volume when just using a patch cord, I could be wrong, but don't remember.

measure the output send level with a meter or scope and see if the level changes.

Edit: Pretty sure the switch changes the send level and return level, so it remains the same.

[Jazz P Bass]

As designed, it is a serial loop circuit.

When nothing is plugged into the loop jacks, the circuit is bypassed.

So jumpering the jacks engages the circuit.

[Enzo]

OK, so the amap works if you don't use the loop. Through the loop it is weak and tinny.

So look at the schematic, the loop return runs through IC2, a common 4558 op amp IC. Did you try a new one?

The switch is likely just fine, you can test it with a meter if you like. I don't expect it to change the volume, like drewl said.

Plug the guitar into the FX return jack to bypass the preamp. Does that still sound weak and tinny?

That whole mess flows in and out through CON3, try resoldering the pins on that.