M5 No DSP Audio Output — True Bypass Works

[neilemborgo]

Hello guys! 

I’m looking for help diagnosing my Line 6 M5 that currently has no DSP audio output.

 

Current symptoms:

 

* Pedal powers on normally

* LCD/display works

* Presets change normally

* True bypass works perfectly

* DSP bypass has NO output

* Most effects have no sound at all

* Some distortion/OD/filter effects produce very weak low-volume output

* Delay/reverb/modulation effects are completely silent

 

Additional important detail:

Originally, the pedal would randomly RESET/reboot whenever changing presets. After opening the backplate and pressing/reseating some ribbon cables/connectors, the reset issue disappeared completely.

 

So now:

 

* preset changing is stable

* but DSP audio is still basically dead

 

Things I already tried:

 

* different known-good 9V center-negative adapters

* Zoom G3Xn adapter

* factory reset

* cleaned ribbon cable contacts/connectors with isopropyl alcohol

* reseated internal ribbon cables multiple times

* tested left mono input/output only

 

What still works:

 

* true bypass path

* pedal boots normally

* UI/display/controls all work

 

What does NOT work:

 

* DSP processed signal path

 

 

Questions for experienced M5 repairers:

 

1. Are there known common failure points on the M5 that match these symptoms?

2. Which voltage rails should I check first?

3. Any known bad capacitors or regulators on the M5?

4. Has anyone seen “true bypass works but DSP path silent” before?

5. Could a ribbon/header issue still cause this even after reseating?

6. Is there a service manual or board schematic available anywhere?

 

Any advice would really help be

fore I start deeper component-level troubleshooting.

 

Thanks!