My mark V FX loop does not take effect.... Please HELP....

My mark V FX loop just stopped to work. I have the "loop active" switch "UP", and all my effects does not take any effect effect.
The "FX" LED is not ON either (the foot switch FX LED is ON).

With the "Loop active" switch in UP position, I have the pedals connected as following:
Guitar --> tunner ---> wahwah --> compressor --> Mark V front panel input
Mark V send --> chorus --> delay --> Mark V return

I don't get any chorus and delay effects. Wah and compressor work fine. Moving delay and chorus before the pre-amp, those pedal work.
Am I missing anything? I have checked the tubes and they look OK. I even changed the effect return tube (V6) but it did not help. I unplugged the foot-switch, and selected"Loop Assign" to "All", but no help. How can I troubleshoot this problem? Can it be the loop active switch causing the problem?
Thousand thanks in advance.

Check the fix switch in the channel settings and the cable routing. Other than that, not sure what to try.

You could start with just plugging in a short cable in the loop (no pedals) to connect send and return. That way, you should get a sound if the loop is active (no concern with a bad pedal or bad cable connection between pedals etc.).

Then, double check the knobs and buttons on the Mark and see what's going on. Make sure that "loop assign" knob on the back of the amp (upper left on the head version) is set to "all on" or however you want it set. It is hopefully just set to a different channel than what you intended?

Mace

Thanks for the help messages.
I have tried to plug in a short cable into send and return, and I have sound.

However, if I unplug the either side, I STILL have sound, e.g. only plug in the "send" side, or only plug in the "return" side.
I remember my previous experience, if I only plug in send, I am not supposed to get any sound.

The issue is the fx loop is totally ignored. It seems like it does not know there is a cable plugged into "send", so it always acts like there is no fx loop cable plugged in, even there is a cable plugged in to send.

cchsiang2008 said:

if I unplug the either side, I STILL have sound, e.g. only plug in the "send" side, or only plug in the "return" side.

Click to expand...

This is normal, I believe. I think the circuitry is "smart" and knows that if the connection is not correct, i.e., the signal is not passing through, it just ignores the loop. I accidentally switched the send and return once (connected the send from the head to the output of the pedal) and I still got sound but no effect. That is why I recommended above to check the cable routing. Also, I mentioned above, the FX switch in the channel settings but I misspoke. On the rear panel, check the "loop assign" switch. It determines which channel (or all channels) go to the FX loop. Or, if it is based on the foot switch, which is where you probably need the switch to be.