dead effects loop on triple! help!

[Yetti]

Hey guys I've got an old rev f triple and when I have the loop engaged on its totally dead. Switch it to bypass and amp comes alive like normall, but still sounds a bit off.
I've tryed mixing tubes up but nuthin. What is it, power tube? A tech told me v2 is the one to change And I may have tested every tube but that one. So ill, see anyone else had same problem?

 

[Gnarkill3k6]

Have you tried to connect the send and return jacks with a patch cable?

 

[domct203]

V4 is your FX Loop Driver. If the amp works with the loop bypassed it is not a power tube problem.

Where do you have your FX Loop SEND Level?

Dom