Im having some problems too with my effect loops.
I am using 2 amps and both have the same problem.
Marshall Mosfet 100 Reverb & Hughes & Kettner Attax 80 Vintage Series.
At first there everything was fine. Then I started using the effect loop. Still nothing was wrong. Untill I unplugged the effect loop and started playing again. My sound was gone! Then it came back with lots of crackles and volume changes (drops). After playing a while it was stable again (or not?!?!). [U]aaarrggg!![/U]
If I use my effect loop again with a pedal, everything is fine again. What I do now when I dont use my effect loop is; I connect a cable from to Send to Return. Then it works okay. But this is not the correct way!
I sent my Marshall to a tech once, but he couldnt fix it. Someone told me once that it could be the jacks that are made of bad quality. maybe..
Does anybody know what the problem might be?