Mark IV issue (FX Loop related?)

[guitarrero]

I'm using a Mark IV (b) and a tc electronic g-system (4-cable-method) for quite a while now and everything worked nicely. Well, until last week... all of a sudden my tone became very flat, silent and dead. The gain was almost completely gone in the "Rhy 2"- and "Lead"-Channel. Well, i tried to locate the problem and unplugged my g-system, played through the amp only and it works and sounds perfect. Switching channels with the G is still possible when i hook up the relais-cables and stuff. But as soon as the guitar signal goes in the G-System first and takes the whole ride through the FX-Loop and the circuit of the g-system the problem remains.
My first guess is that there must be something wrong with the FX Loop, but i'm not sure... does this sound familiar to anyone of you? Anyone with some good advice? Thanks a lot in advance! Please, save me!

 

[Rampage]

It definitely sounds as if there is a problem with the loop or the G-System. I think the simplest place to start is cleaning the effects loop jacks on the MKIV as well as the jacks on the G-System. Something like Deoxit will work perfectly for this. You may also want to swap the V2 preamp tube out with a spare to see if that could be on its way out.

 

[guitarrero]

Cool, thanks so far. Cleaning the jacks is a good idea indeed, yeah. Will try that.
Plus: I have a complete set of brandnew Preamp-Tubes that i wanted to put it anyway... i'll try both within the next few days and keep you guys up to date. Don't think the G might be the source of trouble here, but will also check that for sure. Thankies!

 

[Rob Lockwood]

If you plug the guitar straight in to the amp, make sure that the effects loop is switched on and then just connect a patch cable to join send and return then you are using the whole effects loop, just with no effects in it. If it all still works correctly like this with g system disconnected then it isn't a loop problem. If the loop is bad you should hear the same problem when you switch the loop on - in that case could be the tube that runs the loop.

If you are loosing gain then the problem sounds like it may be a loss of signal going in to the front of the amp rather than a problem in the loop. Could be preamp tubes but if it is all perfect without the effects connected maybe not. I would try checking each part seperately ie amp only as above, then just the g system in front of amp bypassing the loop, then with guitar straight into amp (with different cable) and g system in the loop only.

Hopefully like this you can narrow down where in the signal chain the problem lies.

 

[guitarrero]

Thanks Rob, sounds like some more useful hints, very appreciated!
I can say that the amp works perfectly as long as i plug straight into the amp. That's how i used it the last two band-rehearsals. I'll probably find some time tomorrow for checking everything out and i guess with your help i'll be ablo to locate and solve the problem. Thanks, guys! I'll report what happens!

 

[guitarrero]

Proble solved guys. It wasn't the Amp at all! For some reason the Insert Loop on the G-System turned itself off globally so on every channel everything that came through was a processed signal with the Gs FX but without any Preamp-Flavor... *lol* Anyway, found it, solved it, everything works well again. Those Mesa-Amps are built like a tank apparently...