I have a Ibanez V Series V70CE. It play well. But sometimes when I connect it to the amplifier, there seems to be loose connection and no sound is heard. When I move the wire little bit sound come back.. Have you seen this issue before?

It sounds like the guitar pickup may be faulty. It may be a loose connection on the which could be soldered easily or it may be damaged in some other way.

As this is isolated with your G string, I suspect the individual pickup unit is at fault.

I would try a different guitar cord to prove that the one you use is good. If the plugin socket is rusty or corroded it could do the same. If you use a speaker cable as an instrument cable, that would happen. A loose ground can also and cheap pickups will do it naturally. I know an S series should have decent pickups but, if you got it used or something like that, you never know what was done to it before you got it.

I think this might help you, http://www.ultimate-guitar.com/forum/showthread.php?t=1030494

Is the tuner showing power. If so pull out (remove mounting screws) and check 1/8 plug from under the saddle pickup.

If the sound you when plugging in the headphones is coming from the main speaker, it is just the interupt contacts on the headphones are making or breaking contacts. This can be a good sign if there is volume to it.

Make sure the plugs you are using are the right length for jacks in the Ibanez as there are 5 lengths in use.. If this just developed, the length issue is not a concern. If the jacks are loose, snug them down, it may be a faulty ground relying on the chassis contact with the jack.. It may be a broken wire. It can also be a dirty switch contact or dirty level control, especially the standby switch. make sure they are clean with some contact cleaner. Make sure your level is turned up too,

If you have effects, or a preamp on the guitar, make sure they have fresh batteries or good power connections and are not on mute.

View videos on "guitar setup" on YouTube. Quickest way to learn and also avoid mistakes on guitar setup !!!

There are many things it could be... I often quote the probabilities based on many ears experience and the symptoms the person posts.

Based on what you have posted, there is an intermittent... the unit vibrates while operating... there could be a loose connection, however my guess is you will find bad soldering at one of the pots that control the distortion. The solder often breaks loose as the sometimes support the circuit board from the leads of these pots. Also switches that solder to the board and sometimes jacks for foot switches break loose.

Open the unit and inspect the solder joints. It often takes an experienced eye to spot the tiny grey ring around the solder meniscus that signals a failure.

Make sure that when the audio from the speaker cuts, the headphones are still working (try it several times). If this is correct the problem isn't at the preamp or driver tubes. Try to locate the wire that is connecting the speaker with the amplifier. There must be a bad connection at this cable or at the speaker's coil itself. Try hiting with a solid tool (the back of a screwdriver for example) the parts and the connections (not the tubes, just the mountings) while the amplifier is working in order to locate a loosing contact. Once you find it use a soldering gun to fix the connection.

In case of a problem or clarification, don't hesitate to post.

Thanks and regards
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Stelios
direct FixYa link: http://www.fixya.com/users/technical114

there is an open connection, most likely right off the input jack ,causing this issue. Resolder it or retension the jack connection.