Amplifier problems: Can only hear right channel

[Erik_Twice]

I finally got the cables for my dad's amplifier and it seems to work well, except that I can only hear one channel when I plug in my headphones.

EDIT: I've tested a bit and it seems the signal simply doesn't reach the left side of the amplifier. The actual connections are good and if I plug a couple of headphones that cause interference, the left needle moves like the right one.

The left needle doesn't move when I first power up the amp so this must be a very easy fix. I opened the case and everything seems to be in top shape, it must be a bad conneciton or something like that.

Thoughts?

[JoeAwesome]

I'm not technically minded, but it sounds like an issue with the headphone jack. Have you cleaned that up? Are your headphones with or without a mic option? I think that might bring another issue.

[Erik_Twice]

Ok, I think I might have found something's up: When I first turn on the amplifier, the right speaker indicator goes full to the side and then comes back. But the left one stays still. Perhaps it is not working?

[fastbilly1]

Is the headphone jack mono only?

[Erik_Twice]

Is the headphone jack mono only?

No, it's stereo. I thought so at first but no, it seems there's no signal in the whole left side of the stereo. But when I open it, everything seems fine.

There must be a problem somewhere and it must be a really easy fix, but I have no idea on how to do it!