So what is wrong with my headphone panel jack?

My Zana Deux S has a problem that I can't figure out
Various headphones with 1/4" TRS connections all work in other amps just fine but the ZDS only plays one channel at a time. Either right or left but not both.
I insert the headphone plug, first the tip meets the sleeve and I get sound from the left channel. I insert it all the way and end up with sound just from the right channel.
So the panel jack outputs left and right successfully. I think the amp connections must be good, right?
I disassemble and insert a cable and I can see that mechanically, the connectors are behaving as they should.
I test for continuity and it is there for left, right and ground, the plug and jack are mating correctly.
So why can't I get both channels to play at the same time?
The tip contacts in the right place, the sleeve and ground do as well.
As I type this, I recall the Crack uses resistors between these 3 connections. I don't know about the ZDS.
Any ideas?

 

When the plug is partially inserted, sound comes out of the left headphone but that sound is actually the right channel. Because the left part of the plug is in contact with the right part of the jack.

Maybe the left channel of your ZDS is broken.

 

A simple test in Windows does indicate the left channel is out, time to find out why.

 

Thanks, it's the rca cable (diy), got some work to do...lol, should have tested this first...
spoke too soon, it's not the cables, now I really have some work to do...

 

I use a diy rca switch, one of the outputs just stopped working on the left channel. I am fine on another output. Thanks for pointing me in the right direction.