Sick orchid- HELP

Can any orchid lovers out there tell me what to do with this sick orchid?

It's not too much light or heat because there are 2 others on the same window sill with beautiful green leaves.
There are flowers on the shoot
There does not appear to be any rot. 
It has not been over watered - more likely the reverse. 

Does it need repotting because there are no nutrients left in the bark mixture? 
Do I cut off these yellow leaves ?  Then it will have no leaves!
HELP