Mesa 7i77 - dead encoder channel

Hi
I have a Mesa 7i77 with a dead encoder channel number 5. I dont know why, but the card was new, and i installed it in a new retrofit project. Encoder 5 is the spindle encoder. 

This machine has 6 encoders. all of them differential ABZ, 5v. 
First, i suspected the encoder, so i wired a new one in. no luck
I then plugged the new encoder into the encoder 4 position in the card, and it works, plug it into no 5 position, not working

I changed the 7i77 with another new one and it worked. So i assume that the problem is with that 7i77. 

I can repair it if i can find the defective component, and i would appreciate any pointers on what to look for, where to measure and so on. I have scope if required.

Thanks
Henk

Hi. 
that was the first thing I did. No luck 
I'm positive that the card is faulty. Changed it with another 7i77 and it worked. 

I did check the jumpers as well, although with a diff encoder, it will count if the jumpers are set for single ended as well.

If Neither A,B or index work, its likely the 26LS32 near the ENC 5 connector

I will set up the card on the bench and do some poking around.
Thanks