This is a note to describe the fiber ribbon problem that developed on the Hubs.

There is a channel running down the edge of the heat sink where the ribbons are routed. As boards started spending more and more time in the shelf with fans blowing across them, the fins started slowly wearing their way through the outer jacket, through the colored inner jackets, and eventually through the
fibers themselves. Usually, Hub Minipods would report drastic losses in received light levels before it got that far, but several did continue to function in a slowly degrading state for some time. Many reported received light levels in the low 100uW to double-digit range. (Interestingly, TTC could hold a lock to as low as 180uW.)

In troubleshooting working light level problems, I eventually made my way back on to the Hubs and discovered the problem as described above. The heat sink is the main problem area, but there is also a spot between the Zone 2 connector and the MPOs where the ribbons are sort of held in place by a through-hole
capacitor.

The solution we agreed upon was to wrap the fiber ribbons in Kapton where it would come in direct contact with the heat sink and that capacitor. On the Hubs that have been modified in this way, all light level issues have disappeared.

There are several photos in this folder which show how this had been affecting various Hubs.

I will be placing a note in each Hub’s trailer sheet to confirm that the fiber ribbons have been replaced and protected. I’m starting with the boards that have RODs installed and which are slated for testing and eventual shipment to CERN. The rest will come later, and as fiber ribbons become available from vendors.