FCO 85XBA-1003, Fusible resistor blows prematurely on power up.

This Field Change Order (FCO) addresses the premature blowing of a fusible resistor on power-up, which prevents the H7176-A module from turning on in VAX 8500/8550 (851BA and 855BA) 50 Hz systems. A quick check involves looking for a disk-shaped PTC (Positive Temperature Coefficient thermistor) in place of R31, indicating if the fix has been applied.

The FCO mandates upgrading affected systems and field spares by retrofitting the 70-23575 (VAX 8500) and 70-23145 (VAX 8550) Cab Sub-assemblies to "C1." This is achieved by installing ECO H7176-A-LK003, which includes replacing any H7176-A module below Revision C2 with a new H71760-A Power Supply (Revision C2 or higher, part number EQ-01460-01).

The installation procedure, estimated to take 1 hour, involves a controlled system shutdown, powering off the VAX, utilizing ESD safety precautions, locating and removing the existing H7176-A module (if its revision is below C2), and installing the replacement. Following the module swap, the system is powered back on, the VMS Operating System is brought up, the Site Management Guide is updated, and the FCO activity is reported on a LARS form. This FCO affects an initial population of 433 systems.