I powered up the rebuilt power supply board today and it works perfect.

Closing thoughts on the D5500 are that one of the most important things to do to ensure stability of the closed loop is to make sure the contacts between the mixing box and the power supply are clean. I have been using Deoxit with good results.

The signal carried through that connector is very low voltage and is easily destroyed or corrupted by just a little oxidation on the contacts. The symptom is that the motors keep 'hunting' and moving the filters slowly during the exposure.