Acifica Mastech Power Supplies Failure to honor warranty, Internet

I am having problems with a company from which we have purchased Mastech DC power supplies.

The company is Acifica, Inc., San Jose, CA. We have purchased more than 20 DC power supplies from this company over the last five years, but recently we purchased eight units of a higher voltage model (HY10020EX) that we had not used before.
Although previously purchased models have shown decent reliability, the HY10020EX model has had several failures and the failed units have been repaired by Acifica under the 1 year warranty.

These units have been used in accordance with information supplied by Acifica and the Users Manual. However, now Acifica refuses to repair and return one of our power supplies that was sent to them for warranty repair unless we pay a $100 service fee.