NINTH AMENDMENT TO CREDIT AGREEMENT

EXHIBIT 10.85.9

CLOSING CHECKLIST

RENEWAL AND EXTENSION OF THE

REVOLVING LINE OF CREDIT PROVIDED BY

COMERICA BANK

TO

MICROSEMI CORPORATION

MARCH 29, 2005

NINTH AMENDMENT

TO

CREDIT AGREEMENT

Dated as of March 29, 2005

This NINTH AMENDMENT TO CREDIT AGREEMENT (this “ Amendment ”) is among MICROSEMI CORPORATION , a Delaware corporation (the “ Borrower ”), the several financial institutions party to the Credit Agreement referred to below (collectively, the “ Lenders ”; individually, a “ Lender ”), and COMERICA BANK , as administrative agent for the Lenders (the “ Administrative Agent ”).

PRELIMINARY STATEMENTS:

(1) The Borrower, the Lenders and the Administrative Agent have entered into that certain Credit Agreement dated as of April 2, 1999, as amended by the First Amendment to Credit Agreement dated as of June 25, 1999, the Second Amendment to Credit Agreement dated as of February 14, 2000, the Third Amendment to Credit Agreement dated as of April 2, 2001, the Fourth Amendment to Credit Agreement dated as of May 25, 2002, the Fifth Amendment to Credit Agreement dated as of December 5, 2002, the Sixth Amendment to Credit Agreement dated as of December 10, 2003, the Seventh Amendment to Credit Agreement dated as of March 31, 2004 and the Eighth Amendment to Credit Agreement dated as of March 31, 2004 (as so amended the “ Credit Agreement ”; capitalized terms used and not otherwise defined herein have the meanings assigned to such terms in the Credit Agreement).

(2) The Borrower has requested that the Administrative Agent and the Lenders make certain amendments to the Credit Agreement.

(3) The Administrative Agent and the Lenders are, on the terms and conditions stated below, willing to grant the request of the Borrower.

NOW, THEREFORE in consideration of the premises and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto hereby agree as: follows:

SECTION 1. Amendments to Credit Agreement . Effective as of the date hereof and subject to the satisfaction of the conditions precedent set forth in Section 2 hereof, the Credit Agreement is hereby amended as follows:

(a) The definition of the term “Stated Maturity Date” in Section 1.1 of the Credit Agreement is hereby amended and restated in its entirety to read as follows:

“ Stated Maturity Date means March 31, 2007.”

(b) Section 10.9, Governing Law, shall be amended and restated in its entirely to read as follows:

“Section 10.9. Governing Law; Entire Agreement. THIS AGREEMENT, THE NOTES AND EACH OTHER LOAN DOCUMENT SHALL EACH BE DEEMED TO BE A CONTRACT GOVERNED BY AND CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE LAWS OF THE STATE OF CALIFORNIA (WITHOUT REGARD TO ITS CONFLICT OF LAWS PROVISIONS). This Agreement, the Notes and the other Loan Documents constitute the entire understanding among the parties hereto with respect to the subject matter hereof and supersede prior agreements, written or oral, with respect thereto.”

(c) Section 10.13., Forum Selection and Consent to Jurisdiction, shall be amended and restated in its entirety to read as follows:

“Section 10.13 Forum Selection and Consent to Jurisdiction . Borrower, Lenders and Administrative Agent hereby irrevocably submit to the non-exclusive jurisdiction of any United States Federal Court or California state court sitting in Los Angeles, California in any action or proceeding arising out of or relating to this Agreement or any of the Loan Documents and Borrower, Lenders and Administrative Agent hereby irrevocably agree that all claims in respect of such action or proceeding may be heard and determined in any such United States Federal Court or California state court. Borrower irrevocably consents to the service of any and all process in any such action or proceeding brought in any court in or of the State of California by the delivery of copies of such process to Borrower at its address specified on the signature page hereto or by certified mail directed to such address or such other address as may be designated by Borrower in a notice to the other parties that complies as to delivery with the terms of Section 10.2. Nothing in this Section shall affect the right of Administrative Agent to serve process in any other manner permitted by law or limit the right of the Administrative Agent to bring any such action or proceeding against Borrower or any of Borrower’s Subsidiaries or any of their property in the courts with subject matter jurisdiction of any other jurisdiction. Borrower irrevocably waives any objection to the laying of venue of any such suit or proceeding in the above described courts.”

SECTION 2. Conditions to Effectiveness . The amendments in Section 1 of this Amendment shall be effective as of the date hereof, subject to the Administrative Agent’s receipt of the following on or before March 29, 2005:

(a) counterparts of this Amendment executed by the Administrative Agent, the Borrower and all Lenders;

(b) a reaffirmation of the Guaranty, in form and substance satisfactory to the Agent, duly-executed by each Guarantor; and

(c) such other documentation as the Administrative Agent or any Lender shall reasonably request.

SECTION 3. Representations and Warranties . The Borrower represents and warrants as follows:

(a) Authority: Enforceability . The Borrower has the requisite corporate power and authority to execute, deliver and perform this Amendment, and to perform its obligations under the Credit Agreement as amended hereby. The execution, delivery and performance by the Borrower of this Amendment, and the consummation of the transactions contemplated hereby, have been duly approved by the Board of Directors of the Borrower and no other corporate proceedings on the part of the Borrower are necessary to consummate such transactions. This Amendment has been duly executed and delivered by the Borrower. Each of this Amendment and the Credit Agreement as amended hereby constitutes the legal, valid and binding obligation of the Borrower, enforceable against the Borrower in accordance with its terms.

(b) Loan Document Representations and Warranties . The representations and warranties contained in each Loan Document are true and correct on and as of the date hereof, before and after giving effect to this Amendment, as though made on and as of such date (unless stated to relate solely to an earlier date, in which case such representations and warranties shall be true and correct as of such earlier date).

(c) Absence of Default . No event has occurred and is continuing, or would result from the effectiveness of this Amendment, that constitutes a Default.

SECTION 4. Reference to and Effect on the Loan Documents . (a) Upon and after the effectiveness of this Amendment, each reference in the Credit Agreement to “this Agreement”, “hereunder”, “hereof” or words of like import referring to the Credit Agreement, and each reference in the other Loan Documents to “the Credit Agreement”, “thereunder”, “thereof” or words of like import referring to the Credit Agreement, shall mean and be a reference to the Credit Agreement as modified and amended hereby.

(b) Except as specifically amended above, the Credit Agreement and all other Loan Documents are and shall continue to be in full force and effect and are hereby in all respects ratified and confirmed.

(c) The execution, delivery and effectiveness of this Amendment shall not, except as expressly provided herein, operate as a waiver of any right, power or remedy of any Lender or the Administrative Agent under any of the Loan Documents, nor constitute a waiver of any provision of any of the Loan