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                                  EXHIBIT 10.6


                      FOURTH AMENDMENT TO CREDIT AGREEMENT


         THIS FOURTH AMENDMENT TO CREDIT AGREEMENT (the "Fourth Amendment") is
made and dated as of the 27th day of April, 2000, by and among SANWA BANK
CALIFORNIA ("Sanwa"), those other banks (each, including Sanwa, a "Lender" and,
collectively, the "Lenders"), party with Sanwa to the Agreement defined in
Recital A below, SANWA, as agent for the Lenders (in such capacity, the "Agent")
and as the L/C Bank (as defined in the Agreement), and NTS TECHNICAL SYSTEMS, a
California corporation (the "Borrower").


                                    RECITALS


         A. Pursuant to that certain Credit Agreement dated as of September 8,
1997 among the Agent, the Lenders and the Borrower (as amended, modified, or
waived, the "Agreement"), the Lenders agreed to extend credit to the Borrower on
the terms and subject to the conditions set forth therein. All capitalized terms
not otherwise defined herein shall have the meanings given to such terms in the
Agreement.


         B. The Company has notified the Lenders that the Company has incurred
non-recurring expenses, with an aggregate after tax impact of $1,237,000, as a
result of (1) the legal and settlement costs of a lawsuit brought by Tecstar
(the "Tecstar Lawsuit") and (2) discontinuance of certain operations in
Sacramento, California (the "Discontinued Operations").


         C. As a result of the such losses, the Borrower has notified the
Lenders that, as of the end of if the fiscal quarter ending January 31, 2000,
the Borrower had failed to comply with the requirements of PARAGRAPH 7(j)(5) of
the Agreement pursuant to which the Borrower had agreed to maintain a Debt
Coverage Ratio of at least 1.30 to 1.00.


         D. The Borrower has requested the Agent, the L/C Bank and the Lenders
to waive any Potential Default or Event of Default occurring as a result of the
Borrower's failure to comply with such covenant and has further requested that
the Agreement be amended in certain related respects.

         E. At the request of the Borrower, the Agent, the L/C Bank and the
Lenders have agreed to provide such waivers and amendments to the Agreement as
more particularly described below.


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

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                                    AGREEMENT


         1.       INDUCEMENT REPRESENTATIONS BY THE BORROWER AND WAIVER.

                  (a) INDUCEMENT REPRESENTATIONS. In addition to the other
representations and warranties contained in this Fourth Amendment and the other
terms and provisions hereof, to induce the Agent, the L/C Bank and the Lenders
to grant the waiver requested by the Borrower and to enter into this Fourth
Amendment, the Borrower hereby represents and warrants as of the Effective Date
(as such term is defined in PARAGRAPH 4 below) that:

                           (i) The descriptions of the Tecstar Lawsuit and the
         Discontinued Operations and the results thereof contained in notices
         delivered pursuant to PARAGRAPH 6(f) of the Agreement are complete and
         accurate in all material respects, do not contain any material
         misstatement of fact, and do not fail to contain any statement that
         results in such statements being misleading in any material respect.

                           (ii) Except for the Borrower's breach of PARAGRAPH
         7(j)(5) (the "Existing Default") by reason of its failure to maintain a
         Debt Coverage Ratio for the four fiscal quarters ended January 31, 2000
         of at least 1.30 to 1.00, no Events of Default or Potential Defaults
         have occurred under the Agreement.

                  (b) WAIVER. In reliance on the foregoing representations and
subject to the other terms and conditions of this Fourth Amendment, as of the
Effective Date, the Agent, the L/C Bank and the Lenders waive the Existing
Default. Such waiver is specific in time and intent and does not constitute a
waiver of any other rights or provisions in the Agreement or any related
documents.


         2.       AMENDMENTS REGARDING DEBT COVERAGE RATIO.

                  (a) DEBT COVERAGE RATIO REQUIREMENT. To reduce the Debt
Coverage Ratio that the Borrower is required to maintain as of April 30, 2000
and July 31, 2000, PARAGRAPH 7(j)(5) of the Agreement is amended to read as
follows:

         "(5) permit the Debt Coverage Ratio for the four fiscal quarters ending
         (i) as of April 30, 2000 and July 31, 2000, to be less than 1.20 to
         1.00 or (ii) as of the end of any fiscal quarter thereafter, to be less
         than 1.30 to 1.00."

                  (b) DEFINITION. To delete deferred tax changes from the
calculation of the "Debt Coverage Ratio" , the definition of "Debt Coverage
Ratio" in PARAGRAPH 1 of the Agreement is amended to read as follows:

                           "DEBT COVERAGE RATIO" shall mean the ratio of (i)
         for any period, (a) net profit after taxes realized during such period
         plus (b) depreciation and amortization expense and interest expense
         deducted in determining such net income to (ii) the sum of

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         the current portion of long-term debt as at the end of such period
         plus interest expense and cash dividends paid during the period just
         ended. In calculating the Debt Coverage Ratio, the $1,237,000.00
         after-tax impact of the non-recurring expenses defined in the Fourth
         Amendment to this Agreement as the Tecstar Lawsuit and the
         Discontinued Operations shall be disregarded."

         3. REAFFIRMATION OF SECURITY AGREEMENTS. The Borrower hereby affirms
and agrees that (a) the execution and delivery by the Borrower of and the
performance of its obligations under this Fourth Amendment shall not in any way
amend, impair, invalidate or otherwise affect any of the Obligations of the
Borrower or the rights of the Lenders under the Security Documents or any other
document or instrument made or given by the Borrower in connection therewith,
(b) the term "Obligations" as used in the Security Agreement includes, without
limitation, the Obligations of the Borrower under the Agreement as amended
hereby, and (c) the Security Documents remain in full force and effect and
constitutes a continuing first priority security interest in and lien upon the
Collateral described therein.

         4. EFFECTIVE DATE. This Fourth Amendment shall be effective on the date
(the "Effective Date") when all of the following conditions precedent have been
satisfied:

                  (a) The Borrower shall have delivered or shall have had
delivered to the Agent each of the following (with sufficient copies for each of
the Lenders):

                           (i)   A duly executed copy of this Fourth Amendment;

                           (ii)  Such credit applications, financial statements,
authorizations, environmental reviews of the Property, and such information
concerning the Borrower and its Guarantors and their business, operations and
condition (financial and otherwise) as any Lender may reasonably request;

                           (iii) Certified copies of resolutions of the Board of
Directors of the Borrower approving the execution and delivery of the Fourth
Amendment and related Loan Documents to which it is a party;

                           (iv)  Certificates of the Secretary or an Assistant
Secretary of the Borrower certifying the names and true signatures of its
officers authorized to sign this Fourth Amendment and related Loan Documents to
which it is a party;

                           (v)   An opinion of Sheppard, Mullin, Richter &
Hampton, LLP, counsel to the Parent, the Borrower and their respective
Subsidiaries, in form and substance satisfactory to the Lenders, addressing the
due authorization, execution and delivery of this Fourth Amendment and related
documents and such other matters as the Lenders shall reasonably request.

                  (b) All fees and other amounts payable hereunder prior to such
date shall have been paid, and all acts and conditions (including, without
limitation, the obtaining of any necessary regulatory approvals and the making
of any required filings, recordings or

                                       3
<PAGE>

registrations) required to be done and performed and to have happened precedent
to the execution, delivery and performance of this Fourth Amendment and to
constitute the same legal, valid and binding obligations, enforceable in
accordance with their respective terms, shall have been done and performed and
shall have happened in due and strict compliance with all applicable laws.

                  (c) The representations and warranties made by or on behalf of
the Borrower and each Guarantor in or pursuant to the Loan Documents (and by
executing and delivering this Fourth Amendment, the Borrower represents that all
such representations and warranties) shall be accurate and complete in all
material respects as if made on and as of such date.

                  (d) There shall not have occurred an Event of Default or
Potential Default not otherwise cured or waived.

         If this Fourth Amendment shall not have become effective on or before
June 1, 2000, then this Fourth Amendment shall, at the election of the Lenders
as evidenced by written notice to such effect given by the Lenders to the
Borrower, terminate and be of no further force or effect.

         5. REPRESENTATIONS AND WARRANTIES. As an inducement to the Agent, the
L/C Bank and each Lender to enter into this Fourth Amendment, the Borrower
represents and warrants to the Agent, the L/C Bank and each Lender that:

                  (a) CORPORATE EXISTENCE; COMPLIANCE WITH LAW. The Borrower and
each Guarantor (1) is duly organized, validly existing and in good standing as a
corporation under the laws of the state of its incorporation and is qualified to
do business in each jurisdiction where its ownership of property or conduct of
business requires such qualification and where failure to qualify would have a
material adverse effect on it or its property and/or business or on its ability
to pay or perform the Obligations, (2) has the corporate power and authority and
the legal right to own and operate its property and to conduct business in the
manner in which it does and proposes so to do, and (3) is in compliance with all
Requirements of Law and Contractual Obligations.

                  (b) CORPORATE POWER; AUTHORIZATION; ENFORCEABLE OBLIGATIONS.
The Borrower and each Guarantor has the corporate power and authority and the
legal right to execute, deliver and perform this Fourth Amendment and the
Agreement as amended hereby to which it is a party and has taken all necessary
corporate action to authorize the execution, delivery and performance of this
Fourth Amendment and the Agreement as amended hereby. This Fourth Amendment and
the Agreement as amended hereby have been duly executed and delivered on behalf
of the Borrower and each Guarantor party thereto and constitute such Person's
legal, valid and binding obligations enforceable against it in accordance with
their respective terms, subject to the effect of applicable bankruptcy and other
similar laws affecting the rights of creditors generally and the effect of
equitable principles whether applied in an action at law or a suit in equity.

                  (c) NO LEGAL BAR. The execution, delivery and performance of
this Fourth Amendment and the Agreement as amended hereby, the borrowings
hereunder and the use of the

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proceeds thereof, will not violate any Requirement of Law or any Contractual
Obligations of the Borrower or any Guarantor or create or result in the creation
of any Lien on any assets of the Borrower or any Guarantor except as
contemplated thereby.

                  (d) NO MATERIAL LITIGATION. Except for the Tecstar Lawsuit and
the related lawsuit against The Borrower's insurance carrier, no litigation,
investigation or proceeding of or before any arbitrator or Governmental
Authority is pending or, to the knowledge of the Borrower or any Guarantor,
threatened by or against the Borrower or any Guarantor or against any of the
Borrower's or any Guarantor's properties or revenues which is likely to be
adversely determined and which, if adversely determined, is likely to have a
material adverse effect on the business, operations, property or financial or
other condition of the Parent and its Subsidiaries, taken as a whole.

                  (e) CONSENTS, ETC. No consent, approval, authorization of, or
registration, declaration or filing with any Governmental Authority is required
on the part of the Borrower or any Guarantor in connection with the execution
and delivery of this Fourth Amendment and the Agreement as amended hereby or the
performance of or compliance with the terms, provisions and conditions hereof or
thereof.

                  (f) NO DEFAULT. No Potential Default or Event of Default has
occurred under the Agreement which has not otherwise been cured or waived.

                  (g) FULL DISCLOSURE. None of the representations or warranties
made by the Borrower or any Guarantor in the Loan Documents as of the date such
representations and warranties are made or deemed made, and none of the
statements contained in any exhibit, report, statement or certificate furnished
by or on behalf of the Borrower or any Guarantor in connection with the Loan
Documents contains any untrue statement of a material fact or omits any material
fact required to be stated therein or necessary to make the statements made
therein, in light of the circumstances under which they are made, not misleading
as of the time when made or delivered.

         6.       MISCELLANEOUS PROVISIONS.

                  (a) EXPENSES. In accordance with Paragraph 6(g) of the
Agreement, the Borrower agrees to pay all reasonable out-of-pocket expenses of
the Agent incident to the preparation and negotiation of this Fourth Amendment.

                  (b) ENTIRE AGREEMENT. This Fourth Amendment embodies the
entire agreement and understanding between the parties hereto and supersedes all
prior agreements and understandings relating to the subject matter hereof and
thereof.

                  (c) GOVERNING LAW. This Fourth Amendment shall be governed by
and construed in accordance with the laws of the State of California, without
giving effect to choice of law rules.

                  (d) COUNTERPARTS. This Fourth Amendment may be executed in any
number of counterparts, all of which together shall constitute one agreement.

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         IN WITNESS WHEREOF, the parties hereto have caused this Fourth
Amendment to be executed as of the day and year first above written.



                              NTS TECHNICAL SYSTEMS, a California corporation


                              By: /s/ Lloyd Blonder
                                 ------------------
                              Name: Lloyd Blonder
                              Title: Vice President



                              SANWA BANK CALIFORNIA, as Agent and the L/C Bank


                               By: /s/ Robert Ligon
                                  -----------------
                               Name: Robert Ligon
                               Title: Vice President



                               SANWA BANK CALIFORNIA, as a Lender


                               By: /s/  Robert Ligon
                                  ------------------
                               Name: Robert Ligon
                               Title: Vice President



                               MELLON BANK, N.A., as a Lender


                               By: /s/ Garry Handelman
                                  --------------------
                               Name: Garry Handelman
                               Title: Vice President



                                       6
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                           REAFFIRMATION OF GUARANTIES

Each of the undersigned Subsidiaries and Guarantors agrees to the terms of this
Fourth Amendment and hereby ratifies and reaffirms its Guaranty of the
Obligations of the Borrower and its grant of a security interest in certain
property to secure such Guaranty in favor of the Agent, on behalf of itself, the
Lender, and the L/C Bank and agrees that, notwithstanding this Fourth Amendment
and any other amendment or supplement to the Agreement entered into prior to
this Fourth Amendment, its Guaranty shall remain in full force and effect with
respect to the Agreement as amended hereby.



                                   NATIONAL TECHNICAL SYSTEMS, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   ETCR, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   APPROVED ENGINEERING TEST LABORATORIES, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   ACTON ENVIRONMENTAL TESTING CORPORATION


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President


                                       7
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                                   NATIONAL TECHNICAL SYSTEMS - CERTIFICATION
                                   SERVICES, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   WISE AND ASSOCIATES, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   NTS TECHNICAL SERVICES, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



                                   XX CAL, INC.


                                   By: /s/ Lloyd Blonder
                                      ------------------
                                   Name: Lloyd Blonder
                                   Title: Vice President



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