<PAGE>   1
10(y). First Amendment dated March 28, 1997, to the Credit Agreement dated May
31, 1994.

                       FIRST AMENDMENT TO CREDIT AGREEMENT
                       -----------------------------------

                  THIS FIRST AMENDMENT TO CREDIT AGREEMENT, dated as of March
28, 1997 (this "Amendment"), is among KEITHLEY INSTRUMENTS, INC., an Ohio
corporation (the "Company"), KEITHLEY INSTRUMENTS, GMBH, a corporation organized
and existing under the laws of the Federal Republic of Germany (the "Borrowing
Subsidiary" and, collectively with the Company, the "Borrowers" or each a
"Borrower") the banks set forth on the signature pages hereof (collectively, the
"Banks") and NBD BANK, a Michigan banking corporation, as agent for the Banks
(in such capacity, the "Agent").

                                    RECITALS
                                    --------

                  A. The Borrowers, the Agent and the Banks are parties to a
Credit Agreement, dated as of May 31, 1994 (as now and hereafter amended, the
"Credit Agreement"), pursuant to which the Banks agreed, subject to the terms
and conditions thereof, to extend credit to the Borrowers.

                  B. The  Borrowers  desire to amend  the  Credit  Agreement  
and the Agent and the Banks are willing to do so strictly in accordance with 
the terms hereof.

                                      TERMS
                                      -----

                  In consideration of the premises and of the mutual agreements
herein contained, the parties agree as follows:

                                   ARTICLE 1.
                                   AMENDMENTS
                                   ----------

                  Upon  fulfillment of the conditions set forth in Article 3 
hereof,  the Credit Agreement shall be amended as follows:

                  1.1      Section 1.1 shall be amended as follows:

                           (a) The definition of "Commitment"  shall be amended
 by deleting the last sentence of such definition in its entirety.

                           (b) The  definition  of  "Floating  Rate" shall be 
amended by deleting  clause (a) therein in its entirety and inserting the 
following in place thereof:  "(a) zero percent (0%) per annum".
<PAGE>   2

                           (c) The definition of "Leverage Ratio" shall be 
deleted in its entirety and the following shall be inserted in place thereof:

                           "LEVERAGE RATIO" shall mean, as of any date, the
                           ratio of the Consolidated Funded Debt of the Company
                           and its Subsidiaries to the Consolidated Total
                           Capitalization of the Company and its Subsidiaries.

                           (d) The  definition of  "Multicurrency  Rate" shall 
be amended by deleting  clause (a) in its entirety and inserting the following 
in place thereof:  "(a) Applicable Margin".

                           (e) The definition of  "Termination  Date" shall be 
amended by deleting clause (a) in its entirety and inserting the following in 
place thereof:  "March 28, 2002".

                           (f) The following definitions shall be added in
appropriate alphabetical order:

                           "APPLICABLE MARGIN" shall mean with respect to any
                           Multicurrency Rate Loan or facility fee, as the case
                           may be, the applicable percentage set forth in the
                           table below as adjusted on the first day of each
                           fiscal quarter of the Company based upon the Leverage
                           Ratio for the fiscal quarter immediately preceding
                           the fiscal quarter most recently ended and shall
                           remain in effect until the next change to be effected
                           pursuant to this definition:

                                Applicable Margin
                                -----------------
<TABLE>
<CAPTION>

 ====================================================================================
 Leverage Ratio                                      Multi-currency     Facility Fee
                                                        Rate Loan
 --------------------------------------------------- --------------------------------
<S>                                                  <C>                <C>   
 Less than .30:1.0                                      0.275%             0.125%
 --------------------------------------------------- ------------------ -------------
 Greater than or equal to .30:1.0 but less than 
 .38:1.0                                                0.35%              0.15%
 --------------------------------------------------- ------------------ -------------
 Greater than or equal to .38:1.0                       0.55%              0.20%
 --------------------------------------------------- ------------------ -------------
</TABLE>

                           "FIRST AMENDMENT EFFECTIVE DATE" shall mean 
                           March 28, 1997.

                           "FIXED CHARGE COVERAGE RATIO" shall mean, as of the
                           end of any fiscal quarter, the ratio of (a)
                           Consolidated EBITDA of the Company and its
                           Subsidiaries for such period, plus the maximum amount
                           of all rents and other payments paid or required to
                           be paid during such period under any operating lease
                           in respect of which the Company or any of its
                           Subsidiaries is obligated as lessee or 


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<PAGE>   3


                           user, to (b) the sum of Consolidated Interest Expense
                           for such period, plus the maximum amount of all rents
                           and other payments paid or required to be paid during
                           such period under any operating lease in respect of
                           which the Company or any of its Subsidiaries is
                           obligated as lessee or user.

                           "FUNDED DEBT" of any person shall mean, as of any
                           date, the sum of all interest bearing Indebtedness
                           (including obligations under Capital Leases) of such
                           person, other than Indebtedness which is subordinated
                           to Indebtedness owing to the Agent and the Banks
                           under the Loan Documents in form and substance
                           satisfactory to the Banks minus invested cash in
                           excess of $1,500,000.

                           "NET WORTH" of any person shall mean, as of any date,
                           the amount of any capital stock, paid in capital and
                           similar equity accounts plus (or minus in the case of
                           a deficit) the capital surplus and retained earnings
                           of such person and the amount of any foreign currency
                           translation adjustment account shown as a capital
                           account of such person.

                           "TOTAL CAPITALIZATION" of any person shall mean, as
                           of any date, the sum of the Funded Debt of such
                           person plus the Net Worth of such person.

                           (g) The following  definitions shall be deleted from 
Section 1.1 together with all references to such defined terms in the Credit
Agreement: "Applicable Tranche B Commitment Fee", "Applicable Multicurrency Rate
Margin", "Applicable Floating Rate Margin", "Current Assets" and "Current
Liabilities", "Debt Service Coverage Ratio" and "Tangible Net Worth".

                    1.2    Section  2.1(b)  shall be amended by deleting the  
reference to "Tranche A Commitment" in the last sentence and inserting
"Commitment" in place thereof.

                    1.3    Section 2.1(c) shall be deleted and the following
shall be inserted in place thereof:

                           (a) LIMITATION ON AMOUNT OF LOANS. Notwithstanding
                           anything in this Agreement to the contrary, the
                           aggregate principal amount of the Loans made by any
                           Bank at any time outstanding shall not exceed the
                           amount of its respective Commitment as of the date
                           any such Loan is made.

                    1.4    Section 2.3 shall be deleted and the following shall
be inserted in place thereof:




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<PAGE>   4

                                   2.3 FEES. (a) The Borrowers agree to pay to
                           each Bank a facility fee on the daily average amount
                           of its respective Commitment, for the period from the
                           Effective Date to but excluding the Termination Date,
                           at a per annum rate equal to the Applicable Margin.
                           Accrued facility fees shall be payable quarterly in
                           arrears on the last Business Day of each March, June,
                           September and December, commencing on the first such
                           Business Day occurring after the Effective Date, and
                           on the Termination Date.

                                           (b) The Borrowers agree to pay to the
                           Agent an agency fee for its services as Agent under
                           this Agreement in such amounts as may from time to
                           time be agreed upon by the Company and the Agent.

                    1.5    Sections 5.2(a), (b), (c) and (d) shall be deleted  
in  their  entirety  and the following shall be inserted in place thereof:

                           (a) [Intentionally Reserved.]

                           (b) NET WORTH. Permit or suffer the Consolidated Net
                           Worth of the Company and its Subsidiaries to be less
                           than the sum of (i) $24,000,000 plus (ii) an amount
                           equal to 50% of Cumulative Net Income (without
                           reduction for net loss) of the Company and its
                           Subsidiaries for each fiscal year of the Company
                           ending after the First Amendment Effective Date.

                           (c) FIXED CHARGE COVERAGE RATIO. Permit or suffer the
                           Consolidated Fixed Charge Coverage Ratio of the
                           Company and its Subsidiaries to be less than (i) for
                           the fiscal quarter ending March 31, 1997, 2.0 to 1.0,
                           (ii) for the fiscal quarter ending June 30, 1997,
                           2.25 to 1.0 ,and (iii) as of the last day of each
                           fiscal quarter thereafter, 2.75 to 1.0., in each
                           case, calculated for the four (4) consecutive fiscal
                           quarters then ending.

                           (d) LEVERAGE  RATIO.  Permit or suffer the 
                           Consolidated  Leverage Ratio of the Company and its  
                           Subsidiaries  to be greater  than .45 to 1.0 at any 
                           time.

                    1.6    The "Commitment Amount" set forth in the signature 
block of each Bank shall be amended by deleting the entire parenthetical
reference directly below the reference to such Bank's Commitment Amount, by
deleting the "Commitment Amount" set forth next to the 


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<PAGE>   5


name of each Bank and inserting in place thereof the amount set forth below next
to the name of such Bank:

                                Commitment Amount
                                -----------------

NBD Bank                         $15,000,000
KeyBank National Association $10,000,000

                                   ARTICLE 2.
                                 REPRESENTATIONS
                                 ---------------

                  Each Borrower represents and warrants to the Agent and the
Banks that:

                  2.1   The execution, delivery and performance of this 
Amendment is within its powers, has been duly authorized and is not in
contravention with any law, of the terms of its Articles of Incorporation or
By-laws, or any undertaking to which it is a party or by which it is bound.

                  2.2   This Amendment is the legal, valid and binding 
obligation of the Borrower enforceable against it in accordance with the terms
hereof.

                  2.3    After  giving  effect to the  amendments  herein  
contained, the representations and warranties contained in Article IV of the
Credit Agreement are true on and as of the date hereof with the same force and
effect as if made on and as of the date hereof.

                  2.4    No Event of Default or any event or condition which 
might become an Event of Default with notice or lapse of time, or both, exists
or has occurred and is continuing on the date hereof.

                                   ARTICLE 3.
                           CONDITIONS OF EFFECTIVENESS
                           ---------------------------

                  This Amendment shall not become effective until each of the
following has been satisfied:

                  3.1    This Amendment shall be signed by the Borrowers, the
Agent and the Banks.

                  3.2    The  Company  shall have  delivered  to the Agent  
resolutions adopted by the Board of Directors of the Company, certified by an
officer of the Company as being in full force and effect without amendment as of
the date hereof, authorizing the Company to execute this Amendment.




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<PAGE>   6

                  3.3    An opinion of legal counsel for the Company with  
respect to the matters contained in Sections 2.1 and 2.2 hereof and otherwise in
form and substance satisfactory to the Banks shall have been delivered to the
Agent.

                  3.4    The Borrowers shall have executed new Revolving Credit 
Notes in favor of each Bank reflecting the Commitment Amounts set forth in
Section 1.6 of this Amendment.







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<PAGE>   7



                                   ARTICLE 4.
                                 MISCELLANEOUS.
                                 --------------

                  41.    References  in the Credit Agreement or in any note,  
certificate, instrument or other document to the "Credit Agreement" shall be
deemed to be references to the Credit Agreement as amended hereby and as further
amended from time to time.

                  4.2    The Company  agrees to pay and to save the Agent 
harmless for the payment of all costs and expenses arising in connection with
this Amendment, including the reasonable fees of counsel to the Agent in
connection with preparing this Amendment and the related documents.

                  4.3    The Company  acknowledges  and agrees that the Agent 
and the Banks have fully performed all of their obligations under all documents
executed in connection with the Credit Agreement and all actions taken by the
Agent and the Banks are reasonable and appropriate under the circumstances and
within their rights under the Credit Agreement and all other documents executed
in connection therewith and otherwise available. Each Borrower represents and
warrants that it is not aware of any claims or causes of action against the
Agent or any Bank, any participant lender or any of their successors or assigns.

                  4.4    Except as expressly amended hereby, each Borrower 
agrees that the Credit Agreement, the Notes and all other documents and
agreements executed by the Borrower in connection with the Credit Agreement in
favor of the Agent or any Bank are ratified and confirmed and shall remain in
full force and effect and that it has no set off, counterclaim or defense with
respect to any of the foregoing. Terms used but not defined herein shall have
the respective meanings ascribed thereto in the Credit Agreement.

                  4.5    This Amendment may be signed upon any number of 
counterparts with the same effect as if the signatures thereto and hereto were
upon the same instrument.




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<PAGE>   8


                  IN WITNESS WHEREOF, the parties signing this Amendment have
caused this Amendment to be executed and delivered as of March 28, 1997.

                                 KEITHLEY INSTRUMENTS, INC.

                                     /s/ Ronald M. Rebner
                                 By: _________________________________

                                          Chief Financial Officer
                                     Its: _______________________________

                                 KEITHLEY INSTRUMENTS, GMBH

                                      /s/ H. Hamm
                                 By: _________________________________

                                           Managing Director
                                     Its: _______________________________

                                 NBD BANK, as Agent and Individually as a Bank

                                      /s/ Winifred S. Pinet
                                 By: _________________________________

                                            First Vice President
                                     Its: _______________________________

                                 KEYBANK NATIONAL ASSOCIATION,
                                 SUCCESSOR BY MERGER TO SOCIETY
                                 NATIONAL BANK

                                     /s/ Thomas J. Purcell
                                 By: _________________________________

                                           Vice President
                                     Its: _______________________________




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