<PAGE>
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                                                                 EXHIBIT 10.(nn)

[FIRST NATIONAL BANK  LOGO]

<TABLE>
<CAPTION>
    LOAN NUMBER           LOAN NAME                ACCT. NUMBER       NOTE DATE       INITIALS
<S>                    <C>                       <C>                 <C>             <C>
2000001751/5 Renewal   M-Tron Industries, Inc.                         04/30/04           JRK
     NOTE AMOUNT          INDEX (w/Margin)             RATE          MATURITY DATE   LOAN PURPOSE
   $3,000, 000.00      FNBO National Base Rate         4.5%            04/30/05       Commercial
                                                 Creditor Use Only
</TABLE>

                                PROMISSORY NOTE
                 (Commercial - Revolving Draw - Variable Rate)

DATE AND PARTIES. The date of this Promissory Note (Note) is April 30, 2004. The
parties and their addresses are:

  LENDER:
   FIRST NATIONAL BANK OF OMAHA
   Stop Code 1030
   1620 Dodge St
   Omaha, Nebraska 68197
   Telephone: (402) 633-3555

  BORROWER:
   M-TRON INDUSTRIES, INC.
   a Delaware Corporation
   100 Douglas Avenue
   Yankton, South Dakota 57078

1. DEFINITIONS. As used in this Note, the terms have the following meanings:

   A. PRONOUNS. The pronouns "I," "me," and "my" refer to each Borrower signing
   this Note, individually and together with their heirs, successors and
   assigns, and each other person or legal entity (including guarantors,
   endorsers, and sureties) who agrees to pay this Note. "You" and "Your" refer
   to the Lender, with its participants or syndicators, successors and assigns,
   or any person or company that acquires an interest in the Loan.

   B. NOTE. Note refers to this document, and any extensions, renewals,
   modifications and substitutions of this Note.

   C. LOAN. Loan refers to this transaction generally, including obligations and
   duties arising from the terms of all documents prepared or submitted for this
   transaction such as applications, security agreements, disclosures or notes,
   and this Note.

   D. PROPERTY. Property is any property, real, personal or intangible, that
   secures my performance of the obligations of this Loan.

   E. PERCENT. Rates and rate change limitations are expressed as annualized
   percentages.

2. PROMISE TO PAY. For value received, I promise to pay you or your order, at
your address, or at such other location as you may designate, amounts advanced
from time to time under the terms of this Note up to the maximum outstanding
principal balance of $3,000,000.00 (Principal), plus interest from the date of
disbursement, on the unpaid outstanding Principal balance until this Note
matures or this obligation is accelerated.

I may borrow up to the Principal amount more than one time.

3.ADVANCES. Advances under this Note are made according to the following terms
and conditions.

   A. REQUESTS FOR ADVANCES. My requests are a warranty that I am in compliance
   with all the Loan documents. When required by you for a particular method of
   advance, my requests for an advance must specify the requested amount and the
   date and be accompanied with any agreements, documents, and instruments that
   you require for the Loan. Any payment by you of any check, share draft or
   other charge may, at your option, constitute an advance on the Loan to me.
   All advances will be made in United States dollars. I will indemnify you and
   hold you harmless for your reliance on any request for advances that you
   reasonably believe to be genuine. To the extent permitted by law, I will
   indemnify you and hold you harmless when the person making any request
   represents that I authorized this person to request an advance even when this
   person is unauthorized or this person's signature is not genuine.

   I or anyone I authorize to act on my behalf may request advances by the
   following methods.

      (1) I make a request in person.

      (2) I make a request by phone.

      (3) I make a request by mail.

      (4) I write a check or share draft.

   B. ADVANCE LIMITATIONS. In addition to any other Loan conditions, requests
   for, and access to, advances are subject to the following limitations.

      (1) Obligatory Advances. You will make all Loan advances subject to this
      Agreement's terms and conditions.

      (2) Advance Amount. Subject to the terms and conditions contained in this
      Note, advances will be made in exactly the amount I request.

      (3) Disbursement of Advances. On my fulfillment of this Note's terms and
      conditions, you will disburse the advance in any manner as you and I
      agree.

      (4) Credit Limit. I understand that you will not ordinarily grant a
      request for an advance that would cause the unpaid principal of my Loan to
      be greater than the Principal limit. You may, at your option, grant such a
      request without obligating yourselves to do so in the future.

      (5) Records. Your records will be conclusive evidence as to the amount of
      advances, the Loan's unpaid principal balances and the accrued interest.

4. INTEREST. Interest will accrue on the unpaid Principal balance of this Note
at the rate of 4.5 percent (Interest Rate) until May 1, 2004, after which time
it may change as described in the Variable Rate subsection.

   A. INTEREST AFTER DEFAULT. If you declare a default under the terms of this
   Loan, including for failure to pay in full at maturity, you may increase the
   Interest Rate payable on the outstanding Principal balance of this Note. In
   such event, interest will accrue on the outstanding Principal balance at the
   variable Interest Rate in effect from time to time, plus an additional 6.000
   percent, until paid in full.

   B. MAXIMUM INTEREST AMOUNT. Any amount assessed or collected as interest
   under the terms of this Note or obligation will be limited to the Maximum
   Lawful Amount of interest allowed by state or federal law. Amounts collected
   in excess of the Maximum Lawful Amount will be applied first to the unpaid
   Principal balance. Any remainder will be refunded to me.

M-Tron Industries, Inc.
Nebraska Promissory Note                                            Initials____
NE/4XX270735007252000043    (c)1996 Bankers Systems, Inc.,                Page 1
64005051404N                 St. Cloud, MN  [ILLEGIBLE](TM)

<PAGE>

C. STATUTORY AUTHORITY. The amount assessed or collected on this Note is
authorized by the Nebraska usury laws under Neb. Rev. Stat. S 45.101.03.

D. ACCRUAL. During the scheduled term of this Loan interest accrues using an
Actual/360 days counting method.

E. VARIABLE RATE. The Interest Rate may change during the term of this
transaction.

   (1) Index. Beginning with the first Change Date, the Interest Rate will be
   based on the following index: the base rate in effect from time to time and
   designated by First National Bank of Omaha as its National Base Rate.

   The Current Index is the most recent index figure available on each Change
   Date. You do not guaranty by selecting this index, or the margin, that the
   interest Rate on this Note will be the same rate you charge on any other
   loans or class of loans you make to me or other borrowers. If this Index is
   no longer available, you will substitute a similar index. You will give me
   notice of your choice.

   (2) Change Date. Each date on which the Interest Rate may change is called a
   Change Date. The Interest Rate may change May 15, 2004 and daily thereafter.

   (3) Calculation Of Change. On each Change Data, you will calculate the
   Interest Rate, which will be the Current Index plus 0.500 percent. The result
   of this calculation will be rounded to the nearest .001 percent. Subject to
   any limitations, this will be the Interest Rate until the next Change Date.
   The new Interest Rate will become effective on each Change Date. The Interest
   Rate and other charges on this Note will never exceed the highest rate or
   charge allowed by law for this Note.

   (4) Limitations. The Interest Rate changes are subject to the following
   limitations:

      (a) Lifetime. The Interest Rate will never be less than 4.500 percent.

   (5) Effect Of Variable Rate. A change in the Interest Rate will have the
   following effect on the payments: The amount of the final payment will
   change.

5. PAYMENT. I agree to pay this Note as follows: I agree to pay this Note in 36
payments. This Note is amortized over 60 payments. A payment of $22,369.76 will
be due May 31, 2004. and on the last day of each month thereafter. A final
payment of the entire unpaid balance of Principal and interest will be due April
30, 2007. Any changes in the Interest Rate will affect the amount of this
payment.

Payments will be rounded to the nearest $.01. With the final payment I also
agree to pay any additional fees or charges owing and the amount of any advances
you have made to others on my behalf. Payments scheduled to be paid on, the
29th, 30th or 31st day of a month that contains no such day will, instead, be
made on the last day of such month.

Each payment I make on this Note will be applied first to interest that is due
then 10 principal that is due, and finally to any charges that I owe other than
principal and interest If you and I agree to a different application of
payments, we will describe our agreement on this Note. The actual amount of my
final payment will depend on my payment record.

6. PREPAYMENT. I may prepay this Loan in full or in part at any time. Any
partial prepayment will not excuse any later scheduled payments until I pay in
full.

7. LOAN PURPOSE. The purpose of this Loan is to fund the purchase of new and
used equipment.

8. SECURITY. This Loan is secured by the following, previously executed,
security instruments or agreements: Restated Loan/Security Agreement dated
August 31, 2001. Letter of Credit #RS1361047 issued by Fleet National Bank on
behalf of Lynch Corporation in the amount of $1,000,000.00 dated September 9,
2002.

 9. DEFAULT. I will be in default if any of the following occur:

   A. PAYMENTS. I fail to make a payment in full when due.

   B. INSOLVANCY OR BANKRUPTCY. I make an assignment for the benefit of
   creditors or become Insolvent, either because my liabilities exceed my assets
   or I am unable to pay my debts as they become due; or I petition for
   protection under federal, state or local bankruptcy, insolvency or debtor
   relief laws, or am the subject of a petition or action under such laws and
   fail to have the petition of action dismissed within a reasonable period of
   time not to exceed 60 days.

   C. BUSINESS TERMINATION. I merge, dissolve, reorganize, and my business or
   existence, or a partner or majority owner dies or is declared legally
   incompetent.

   D. FAILURE TO PERFORM. I fail to perform any condition or to keep any promise
   or covenant of this Note.

   E. OTHER DOCUMENTS. A default occurs under the terms of any other transaction
   document.

   F. OTHER AGREEMENTS. I am in default or any other debt or agreement I have
   with you.

   G. MISREPRESENTATION. I make any verbal Of written statement or provide any
   financial information that is untrue, inaccurate, or conceals a material fact
   at the time it is made or provided.

   H. JUDGMENT. I fail to satisfy or appeal any judgment against me.

   I. FORFEITURE. The Property is used in a manner or for a purpose that
   threatens confiscation by a legal authority.

   J. NAME CHANGE. I change my name or assume an additional name without
   notifying you before making such a change.

   K. PROPERTY TRANSFER. I transfer all or a substantial part of my money or
   property.

   L. PROPERTY VALUE. The value of the Property declines or is impaired.

   M. MATERIAL CHANGE. Without first notifying you, there is a material change
   in my business, including ownership, management, and financial conditions.

   N. INSECURITY. You reasonably believe that you are insecure.

10. ASSUMPTIONS. Someone buying the Property cannot assume the obligation. You
may declare the entire balance of the Note to be immediately due and payable
upon the creation of, or contract for the creation of, any lien, encumbrance, or
transfer of the property.

11. WAIVERS AND CONSENT. To the extent not prohibited by law, I waive protest,
presentment for payment, demand, notice of acceleration, notice of intent to
accelerate and notice of dishonor.

   A. ADDITIONAL WAIVERS BY BORROWER. In addition, I, and any party to this Note
   and Loan, to the extent permitted by law. consent to certain actions you may
   take, and generally waive defenses that may be available based on these
   actions or based on the status of a party to this Note.

      (1) You may renew or extend payments on this Note, regardless of the
      number of such renewals or extensions.

      (2) You may release any Borrower, endorser, guarantor, surety,
      accommodation maker or any other co-signer.

      (3) You may release, substitute or impair any Property securing this Note.

      (4) You, or any institution participating in this Note, may invoke your
      right of set-off.

      (5) You may enter into any sales, repurchases or participations of this
      Note to any person in any amounts and I waive notice of such sales,
      repurchases or participations.

      (6) I agree that any of us signing this Note as a Borrower is authorized
      to modify the terms of this Note or any instrument securing, guarantying
      or relating to this Note.

   B. NO WAIVER BY LENDER. Your course of dealing, or your forbearance from, or
   delay in, the exercise of any of your rights, remedies, privileges or right
   to insist upon my strict performance of any provisions contained in this
   Note, or other Loan documents, shall not be construed as a waiver by you,
   unless any such waiver is in writing and is signed by you.

12. REMEDIES, After I default, and after you give any legally required notice
and opportunity to cure the default, you may at your option do any one or more
of the following.

   A. ACCELERATION. You may make all or any part of the amount owing by the
   terms of this Note immediately due.

   B. SOURCES. You may use any and all remedies you have under state or federal
   law or in any instrument securing this Note.

M-Tron Industries, Inc.
Nebraska Promissory Note                                            Initials____
NE/4XX270735007252000043    (c)1996 Bankers Systems, Inc.,                Page 2
64003051404N                 St. Cloud, MN  [ILLEGIBLE](TM)

<PAGE>

   C. INSURANCE BENEFITS. You may make a claim for any and all insurance
   benefits or refunds that may be available on my default.

   D. PAYMENTS MADE ON MY BEHALF. Amounts advanced on my behalf will be
   immediately due and may be added to the balance owing under the terms of this
   Note, and accrue interest at the highest post-maturity interest rate.

   E. TERMINATION. You may terminate my right to obtain advances and may refuse
   to make any further extensions of credit.

   F. SET-OFF. You may use the right of set-off. This means you may set-off any
   amount due and payable under the terms of this Note against any right I have
   to receive money from you.

   My right to receive money from you includes any deposit or share account
   balance I have with you: any money owed to me on an item presented to you or
   in your possession for collection or exchange; end any repurchase agreement
   or other non-deposit obligation, "Any amount due and payable under the terms
   of this Note" means the total amount to which you are entitled to demand
   payment under the terms of this Note at the time you set-off.

   Subject to any other written contract, if my right to receive money from you
   is also owned by someone who has not agreed to pay this Note, your right of
   set-off will apply to my interest in the obligation and to any other amounts
   I could withdraw on my sole request or endorsement.

   Your right of set-off does not apply to an account or other obligation where
   my rights arise only in a representative capacity, It also does not apply to
   any individual Retirement Account or other tax-deferred retirement account.

   You will not be liable for the dishonor of any check when the dishonor occurs
   because you set-off against any of my accounts. I agree to hold you harmless
   from any such claims arising as a result of your exercise of your right of
   set-off.

   G. WAIVER. Except as otherwise required by law. by choosing any one or more
   of these remedies you do not give up your right to use any other remedy. You
   do not waive a default if you choose not to use a remedy. By electing not to
   use any remedy, you do not waive your right to later consider the event a
   default and to use any remedies if The default continues or occurs again.

13. COLLECTION EXPENSES AND ATTORNEYS' FEES. On or after Default, to the extent
permitted by law, I agree to pay all expenses of collection, enforcement or
protection of your rights and remedies under this Note. Expenses include, but
are not limited to, attorneys' fees, court costs and other legal expenses. These
expenses are due and payable immediately, If not paid immediately, these
expenses will bear interest from the date of payment until paid in full at the
highest interest rate In effect as provided for in the terms of this Note. All
fees and expenses will be secured by the Property I have granted to you, if any.
To the extent permitted by the United States Bankruptcy Code. I agree to pay the
reasonable attorneys' fees you incur to collect this Debt as awarded by any
court exercising jurisdiction under the Bankruptcy Code.

14. WARRANTIES AND REPRESENTATIONS. I make to you the following warranties and
representations which will continue as long as this Note is in effect:

   A. POWER. I am duly organised, and validly existing and in good standing in
   all jurisdictions in which I operate. I have the power and authority to enter
   into this transaction and to carry on my business or activity as it is now
   being conducted and, as applicable, am qualified to do so in each
   jurisdiction in which I operate.

   B. AUTHORITY. The execution, delivery end performance of this Note and the
   obligation evidenced by this Note are within my powers, have been duly
   authorized, have received all necessary governmental approval, will not
   violate any provision of law, or order of court or governmental agency, and
   will not violate any agreement to which I am a party or to which I am or any
   of my Property is subject.

   C. NAME AND PLACE OF BUSINESS. Other than previously disclosed in writing to
   you I have not changed my name or principal place of business within the last
   10 years and have not used any other trade or fictitious name. Without your
   prior written consent, I do not and will not use any other name end will
   preserve my existing name, trade names and franchises.

15. APPLICABLE LAW. This Note is governed by the laws of Nebraska, the United
States of America and to the extent required, by the laws of the jurisdiction
where the Property is located. In the event of a dispute, the exclusive forum,
venue and place of jurisdiction will be in Nebraska, unless otherwise required
by law.

16. JOINT AND INDIVIDUAL LIABILITY AND SUCCESSORS. My obligation to pay this
Loan is independent of the obligation of any other person who has also agreed to
pay it, You may sue me alone, or anyone else who is obligated on this Loan, or
any number of us together, to collect this Loan, Extending this Loan or new
obligations under this Loan, will not affect my duty under this Loan and I will
still be obligated to pay this loan. The duties and benefits of this Loan will
bind and benefit the successors and assigns of you and me.

17. AMENDMENT, INTEGRATION AND SEVERABILITY. This Note may not be amended or
modified by oral agreement. No amendment or modification of this Note is
effective unless made in writing and executed by you and me. This Note is the
complete and final expression of the agreement. If any provision of this Note is
unenforceable, then the unenforceable prevision will be severed and the
remaining provisions will still be enforceable.

18. INTERPRETATION. Whenever used, the singular includes the plural and the
plural includes the singular. The section headings are for convenience only and
are not to be used to interpret or define the terms of this Note.

19. NOTICE, FINANCIAL REPORTS AND ADDITIONAL DOCUMENTS. Unless otherwise
required by law, any notice will be given by delivering it or mailing it by
first class mail to the appropriate party's address listed in the DATE AND
PARTIES section, or to any other address designated in writing. Notice to one
party will be deemed to be notice to all parties. I will inform you in writing
of any change in my name, address or other application information. I will
provide you any financial statement or information you request. All financial
statements and information I give you will be correct and complete. I agree to
sign, deliver, and file any additional documents or certifications that you may
consider necessary to perfect, continue, and preserve my obligations under this
Loan and to confirm your lien status on any Property. Time is of the essence.

20. CREDIT INFORMATION. I agree to supply you with whatever information you
reasonably request. You will make requests for this information without undue
frequency, and win give me reasonable time in which to supply the Information.

21. ERRORS AND OMISSIONS. I agree, if requested by you, to fully cooperate in
the correction, if necessary, in the reasonable discretion of you of any and all
loan closing documents so that all documents accurately describe the loan
between you and me. I agree to assume all costs including by way of illustration
and not limitation, actual expenses, legal fees and marketing losses for failing
to reasonably comply with your requests within thirty (30) days.

M-Tron Industries, Inc.
Nebraska Promissory Note                                            Initials____
NE/4XX270735007252000043    (c)1996 Bankers Systems, Inc.,                Page 3
64003051404N                 St. Cloud, MN  [ILLEGIBLE](TM)

<PAGE>

A CREDIT AGREEMENT MUST BE IN WRITING TO BE ENFORCEABLE UNDER NEBRASKA LAW. TO
PROTECT YOU AND US FROM ANY MISUNDERSTANDINGS OR DISAPPOINTMENTS, ANY CONTRACT,
PROMISE, UNDERTAKING, OR OFFER TO FOREBEAR REPAYMENT OF MONEY OR TO MAKE ANY
OTHER FINANCIAL ACCOMMODATION IN CONNECTION WITH THIS LOAN OF MONEY OR GRANT OR
EXTENSION OF CREDIT, OR ANY AMENDMENT OF CANCELLATION OF, WAIVER OF, OR
SUBSTITUTION FOR ANY OR ALL OF THE TERMS OR PROVISIONS OF ANY INSTRUMENT OR
DOCUMENT EXECUTED IN CONNECTION WITH THIS LOAN OF MONEY OR GRANT OR EXTENSION OF
CREDIT, MUST BE IN WRITING TO BE EFFECTIVE.

   BORROWER:

      M-Tron Idustries, Inc.

         By /s/ DAVID L. REIN
         -----------------------------------
         Authorized Signer

22. SIGNATURES. By signing. I agree to the terms contained in this Note. I also
acknowledge receipt of copy of this Note.

   BORROWER:

      M-Tron Idustries, Inc.

         By /s/ DAVID L. REIN
         -----------------------------------
         Authorized Signer

   LENDER:

      First National Bank of Omaha

      By /s/ James R. Kamm
         -----------------------------------
         James R. Kamm, Vice President

M-Tron Industries, Inc.
Nebraska Promissory Note                                            Initials____
NE/4XX270735007252000043    (c)1996 Bankers Systems, Inc.,                Page 4
64005051404N                 St. Cloud, MN  EXPERTS(TM)