<PAGE>   1
                                                                   Exhibit 10.20

                      THIRD AMENDMENT TO PROMISSORY NOTES
                     AND SIXTH AMENDMENT TO LOAN AGREEMENT

     THIS THIRD AMENDMENT TO PROMISSORY NOTES AND SIXTH AMENDMENT TO LOAN
AGREEMENT ("Amendment") is made as of June 29, 2001, by and between U.S. BANK
NATIONAL ASSOCIATION, a national banking association ("Bank") and QUALMARK
CORPORATION, a Colorado corporation ("Borrower").

                                   RECITALS:

     A.   Bank made a revolving loan to Borrower, as evidenced by that certain
Promissory Note dated December 22, 1998 made by Borrower payable to Bank in the
original principal amount of $3,000,000 (the "Original Revolving Note"), as
amended and supplemented by that certain Modification/Extension Agreement
effective as of August 23, 1999 between Bank and Borrower (the "First
Amendment") and that certain Second Amendment to Promissory Notes and Fifth
Amendment to Loan Agreement dated as of February 1, 2001 (the "Second
Amendment"). Pursuant to the First Amendment, the principal amount of the
Revolving Note was reduced to $2,000,000. Pursuant to the Second Amendment, the
principal amount of the Revolving Note was reduced to $1,000,000. The Original
Revolving Note, as amended and supplemented by the First Amendment and the
Second Amendment, is hereinafter called the "Revolving Note."

     B.   Bank made a term loan to Borrower, as evidenced by that certain
Promissory Note dated December 22, 1998 made by Borrower payable to Bank in the
original principal amount of $2,000,000 (the "Original Term Note"), as amended
and supplemented by that certain Modification/Extension Agreement effective as
of August 23, 1999 between Bank and Borrower (the "Amendment to Term Note") and
the Second Amendment. Pursuant to the Amendment to Term Note, the principal
amount of the Term Note was reduced to $1,261,017.47. Pursuant to the Second
Amendment, the principal amount of the Term Note was increased to
$2,000,000.00. The Original Term Note, as amended and supplemented by the
Amendment to Term Note and the Second Amendment, is hereinafter called the
"Term Note."

     C.   The Revolving Note and the Term Note (collectively, the "Notes") are
subject to the provisions of that certain Revolving Credit and Term Loan
Agreement dated as of December 22, 1998 between Borrower and Bank, as amended
by (i) Waiver and Amendment to Loan Agreement dated as of March 15, 1999, (ii)
Second Amendment to Loan Agreement dated as of August 23, 1999, (iii) Third
Amendment to Loan Agreement dated as of March 31, 2000, (iv) Fourth Amendment
to Loan Agreement dated as of August 2, 2000, and (v) the Second Amendment (as
so amended, the "Loan Agreement").

     D.   All obligations of the Borrower to Bank are secured by a security
interest in (among other collateral) the Borrower's inventory, equipment,
rights to payment and general intangibles pursuant to Borrower's Security
Agreement between Borrower and Bank and the Addendum thereto, each dated
December 22, 1998 (the "Security Agreement"), and that security interest has
been properly perfected. In addition, Borrower specifically assigned six (6)
trademarks and ten (10) patents pursuant to an Assignment of Security Interest
in United States Trademarks and Patents (the
<PAGE>   2
"Assignment," and collectively with the Security Agreement, the "Security
Documents") dated as of December 22, 1998 between Borrower, as assignor, and
Bank, as assignee. The Assignment was properly recorded with the United States
Patent and Trademark Office effective on January 14, 1999.

     E.   Borrower and Bank desire to amend and supplement the Revolving Note,
the Term Note and the Loan Agreement, all as specifically hereinafter set forth.

                                   AGREEMENT:

     NOW, THEREFORE, in consideration of the foregoing recitals, the agreements
set forth herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto hereby agree as
follows:

     1.   Incorporation of Recitals. The foregoing recitals are hereby
incorporated herein and made a part hereof.

     2.   Confirmation of Indebtedness. The outstanding principal balance under
the Revolving Note as of June 27, 2001 is $0. The outstanding principal balance
under the Term Note as of June 27, 2001 is $1,895,000.00. Interest has and
continues to accrue under the Notes in accordance with the terms thereof.

     3.   Amendments to Revolving Note. The terms of the Revolving Note are
changed as follows:

          A.   Exhibit A to the Revolving Note, entitled "Interest Rate--Reserve
          Adjusted LIBOR Rate-Fixed Interest Periods" is deleted in its
          entirety. All other references to a LIBOR interest rate option in the
          Revolving Note are deleted.

     4.   Amendments to Term Note. The terms of the Term Note are changed as
follows:

          A.   Exhibit A to the Revolving Note, entitled "Interest Rate--Reserve
          Adjusted LIBOR Rate-Fixed Interest Periods" is deleted in its
          entirety. All other references to a LIBOR interest rate option in the
          Revolving Note are deleted.

          B.   Commencing on July 1, 2001, Borrower agrees to pay all interest
          accrued under the Term Note, plus $45,000.00 in principal, on the last
          day of each month until maturity.

     5.   Amendment to Loan Agreement. The Loan Agreement is amended as follows:

          A.   Section 1.01.4 of the Loan Agreement is deleted in its entirety
          and replaced with the following:


                                       2


<PAGE>   3
          "1.01.04 Interest. Borrower agrees to pay interest on the Line Balance
          from time to time as provided herein. Interest will accrue on the
          daily outstanding balance of each Advance at a fluctuating rate per
          annum equal at all time to the sum of the Reference Rate plus one
          percent (1%), which rate will change when and as the Reference Rate
          changes. Accrued interest shall be due and payable (i) on the last day
          of each calendar month, (ii) at final maturity of the Line Note, and
          (iii) on demand after such maturity."

          B.   Section 1.02.3 of the Loan Agreement is deleted in its entirety
          and replaced with the following:

          1.02.3 Interest. Borrower agrees to pay interest on the Term Loan from
          time to time as provided herein. Interest will accrue on the daily
          outstanding Term Loan balance at a fluctuating rate per annum equal at
          all time to the sum of the Reference Rate plus one percent (1%), which
          rate will change when and as the Reference Rate changes. Accrued
          interest shall be due and payable (i) on the last day of each calendar
          month, (ii) at final maturity of the Term Loan, and (iii) on demand
          after such maturity."

          C.   Section 1.02.4 of the Loan Agreement is amended by deleting the
          first sentence thereof and replacing it with the following:

          "Borrower agrees to repay the Term Loan made hereunder in monthly
          payments in the amount of $45,000, each due on the last day of each
          calendar month, commencing July 31, 2001, and the final installment of
          all unpaid principal and accrued interest due and payable in full at
          the final maturity of the Term Loan, which will be March 15, 2002,
          subject to acceleration upon the occurrence of an Event of Default."

          D.   Section 5.09 is deleted in its entirety and replaced with the
          following:

          "5.09 Financial Condition. Maintain the financial condition of
          Borrower, determined in accordance with GAAP, so that it meets the
          following requirements all determined on a quarterly basis as of the
          end of each fiscal quarter end, where indicated, for the preceding or
          trailing 12 month period (the "TTMP"):

               (a)  Commencing with the quarter ending June 30, 2001 and
          thereafter:

                    i.   Borrower's Tangible Net Worth will be not less than
                         $3,400,000 as of the end of each fiscal quarter; and

                    ii.  Borrower's ratio of Free Cash Flow to Debt Service will
                         be not less than 1.2:1 for the TTMP.


                                       3

<PAGE>   4
               (b)  Commencing with the quarter ending June 30, 2001:

                    i.   Borrower's EBITDA will not be less than $250,000 at
                         9/30/01 and 12/31/01.

               For purposes hereof, "Free Cash Flow" means EBITDA less cash
               taxes, cash capital expenditures and cash dividends; "Debt
               Service" means interest expense plus all mandatory principal
               payments on Debt; and "Tangible Net Worth" means stockholders'
               equity (including preferred stock) less intangible assets."

          E.   Exhibit A of the Loan Agreement, entitled "Interest Rate --
          Reserve Adjusted LIBOR Rate-Fixed Interest Periods," is deleted in
          its entirety.

     6.   Limited Waiver. Subject to the Borrower's agreement hereto, Bank
hereby waives any default which has occurred by reason of Borrower's failure to
comply with the covenants set forth in section 5.09 of the Loan Agreement as of
June 30, 2001.

     7.   Conditions to Effectiveness of Amendment. This Amendment shall become
effective as of the date first above written when the Bank shall have received
and, as applicable, executed each of the following: (i) an original of this
Amendment, duly executed by the Borrower; (ii) an extension fee equal to
$2,500; (iii) attorneys' fees incurred by Bank equal to $750.00; and (iv) such
additional information or documentation as the Bank may require. If the Bank
has not received all of the foregoing on or before July 6, 2001, then the
Bank's offer to make the agreements set forth herein may be terminated, at the
Bank's option, by giving notice to the Borrower.

     8.   General Release. Borrower, for and on behalf of itself and its legal
representatives, heirs, successors and assigns, does hereby waive, release,
relinquish and forever discharge Bank and its past and present directors,
officers, agents, employees, parents, subsidiaries, affiliates, insurers,
attorneys, representatives and assigns, and each and all thereof (collectively,
the "Released Parties"), of and from any and all manner of action or causes of
action, suits, claims, demands, judgments, damages, levies, and the execution
of whatsoever kind, nature and/or description arising on or before the date
hereof, including, without limitation, any claims, losses, costs or damages,
including compensatory and punitive damages, in each case whether known or
unknown, liquidated or unliquidated, fixed or contingent, direct or indirect,
which Borrower, or its legal representatives, heirs, successors or assigns,
ever had or now has or may claim to have against any of the Released Parties,
with respect to any matter whatsoever, including, without limitation, this
Amendment and any other instruments and agreements executed by Borrower in
connection herewith, arising on or before the date hereof.

     9.   Effect of Amendment; Representations and Warranties; Nonwaiver. Bank
and Borrower agree that the Term Note, the Revolving Note and the Loan
Agreement, all as amended by this Amendment, and the Security Documents remain
in full force and effect. Borrower


                                       4
<PAGE>   5
represents and warrants that its has the power and legal right and authority to
enter into this Amendment, and that neither this Amendment, nor the agreements
contained herein, contravene or constitute a default under any agreement,
instrument or indenture to which Borrower is a party or signatory, or, to the
best knowledge of Borrower, any other agreement or requirement of law. Borrower
represents and warrants that no consent, approval or authorization of or
registration or declaration with any party, including but not limited to any
governmental authority, is required in connection with the execution and
delivery by Borrower of this Amendment, or the performance of its obligations
herein described.

     10.  Ratification of Notes, Loan Agreement and Security Documents. All of
the terms, conditions, provisions, agreements, requirements, promises,
obligations, duties, covenants and representations under the Revolving Note,
the Term Note, the Loan Agreement, all as amended and supplemented by this
Amendment, and the Security Documents are hereby ratified and affirmed in all
respects by Borrower. Borrower further represents and warrants that the
Security Documents continue to secure the obligations of Borrower under the
Revolving Note, the Term Note, the Loan Agreement, all as amended and
supplemented by this Amendment.

     11.  Further Assurances. The Borrower shall promptly correct any defect or
error that may be discovered in any Security Document or in the execution,
acknowledgment or recordation thereof. Promptly upon request by the Bank, the
Borrower also shall do, execute, acknowledge, deliver, record, re-record, file,
re-file, register and re-register, any and all deeds, conveyances, mortgages,
deeds of trust, trust deeds, assignments, estoppel certificates, financing
statements and continuations thereof, notices of assignment, transfers,
certificates, assurances and other instruments as Bank may reasonably require
from time to time in order: (a) to carry out more effectively the purposes of
the Security Documents; (b) to perfect and maintain the validity, effectiveness
and priority of any security interests intended to be created by the Security
Documents; (c) to grant the Bank, to secure the Notes, a perfected lien in any
intellectual property or other assets owned by the Borrower; and (d) to better
assure, convey, grant, assign, transfer, preserve, protect and confirm unto
Bank the rights granted now or hereafter intended to be granted to Bank under
any Security Document or under any other instrument executed in connection with
any Security Document or that the Borrower may be or become bound to convey,
mortgage or assign to Bank in order to carry out the intention or facilitate
the performance of the provisions of any Security Document. The Borrower shall
furnish evidence satisfactory to Bank of every such recording, filing or
registration.

     12.  Merger and Integration, Superseding Effect. This Amendment embodies
the entire agreement and understanding between the parties hereto with respect
to the subject matter hereof and supersedes and has merged into it all prior
and written agreements in the same subjects by and between the parties hereto
with the effect that this Amendment shall control.

     13.  Counterparts. This Amendment may be executed in different
counterparts with the same effect as if the signatures thereon were in the same
instrument, and will be effective upon delivery of all such counterparts to
Bank.



                                       5
<PAGE>   6
     14.  Governing Law. THE LAWS OF THE STATE OF COLORADO GOVERN THIS
AMENDMENT.

     15.  Advice of Counsel. Borrower acknowledges and agrees that it has
received the advice of independent counsel selected by it, or the opportunity
to obtain such advice, before entering into this Amendment and any other
instruments and agreements executed by Borrower in connection herewith, and has
not relied upon Bank or any of its officers, directors, employees, agents or
attorneys concerning any aspect of the transactions contemplated by this
Amendment or any of the other said instruments and agreements.

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

                                   U.S. BANK NATIONAL ASSOCIATION



                                   By    /s/ Daniel J. Falstad
                                     ---------------------------------
                                     Daniel J. Falstad, Vice President


                                   QUALMARK CORPORATION

                                   By    /s/ Anthony A. Scalese
                                     ---------------------------------
                                     Anthony A. Scalese,
                                     VP Finance & Administration
                                     (Print Name and Title)


STATE OF COLORADO)
                 ) ss.
COUNTY OF ADAMS  )

     The foregoing instrument was acknowledged before me this 6 day of June,
2001, by Anthony A. Scalese, the VP Finance of Qualmark Corporation, a Colorado
corporation, on behalf of the corporation.


[NOTARY PUBLIC SEAL                              /s/ Teresa Gentry
 TERESA GENTRY                                   ------------------------
 STATE OF COLORADO]                                   Notary Public


                                    MY COMMISSION EXPIRES 04/23/2005



6