Document:

<PAGE>   1

                                                                   Exhibit 10.28

                                    FROEDTERT
                           Memorial Lutheran Hospital

July 12, 1999

Craig K. Tagawa
Chief Executive Officer
GK Financing, LLC
Four Embarcadero Center
Suite 3620
San Francisco, CA  94111-4155

Dear Craig:

This letter serves to amend our May 28, 1999 Lease Agreement for a Gamma Knife
Unit between GK Financing (GKF) and Froedtert Memorial Lutheran Hospital (FMLH).
GKF and FMLH agree to change the delivery date identified in Section 4, from
July 1, 1999 to October 15, 1999. All other provisions of the lease continue to
apply, including the penalty provision for delivery more than 30 days after the
amended delivery date.

Please confirm this amendment by signing the enclosed copy of this letter and
returning it to me.

Very truly yours,

FROEDTERT MEMORIAL LUTHERAN HOSPITAL

/s/ Blaine J. O'Connell
------------------------
Blaine J. O'Connell
Sr. Vice President & CFO

GK FINANCING, LLC

/s/ Craig K. Tagawa
----------------------------------------
Craig K. Tagawa, Chief Executive Officer

cc:     Charles W. Runge

  9200 West Wisconsin Avenue P.O. Box 26099 Milwaukee, WI 53226-3596 Telephone:
                                  414-259-3000
       Staffed by physicians of the Medical College of Wisconsin. Member,
                             Horizon Healthcare Inc.<PAGE>   1

                                                                   Exhibit 10.29

                                    FROEDTERT
                           Memorial Lutheran Hospital

August 24, 1999

Craig K. Tagawa
Chief Executive Officer
GK Financing, LLC
Four Embarcadero Center, Suite 3620
San Francisco, CA  94111-4155

Dear Craig:

This letter serves to amend our May 28, 1999 Lease Agreement for a Gamma Knife
Unit between GK Financing, LLC (GKF) and Froedtert Memorial Lutheran Hospital
(FMLH). GKF and FMLH agree to change the delivery date identified in Section 4,
from July 1, 1999 to December 8, 1999. All other provisions of the lease
continue to apply, including the penalty provision for delivery more than 30
days after the amended delivery date.

This agreement supercedes our July 12, 1999 amendment letter, in which we agreed
to change the delivery date from July 1, 1999 to October 15, 1999.

Please confirm this amendment by signing the enclosed copy of this letter and
returning it to me.

Very truly yours,

FROEDTERT MEMORIAL LUTHERAN HOSPITAL

/s/ Blaine J. O'Connell
------------------------
Blaine J. O'Connell
Sr. Vice President & CFO

GK FINANCING, LLC

/s/ Craig K. Tagawa
-----------------------
Craig K. Tagawa
Chief Executive Officer

cc:     Charles W. Runge

  9200 W. Wisconsin Avenue P.O. Box 26099 Milwaukee, WI 53226-3596 414/259-3000
                                www.froedtert.com
          Primary affiliate of the Medical College of Wisconsin Member,
                            Horizon Healthcare Inc.<PAGE>   1
                                                                    Exhibit 4(c)
================================================================================
================================================================================

                               ALLEN TELECOM INC.
                                   AS BORROWER

                            THE LENDERS NAMED HEREIN
                                   AS LENDERS

                                    NBD BANK
                             AS DOCUMENTATION AGENT

                                       AND

                                 [KEYBANK LOGO]

                          KEYBANK NATIONAL ASSOCIATION
                                  as a Lender,
                the Swing Line Lender, a Letter of Credit Issuer
 and as the Syndication Agent, the Administrative Agent and the Collateral Agent

                              ---------------------

                                 AMENDMENT NO. 1
                                   dated as of
                                  July 30, 1999
                                       to
                                CREDIT AGREEMENT
                                   dated as of
                                December 31, 1998
                              ---------------------

================================================================================
================================================================================

<PAGE>   2

                       AMENDMENT NO. 1 TO CREDIT AGREEMENT

         THIS AMENDMENT NO. 1 TO CREDIT AGREEMENT, dated as of July 30, 1999
("THIS AMENDMENT"), among the following:

                  (i) ALLEN TELECOM INC., a Delaware corporation (herein,
         together with its successors and assigns, the "BORROWER");

                  (ii) the Lenders party hereto;

                  (iii) NBD BANK as a Lender and as Documentation Agent (the
         "DOCUMENTATION AGENT"); and

                  (iv) KEYBANK NATIONAL ASSOCIATION, a national banking
         association, as a Lender, the Swing Line Lender, the Letter of Credit
         Issuer, and as the Syndication Agent, the Administrative Agent and the
         Collateral Agent under the Credit Agreement:

         PRELIMINARY STATEMENTS:

         (1) The Borrower, the Lenders named therein, the Swing Line Lender, the
Letter of Credit Issuers, the Documentation Agent, the Syndication Agent and the
Administrative Agent entered into the Credit Agreement, dated as of December 31,
1998 (the "CREDIT AGREEMENT"; with the terms defined therein, or the definitions
of which are incorporated therein, being used herein as so defined).

         (2) The parties hereto desire to change certain of the terms and
provisions of the Credit Agreement, all as more fully set forth below.

         NOW, THEREFORE, the parties hereby agree as follows:

         10 AMENDMENTS, ETC. The dollar amount "$25,000,000" which appears in
clause (x) of section 3.1(b) of the Credit Agreement is changed to
"$50,000,000", but such change is effective only during the period from July 30,
1999 through December 31, 2000. For the avoidance of doubt the parties confirm
that no Letter of Credit will be issued or increased in amount which could
result in Letter of Credit Outstandings at any time after December 31, 2000 in
excess of $25,000,000.

         20 REPRESENTATIONS AND WARRANTIES. The Borrower represents and warrants
to the Lenders, the Swing Line Lender, the Letter of Credit Issuer, the
Documentation Agent, the Administrative Agent and the Collateral Agent as
follows:

                  (a) AUTHORIZATION AND VALIDITY OF AMENDMENT, ETC. This
         Amendment has been duly authorized by all necessary corporate action on
         the part of the Borrower, has been duly executed and delivered by a
         duly authorized officer of the Borrower, and constitutes the valid and
         binding agreement of the Borrower, enforceable against the Borrower in
         accordance with its terms, except to the extent that the enforceability
         thereof may be limited by applicable bankruptcy, insolvency,
         reorganization, moratorium or other similar laws generally affecting
         creditors' rights and by equitable principles (regardless of whether
         enforcement is sought in equity or at law).

<PAGE>   3

                  (b) REPRESENTATIONS AND WARRANTIES. The representations and
         warranties of the Credit Parties contained in the Credit Agreement or
         in the other Credit Documents are true and correct in all material
         respects on and as of the date hereof as though made on and as of the
         date hereof, except to the extent that such representations and
         warranties expressly relate to an earlier specified date, in which case
         such representations and warranties are hereby reaffirmed as true and
         correct in all material respects as of the date when made.

                  (c) NO EVENT OF DEFAULT. No condition or event has occurred or
         exists which constitutes or which, after notice or lapse of time or
         both, would constitute an Event of Default.

                  (d) COMPLIANCE. The Borrower is in full compliance with all
         covenants and agreements contained in the Credit Agreement, as amended
         hereby, and the other Credit Documents to which it is a party; and
         without limitation of the foregoing, each Subsidiary of the Borrower
         which, as of the date hereof, is required to be a Subsidiary Guarantor,
         has as on or prior to the date hereof become a Subsidiary Guarantor
         under the Subsidiary Guaranty.

                  (e) FINANCIAL STATEMENTS, ETC. The Borrower has furnished to
         the Lenders and the Administrative Agent complete and correct copies
         of:

                           (i) the audited consolidated balance sheets of the
                  Borrower and its consolidated subsidiaries as of December 31,
                  1997, and December 31, 1998, and the related audited
                  consolidated statements of income, stockholders' equity, and
                  cash flows for the fiscal years then ended, accompanied by the
                  unqualified report thereon of the Borrower's independent
                  accountants; and

                           (ii) the unaudited condensed consolidated balance
                  sheets of the Borrower and its consolidated subsidiaries as of
                  March 31, 1999, and the related unaudited condensed
                  consolidated statements of income and of cash flows of the
                  Borrower and its consolidated subsidiaries for the fiscal
                  quarter or quarters then ended, as contained in the Form 10-Q
                  Quarterly Report of the Borrower filed with the SEC.

         All such financial statements have been prepared in accordance with
         GAAP, consistently applied (except as stated therein), and fairly
         present the financial position of the Borrower and its consolidated
         subsidiaries as of the respective dates indicated and the consolidated
         results of their operations and cash flows for the respective periods
         indicated, subject in the case of any such financial statements which
         are unaudited, to the absence of footnotes and to normal audit
         adjustments which the Borrower reasonably believes will not involve a
         Material Adverse Effect.

         30 RATIFICATIONS. Except as expressly modified and superseded by this
Amendment, the terms and provisions of the Credit Agreement are ratified and
confirmed and shall continue in full force and effect.

         40 BINDING EFFECT. This Amendment shall become effective on a date (the
"EFFECTIVE DATE"), on or before July 30, 1999, if the following conditions shall
have been satisfied on and as of such date:

                  (a) this Amendment shall have been executed by the Borrower
         and the Administrative Agent, and counterparts hereof as so executed
         shall have been delivered to the Administrative Agent; and

                                        2

<PAGE>   4

                  (b) the Administrative Agent shall have been notified by the
         Required Lenders that such Lenders have executed this Amendment (which
         notification may be by facsimile or other written confirmation of such
         execution);

and thereafter this Amendment shall be binding upon and inure to the benefit of
the Borrower, each Lender, the Swing Line Lender, the Letter of Credit Issuers,
the Documentation Agent, the Syndication Agent, the Administrative Agent and the
Collateral Agent and their respective successors and assigns. After this
Amendment becomes effective, the Administrative Agent will promptly furnish a
copy of this Amendment to each Lender and the Borrower and advise them of the
Effective Date.

         50 MISCELLANEOUS. 5.1. SURVIVAL OF REPRESENTATIONS AND WARRANTIES. All
representations and warranties made in this Amendment shall survive the
execution and delivery of this Amendment, and no investigation by the
Administrative Agent or any Lender or any subsequent Loan or other Credit Event
shall affect the representations and warranties or the right of the
Administrative Agent or any Lender to rely upon them.

         5.2. REFERENCE TO CREDIT AGREEMENT. The Credit Agreement and any and
all other agreements, instruments or documentation now or hereafter executed and
delivered pursuant to the terms of the Credit Agreement as amended hereby, are
hereby amended so that any reference therein to the Credit Agreement shall mean
a reference to the Credit Agreement as amended hereby.

         5.3. EXPENSES. As provided in the Credit Agreement, but without
limiting any terms or provisions thereof, the Borrower shall pay on demand all
reasonable costs and expenses incurred by the Administrative Agent in connection
with the preparation, negotiation, and execution of this Amendment, including
without limitation the reasonable costs and fees of the Administrative Agent's
special legal counsel, regardless of whether this Amendment becomes effective in
accordance with the terms hereof, and all reasonable costs and expenses incurred
by the Administrative Agent or any Lender in connection with the enforcement or
preservation of any rights under the Credit Agreement, as amended hereby.

         5.4. SEVERABILITY. Any term or provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the term or provision so held to be invalid or unenforceable.

         5.5. APPLICABLE LAW. This Amendment shall be governed by and construed
in accordance with the laws of the State of Ohio.

         5.6. HEADINGS. The headings, captions and arrangements used in this
Amendment are for convenience only and shall not affect the interpretation of
this Amendment.

         5.7. ENTIRE AGREEMENT. This Amendment is specifically limited to the
matters expressly set forth herein. This Amendment and all other instruments,
agreements and documentation executed and delivered in connection with this
Amendment embody the final, entire agreement among the parties hereto with
respect to the subject matter hereof and supersede any and all prior
commitments, agreements, representations and understandings, whether written or
oral, relating to the matters covered by this Amendment, and may not be
contradicted or varied by evidence of prior, contemporaneous or subsequent oral
agreements or discussions of the parties hereto. There are no oral agreements
among the parties hereto relating to the subject matter hereof or any other
subject matter relating to the Credit Agreement.

         5.8. JURY TRIAL WAIVER. EACH OF THE PARTIES TO THIS AMENDMENT HEREBY
IRREVOCABLY WAIVES ALL RIGHT TO A TRIAL BY JURY IN ANY ACTION, PROCEEDING

                                       3
<PAGE>   5

OR COUNTERCLAIM ARISING OUT OF OR RELATING TO THIS AMENDMENT, THE OTHER CREDIT
DOCUMENTS OR THE TRANSACTIONS CONTEMPLATED HEREBY OR THEREBY. EACH PARTY HERETO
HEREBY (A) CERTIFIES THAT NO REPRESENTATIVE, AGENT OR ATTORNEY OF ANY OTHER
PARTY HAS REPRESENTED, EXPRESSLY OR OTHERWISE, THAT SUCH OTHER PARTY WOULD NOT,
IN THE EVENT OF LITIGATION, SEEK TO ENFORCE THE FOREGOING WAIVER, AND (B)
ACKNOWLEDGES THAT IT AND THE OTHER PARTIES HERETO HAVE BEEN INDUCED TO ENTER
INTO THIS AMENDMENT BY, AMONG OTHER THINGS, THE MUTUAL WAIVERS AND
CERTIFICATIONS IN THIS SECTION.

         5.9. COUNTERPARTS. This Amendment may be executed by the parties hereto
separately in one or more counterparts, each of which when so executed shall be
deemed to be an original, but all of which when taken together shall constitute
one and the same agreement.

                                        4

<PAGE>   6

         IN WITNESS WHEREOF, this Amendment has been duly executed and delivered
as of the date first above written.

<TABLE>
<CAPTION>

<S>                                           <C>
ALLEN TELECOM INC.                               KEYBANK NATIONAL ASSOCIATION,
                                                    INDIVIDUALLY AS THE SWING LINE LENDER,
                                                    A LENDER, A LETTER OF CREDIT ISSUER,
                                                    AND AS THE SYNDICATION AGENT AND
BY:_______________________________                  THE ADMINISTRATIVE AGENT
     VICE PRESIDENT--FINANCE

                                                 BY:_______________________________
                                                         SENIOR VICE PRESIDENT

BANK ONE, MICHIGAN                               FIRSTAR BANK, NATIONAL
   (SUCCESSOR TO NBD BANK),                      ASSOCIATION (FORMERLY STAR BANK,
     INDIVIDUALLY AS A LENDER AND                NATIONAL ASSOCIATION)
     AS DOCUMENTATION AGENT

                                                 BY:_______________________________
BY:_______________________________                    TITLE:
     TITLE:

DRESDNER BANK AG,                                SANPAOLO  IMI,  S. p. A.,
   NEW YORK AND GRAND CAYMAN BRANCHES,              NEW YORK BRANCH
   INDIVIDUALLY AS A LENDER AND AS
   A LETTER OF CREDIT ISSUER
                                                 BY:_______________________________
                                                      TITLE:

BY:_______________________________
     TITLE:

BY:_______________________________
     TITLE:

FIFTH THIRD BANK, NORTHEASTERN                   LaSALLE NATIONAL BANK
OHIO

                                                 BY:_______________________________
BY:_______________________________                    TITLE:
     TITLE:
</TABLE>

                                        5

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