FOURTH AMENDMENT TO TERM LOAN AGREEMENT

     THIS FOURTH  AMENDMENT  TO TERM LOAN  AGREEMENT  ("Amendment")  dated as of
October  1,  1996,  by and  between  AMSC  SUBSIDIARY  CORPORATION,  a  Delaware
corporation  ("Borrower"),  with offices at 10802 Parkridge  Boulevard,  Reston,
Virginia 22091, and NTFC CAPITAL CORPORATION,  a Delaware Corporation  (formerly
known as Northern Telecom Finance Corporation)  ("Lender"),  with offices at 220
Athens Way, Nashville, Tennessee 37228.

                                   BACKGROUND:

     A. Borrower and Lender  executed that certain Term Loan Agreement  dated as
of May 28, 1993, as amended by the First  Amendment to Term Loan Agreement dated
as of April 8, 1994,  the Second  Amendment to Term Loan  Agreement  dated as of
August 1, 1995, the Third  Amendment to Term Loan Agreement dated as of November
7, 1995 (as so amended,  the "Original  Loan  Agreement")  providing for certain
loans to be made to Borrower by Lender (the "Loans").  The Loans are represented
by the Amended and Restated  Equipment Note dated as of April 8, 1994,  amending
and restating the Note originally  dated as of May 28, 1993 (as so amended,  the
"Original Note").

     B.  Borrower  has  requested   Lender  to  make  certain   changes  in  the
amortization  schedule  and  interest  on the Loans,  and to  eliminate  certain
financial  covenants in the Original  Loan  Agreement,  and Lender is willing to
make such changes, on the terms and conditions set forth herein.

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

     1.  Definitions.  All capitalized terms used herein which are not otherwise
defined  shall  have the  meanings  given to such  terms  in the  Original  Loan
Agreement, as amended hereby.
     2.  Amendment to Section 1.01.  Section 1.01 of the Original Loan Agreement
is hereby  amended by amending each of the following  definitions to read in its
entirety as follows:

              "Interest Payment Date ": the First Interest Payment Date for the
         Loan and the first day of each Interest Period.

              "Interest Rate": a variable  interest rate equal to (a) LIBOR plus
         4.5% through and including September 30, 1996, and (b) LIBOR plus 2.5 %
         from and after  October 1, 1996, in each case adjusted on the first day
         of each Interest Period.

              "Maturity Date": December 31, 1997,  on  which  all  principal,
         interest, premium, expenses, fees,  penalties  and  other  amounts due
         under the Note shall be finally due and payable.

     3.  Amendment to Section 2,05.  Section 2.05 of the Original Loan Agreement
is hereby amended to read in its entirety as follows:

               2.05. Principal  Payments.  The  entire  outstanding  principal
               amount of the Note and all accrued but unpaid interest and all
               other unpaid amounts due thereunder shall be paid on the Maturity
               Date.

     4.  Amendment to Section 2.07.  Section 2.07 of the Original Loan Agreement
is hereby amended to read in its entirety as follows:

               2.07. Prepayments.

               (A)  Voluntary  Prepayments.  Commencing  on or after the Initial
Payment Date,  Borrower  may, at its option,  at any time and from time to time,
prepay the Loan in whole or in part, upon (i) at least thirty (30) Business Days
prior  written  notice to Lender  specifying  the date and amount of  prepayment
(together  with all  accrued but unpaid  interest  thereon),  (ii)  payment of a
prepayment  premium in an amount  equal to one  percent  (1 %) of the  principal
amount  prepaid,  and (iii)  payment of all accrued  and unpaid  interest on the
principal amount prepaid and all other charges then outstanding.

               (B) Application of Prepayments. Any prepayments of the Note shall
be applied  first to  interest  and then to the  installments  of  principal  in
reverse chronological order under the Note.

     5. Deletion of Sections 6.10  (Leverage  Ratio) and 6.11 (Cash Flow Ratio).
'The Original Loan  Agreement is hereby  further  amended by deleting  therefrom
Sections 6.10 and 6.11 in their entirety.  Lender hereby and forever waives each
and every  Default or Event of Default based upon or arising from any failure of
the  Borrower  to comply  with the terms of Section 6. 10 of the  Original  Loan
Agreement on or prior to the date hereof.

     6. Amended and Restated Note.  Contemporaneously with the execution of this
Amendment,  Borrower  shall execute an Amended and Restated Note to  incorporate
the terms hereof, in form and substance satisfactory to Lender.

     7. Representations and Warranties of Borrower.  The Borrower represents and
warrants to Lender that the  Borrower has not executed any other deeds of trust,
mortgages,  security  agreements  or financing  statements in favor of any other
person or entity  affecting  the  Collateral;  that no person or entity  has any
rights to claim a lien upon the Collateral  superior to the lien of Lender; that
no Default or Event of Default has occurred  under the Original  Loan  Agreement
(except  Defaults  and Events of Default  that are waived  herein);  and that no
event has occurred and no claim,  offset or other  condition  exists which would
relieve the Borrower of any of its obligations to Lender under the Original Loan
Agreement or other documents executed by the Borrower in connection therewith.

     8. Lender's Fees and Expenses. Borrower shall pay to Lender, on demand, all
costs and  expenses,  including  reasonable  legal  fees,  incurred by Lender in
connection with the preparation,  negotiation,  execution or  implementation  of
this Amendment.

     9. Full Force and  Effect.  Except as  specifically  modified  herein,  the
Original Loan Agreement shall continue in full force and effect as written,  and
nothing  herein is  intended  to, nor shall it,  release,  diminish or waive the
rights of the parties under the Original Loan  Agreement,  the Note or the other
Loan Documents.

     10.  Counterparts.  This  Amendment  may  be  executed  in  any  number  of
counterparts  (by  facsimile  transmission  or  otherwise)  and by the different
parties hereto on separate counterparts,  each of which, when so executed, shall
be deemed an original,  but all such  counterparts  shall constitute but one and
the same instrument.

     IN WITNESS  WHEREOF,  this Fourth Amendment to Term Loan Agreement has been
executed  as  of  the  day  first  above  written  by  the  parties'  authorized
representatives.


LENDER:                                       BORROWER:

NTFC CAPITAL CORPORATION                      AMSC SUBSIDIARY CORPORATION

By:   /s/ L.W. Middleton                      By:  /s/ Richard J. Burnheimer

Title:  Secretary                             Title: Vice President & Treasurer




                            AMENDED AND RESTATED NOTE

$5,933,095.61                               Originally dated as of: May 28, 1993
                                                 Previously Amended and Restated
                                                           as of:  April 8, 1994
                                      Amended and Restated as of October 1, 1996


     FOR VALUE RECEIVED, AMSC SUBSIDIARY CORPORATION  ("Borrower"),  promises to
pay to the order of NORTHERN  TELECOM FINANCE  CORPORATION (the "Lender") at its
offices located at 220 Athens Way, Nashville, Tennessee,  37228-1399, or to such
other Person and such other location  specified in writing by the holder hereof,
in lawful  money of the United  States of America and in  immediately  available
funds the principal  amount of Five Million Nine Hundred  Thirty- Three Thousand
Ninety Five Dollars and Sixty-One Cents ($5,933,095.61),  together with interest
thereon and other  amounts due as provided  below.  Notations  on the  Schedules
attached hereto are for convenience  only, and the failure of the Lender to make
any notation on any  Schedule,  or any  incorrect  notation by the Lender on any
Schedule,  shall not diminish the  obligations  of the Borrower under this Note.
This Note shall mature on December 31, 1997 (the "Maturity Date").

     The  "Initial  Payment  Date" means the first  Business Day of the calendar
month following the month in which the Termination  Date falls. The "Termination
Date" means the earliest of the  following  three dates:  (a) December 31, 1994,
but  only if  Final  Acceptance  (as  defined  in the  Loan  Agreement)  ("Final
Acceptance")  has  occurred on or before  such date;  or (b) the last day of the
month of the date of Final Acceptance if Final Acceptance occurs between January
1, 1995 and February 28, 1995; or (c) March 31, 1995.

     This Note has been made and  delivered  pursuant to that  certain Term Loan
Agreement  dated as of May 28, 1993 by and between the  Borrower and the Lender,
as amended by the First  Amendment to Term Loan  Agreement  dated as of April 8,
1994, the Second  Amendment to Term Loan  Agreement  dated as of August 1, 1995,
the Third Amendment to Term Loan Agreement dated as of November 7, 1995, and the
Fourth  Amendment to Term Loan  Agreement of even date herewith (as the same may
be modified,  amended or supplemented  from time to time, the "Loan  Agreement")
and is the Note  described in Section  2.03(a)  thereof.  Any term not otherwise
defined  in  this  Note  shall  have  the  meaning  ascribed  to it in the  Loan
Agreement.  Reference  is made to the Loan  Agreement,  which among other things
provides for the  acceleration  of the maturity  hereof upon the  occurrence  of
certain  events and for  prepayments in certain  circumstances  and upon certain
terms and  conditions.  This Note is secured by the Collateral  described in the
Security Documents.

     All advances hereunder shall bear interest at the Interest Rate (as defined
below)  from the date of such  Advance  until  such  amount  is due and  payable
(whether  on any  Payment  Date,  at the  Maturity  Date,  by  acceleration,  or
otherwise).

     The  "Interest  Rate" shall be a variable  interest rate equal to (a) LIBOR
plus 4.5 % through and including  September  30, 1996,  and (b) LIBOR plus 2.5 %
from and after  October 1, 1996,  in each case adjusted on the first day of each
Interest Period.

     "LIBOR":  in respect of any Interest Period, the rate of interest per annum
shall be the rate quoted in the "money rates" column of The Wall Street  Journal
for the three-month LIBOR (London  Interbank Offered Rates).  This rate is to be
determined on the second  Business Day before the  commencement of such Interest
Period  (each such second  Business Day before the  commencement  of an Interest
Period being hereinafter referred to as an "Interest Determination Date").

     "Interest  Period":  each three (3) calendar month period beginning January
1, April 1, July I and October I of each calendar year provided that:

               (A) if any Interest  Period  pertaining to a Loan would otherwise
         end on a day which is not a Business Day, that Interest Period shall be
         extended to the next succeeding  Business Day unless the result of such
         extension would be to carry such Interest Period into another  calendar
         month, in which event such Interest Period shall end on the immediately
         preceding Business Day;

               (B) any Interest  Period  pertaining to a Loan that begins on the
         last  Business Day of a calendar  month (or on a day for which there is
         no  numerically  corresponding  day in the calendar month at the end of
         such  Interest  Period)  shall end on the last Business Day of the last
         calendar month of such Interest Period;

               (C) no Interest Period shall extend beyond the Maturity Date; and

               (D) no Interest Period shall extend beyond any date upon which is
         due any scheduled  principal  payment in respect of the Loan unless the
         aggregate  principal amount of the Loan is equal to or in excess of the
         amount of such principal payment.

     Interest  shall  accrue  at the  Interest  Rate  on all  principal  amounts
outstanding  hereunder and shall be payable quarterly in arrears,  commencing on
January  1,  1997,  and  continuing  on the  first day of each  Interest  Period
thereafter,  and shall be payable on the Maturity  Date.  Interest shall also be
payable on the date of any  prepayment  of this Loan pursuant to Section 2.07 of
the Loan  Agreement  for the  portion  of the Loan so prepaid  and upon  payment
(including  prepayment) in full thereof and, after the occurrence and during the
continuance of any Event of Default, interest shall be payable on demand.

     All  outstanding  principal  and interest and all other  amounts  otherwise
payable hereunder shall be due and payable on the Maturity Date.

     Notwithstanding  the foregoing,  if the Borrower shall fail to pay any then
due principal  amount or interest or other amount  payable by the Borrower under
the Loan  Agreement  or under this Note within ten (10) days after the due date,
such amount shall bear  interest  from the original due date at a rate per annum
that is equal to the  lesser  of (i) five  percent  (5 %)  higher  than the then
applicable  Interest  Rate or (ii) the maximum  permissible  interest rate under
applicable Law until such overdue principal amount,  interest or other amount is
paid in full  (both  before  and after  judgment)  whether  or not any notice of
default in the payment thereof has been delivered under the Loan Agreement.

     Notwithstanding  any  provision  of this Note or the Loan  Agreement to the
contrary, it is the intent of the Lender and the Borrower that the Lender or any
subsequent  holder of this Note shall never be  entitled  to  receive,  collect,
reserve  or apply,  as  interest,  any amount in excess of the  maximum  rate of
interest permitted to be charged by applicable Law, as amended or enacted,  from
time to time. In the event Lender,  or any subsequent  holder of this Note, ever
receives,  collects,  reserves or applies,  as interest,  any such excess,  such
amount which would be excessive interest shall be deemed a partial prepayment of
principal and treated as such, or, if the principal  indebtedness  and all other
amounts due are paid in full,  any remaining  excess funds shall  immediately be
paid  to the  Borrower.  In  determining  whether  or not the  interest  paid or
payable,  under any specific  contingency,  exceeds the highest lawful rate, the
Borrower and the Lender shall, to the maximum extent  permitted under applicable
law, (a) exclude voluntary  prepayments and the effects thereof as it may relate
to any fees charged by the Lender,  and (b)  amortize,  prorate,  allocate,  and
spread, in equal parts, the total amount of interest  throughout the entire term
of the Note;  provided  that if the Note is paid and  performed in full prior to
the end of the full contemplated  term hereof,  and if the interest received for
the actual  period of existence  hereof  exceeds the maximum  lawful  rate,  the
Lender or any  subsequent  holder of the Note shall  refund to the  Borrower the
amount of such excess or credit the amount of such excess  against the principal
portion of the Note,  as of the date it was  received,  and, in such event,  the
Lender  shall  not be  subject  to  any  penalties  provided  by  any  laws  for
contracting  for,  charging,  reserving or  receiving  interest in excess of the
maximum lawful rate.

     Upon the  occurrence of any one or more Events of Default  specified in the
Loan Agreement,  all amounts then remaining unpaid on this Note shall be, or may
be  declared  to be,  immediately  due  and  payable  as  provided  in the  Loan
Agreement,  without  further  notice,  at the option of the holder  hereof.  The
holder may waive any Event of Default before or after the same has been declared
and  restore  this Note to full force and effect  without  impairing  any rights
hereunder,  such  right of waiver  being a  continuing  one,  but one waiver not
implying any additional or subsequent waiver.

     Demand,  presentment,  notice and protest are expressly waived,  except for
notices otherwise expressly required in the Loan Agreement.

     In the event this Note is placed in the hands of one or more  attorneys for
collection or enforcement or protection of the holder's rights  described in the
Loan Agreement,  the Borrower  agrees to pay all reasonable  attorneys' fees and
all court and other  out-of-pocket  costs  incurred by the holder  hereof (which
shall be due on demand).

     This Note may be prepaid in accordance  with the provisions of Section 2.07
of the Loan Agreement.

     This Note is  governed by and shall be  construed  in  accordance  with the
internal laws of the State of New York.

     This Note may not be changed, extended or terminated except in writing.

     This Note may be assigned  in  accordance  with  Section 8. 1 8 of the Loan
Agreement.  In the  event  of any  conflict  between  this  Note  and  the  Loan
Agreement, the provisions of the Loan Agreement shall control.

     This  Amended  and  Restated  Note  is  an  amendment,   modification   and
restatement of that certain Note in the original  principal amount of $3,750,000
(plus  capitalized  interest),  dated as of May 28, 1993,  issued by Borrower to
Lender,  previously  amended and restated by an Amended and  Restated  Equipment
Note in the original principal amount of $7,500,000 (plus capitalized  interest)
dated as of April 8, 1994 (as  previously  amended,  the  "Original  Note).  The
principal  amount  of  this  Note is the  outstanding  principal  amount  of the
Original Note, including interest capitalized and added to principal as provided
therein.  This Note is not a novation,  release or discharge of the indebtedness
evidenced by the Original Note.

     Executed as of October 1, 1996.

                                            AMSC SUBSIDIARY CORPORATION
                                            By:     /s/ Richard J. Burnheimer
                                            Title:  Vice President & Treasurer