<PAGE>

EXHIBIT 10.21

                                AMENDMENT NO. 2

  AMENDMENT NO. 2 dated as of July 15, 1999, between FRONTIERVISION OPERATING
PARTNERS, L.P., a limited partnership duly organized and validly existing under
the laws of the State of Delaware (the "Company"); each of the Subsidiaries of
                                        -------
the Company identified under the caption "SUBSIDIARY GUARANTORS" on the
signature pages hereto (individually, a "Subsidiary Guarantor" and, collectively
                                         --------------------
the "Subsidiary Guarantors" and, together with the Company, the "Obligors"); and
     ---------------------                                       --------
THE CHASE MANHATTAN BANK, in its capacity as Administrative Agent pursuant to
authority granted by the Majority Lenders pursuant to Section 11.04 of the
Credit Agreement (as defined below).

  The Company, certain lenders, The Chase Manhattan Bank, as Administrative
Agent, J.P. Morgan Securities Inc., as Syndication Agent, and CIBC Inc., as
Documentation Agent, are parties to a Second Amended and Restated Credit
Agreement dated as of December 19, 1997 (as heretofore amended, the "Credit
                                                                     ------
Agreement").  The Obligors and the Administrative Agent (pursuant to the
---------
authority granted by, and having obtained all necessary consents of, the
Majority Lenders) wish to amend the Credit Agreement in certain respects and,
accordingly, the parties hereto hereby agree as follows:

  Section 1.  Definitions.  Except as otherwise defined in this Amendment No. 2,
              -----------
terms defined in the Credit Agreement are used herein as defined therein.

  Section 2.  Amendments.  The Credit Agreement shall be amended as follows:
              ----------

  Section 2.01.  Certain Defined Terms.  Section 1.01 of the Credit Agreement is
                 ---------------------
amended by adding the following new defined terms (to the extent not already
included in said Section 1.01) and inserting the same in the appropriate
alphabetical locations, and amending the following defined terms (to the extent
already included in said Section 1.01) to read in their entirety, as follows:

          "1999 Acquisition Transaction" shall mean the acquisition by Adelphia
           ----------------------------
     pursuant to the Purchase Agreement dated as of February 22, 1999 (the

     Purchase Agreement") among FrontierVision LP, FVP GP, L.P., the Direct and
     ------------------
     Indirect Limited Partners of FrontierVision LP party thereto and Adelphia
     of all the partnership interests in FrontierVision LP.

           "Adelphia" shall mean Adelphia Communications Corporation.
            --------

          "Change of Control" shall mean any event that requires the Company or
           -----------------
     FrontierVision Capital pursuant to the provisions of the Senior
     Subordinated Debt Documents (or any other agreement or instrument relating
     to or providing for any other Subordinated Indebtedness), or that requires
     FrontierVision Holdings, FrontierVision Holdings Capital Corporation or
     FrontierVision Holdings II Capital Corporation pursuant to the Senior
     Discount Debt Documents, to redeem or repurchase, or make an offer to
     redeem or repurchase, all or any portion of the Subordinated Indebtedness,
     or the Senior Discount

                                Amendment No. 2
                                ---------------

<PAGE>
                                      -2-

     Debt, as a result of a change of control (as defined in the Senior
     Subordinated Debt Documents or any other agreement or instrument relating
     to or providing for any other Subordinated Indebtedness or the Senior
     Discount Debt Documents), provided that the term "Change of
                               --------
     Control" shall not include any such event that occurs as a result of the
     1999 Acquisition Transaction.

          "Debt Ratio" shall mean, as at any date (but subject in any event to
           ----------
      the provisions of Section 8.10(e) hereof), the ratio of:

                (a) the sum of the aggregate amount of all Indebtedness (other
          than Junior Subordinated Indebtedness) of the Company and its
          Restricted Subsidiaries and all letters of credit contemplated by
          Section 8.07(e) hereof, but excluding all performance bonds
          contemplated by said Section as at such date minus, for purposes of
                                                       -----
          Section 8.10(b) only (and not for purposes of the definition of
          "Applicable Margin"), for any date on or before March 30, 2000,
          $20,000,000 to
                      --

                (b) the product of EBITDA for the fiscal quarter ending on, or
          most recently ended prior to such date times four.
                                                 -----

          "Debt Service" shall mean, for any period, the sum, for the Company
           ------------
     and its Restricted Subsidiaries (determined on a consolidated basis without
     duplication in accordance with GAAP), of the following: (a) in the case of
     Loans under this Agreement, the aggregate amount of payments of principal
     of such Loans that, giving effect to Commitment reductions or terminations
     scheduled to be made during such period pursuant to Section 2.03 hereof,
     were required to be made pursuant to Section 3.01 hereof during such period
     plus (b) in the case of all other Indebtedness, all regularly scheduled
     ----
     payments or prepayments of principal of such Indebtedness (including,
     without limitation, the principal component of any payments in respect of
     Capital Lease Obligations but excluding prepayments of principal in respect
     of Junior Subordinated Indebtedness) made or payable during such period
     plus (c) all Interest Expense for such period (excluding, however, non-cash
     ----
     amortization of loan facility fees and other deferred debt costs, in each
     case to the extent included in determining Interest Expense for such
     period).

          "Initial Equityholders" shall mean, collectively, (i) J.P. Morgan
           ---------------------
     Investment Corp., (ii) 1818 II Cable Corp., (iii) Olympus Cable Corp., (iv)
     First Union Capital Partners, Inc., (v) any Control Affiliate of any of the
     foregoing entities, (vi) any limited partnership of which any Control
     Affiliate of any of the foregoing entities is the sole general partner (so
     long as the aggregate equity interests of FrontierVision LP that shall have
     been transferred to all such limited partnerships by any such entity shall
     not exceed 25% of the aggregate equity interests held by such entity in
     FrontierVision LP) and (vii) following consummation of the 1999 Acquisition
     Transaction, Adelphia.

          "Interest Expense" shall mean, for any period, the sum, for the
           ----------------
     Company and its Restricted Subsidiaries (determined on a consolidated basis
     without duplication in


                                Amendment No. 2
                                ---------------
<PAGE>
                                      -3-

     accordance with GAAP), of the following: (a) all interest in respect of
     Indebtedness (including, without limitation, the interest component of any
     payments in respect of Capital Lease Obligations but excluding all interest
     in respect of Junior Subordinated Indebtedness) accrued or capitalized
     during such period (whether or not actually paid during such period) plus
                                                                          ----
     (b) the net amount payable (or minus the net amount receivable) under
                                    -----
     Interest Rate Protection Agreements during such period (whether or not
     actually paid or received during such period).

          "Junior Subordinated Indebtedness" shall mean the Indebtedness of the
           --------------------------------
     Company in respect of junior subordinated notes, in the form attached
     hereto as Exhibit J, issued from time to time to Adelphia.

          "Subordinated Indebtedness" shall mean, collectively, (i) the
           -------------------------
     Indebtedness of the Company and FrontierVision Capital in respect of the
     senior subordinated notes of the Company and FrontierVision Capital due
     2006 issued pursuant to the Senior Subordinated Debt Indenture and (ii) the
     Junior Subordinated Indebtedness."

          Section 2.02. EBITDA. Paragraph (b) of the definition EBITDA in
                        ------
Section 1.01 of the Credit Agreement is hereby amended to read in its entirety
as follows:

          "(b)  all operating expenses for such period, including, without
     limitation, technical, programming, selling and general administration
     expenses incurred by the Company and its Restricted Subsidiaries during
     such period, but excluding (to the extent included in operating expenses)
     depreciation, amortization, Interest Expense, interest in respect of Junior
     Subordinated Indebtedness, any non-cash charges (including, without
     limitation, non-cash pension expenses and any Tax Payment Amount for the
     relevant period), any non-recurring expenses incurred in connection with
     the 1999 Acquisition Transaction, including but not limited to employee
     severance expenses (provided that such expenses are recognized in
     accordance with GAAP prior to the fiscal year ending December 31, 1999)
     plus"
     -----

          Section 2.03. Financial Statements Etc. Section 8.01 (a), (b) and (f)
                        ------------------------
of the Credit Agreement are hereby amended to read their entirety as follows:

          "(a) as soon as available and in any event within 90 days after the
     end of each of the first three quarterly fiscal periods of each fiscal year
     of the Company, consolidated statements of income, changes in partners'
     capital and cash flows of the Company and its Subsidiaries (and, separately
     stated, for the Company and its Restricted Subsidiaries) for such period
     and for the period from the beginning of the respective fiscal year to the
     end of such period, and the related consolidated balance sheets of the
     Company and its Subsidiaries (and, separately stated, for the Company and
     its Restricted Subsidiaries) as at the end of such period, setting forth in
     each case in comparative form the corresponding consolidated figures for
     the corresponding periods in the preceding fiscal year (except that, in the
     case of balance sheets, such comparison shall be to the last day of the
     prior fiscal year), accompanied by a certificate of a Senior Officer, which
     certificate shall state that said

                                Amendment No. 2
                                ---------------
<PAGE>
                                      -4-

     consolidated financial statements fairly present the consolidated financial
     condition and results of operations of the Company and its Subsidiaries (or
     the Company and its Restricted Subsidiaries, as the case may be), in each
     case in accordance with generally accepted accounting principles,
     consistently applied, as at the end of, and for, such period (subject to
     normal year-end audit adjustments), provided that the requirements of this
                                         --------
     Section 8.01(a) with respect to financial statements of the Company and its
     Subsidiaries may be satisfied by delivery by the Company (in accordance
     with this Section 8.01(a)) of the Company's quarterly report filed on Form
     10-Q with the Securities and Exchange Commission;

           (b) as soon as available and in any event within 120 days after the
     end of each fiscal year of the Company, consolidated statements of income,
     changes in partners' capital and cash flows of the Company and its
     Subsidiaries (and, separately stated, for the Company and its Restricted
     Subsidiaries) for such fiscal year and the related consolidated balance
     sheets of the Company and its Subsidiaries (and, separately stated, for the
     Company and its Restricted Subsidiaries) as at the end of such fiscal year,
     setting forth in each case in comparative form the corresponding
     consolidated figures for the preceding fiscal year, accompanied by an
     opinion thereon of independent certified public accountants of recognized
     national standing, which opinion shall state that said consolidated
     financial statements fairly present the consolidated financial condition
     and results of operations of the Company and its Subsidiaries (or the
     Company and its Restricted Subsidiaries, as the case may be) as at the end
     of, and for, such fiscal year in accordance with generally accepted
     accounting principles, and a statement of such accountants to the effect
     that, in making the examination necessary for their opinion, nothing came
     to their attention that caused them to believe that the Company was not in
     compliance with Sections 8.07, 8.08, 8.09 or 8.10 hereof as at the end of
     such fiscal year, insofar as such Sections relate to accounting matters in
     accordance with generally accepted accounting principles, consistently
     applied, as at the end of, and for, such fiscal year, provided that the
                                                           --------
     requirements of this Section 8.01(b) with respect to financial statements
     of the Company and its Subsidiaries may be satisfied by delivery by the
     Company (in accordance with this Section 8.01(b)) of the Company's annual
     report filed on Form 10-K with the Securities and Exchange Commission;

          (f)  concurrently with the delivery of financial statements referred
     to in paragraphs (a) and (b) above, a Quarterly Officer's Report as at the
     end of such periods;"

          Section 2.04. Indebtedness. Section 8.07 of the Credit Agreement is
                        ------------
hereby amended by deleting the word "and" at the end of paragraph (e) thereof,
adding the word "and" at the end of paragraph (f) thereof and by adding new
paragraph (g) at the end thereof to read in its entirety as follows:

          "(g) Junior Subordinated Indebtedness."

          Section 2.05. Subordinated Indebtedness; Other Equity Interests.
                        -------------------------------------------------
Section 8.13(b) of the Credit Agreement is hereby amended by adding the
following paragraph at the end thereof to read in its entirety as follows:

                                Amendment No. 2
                                ---------------
<PAGE>
                                      -5-

          "Notwithstanding the foregoing, the Company may make payments in
     respect of principal and interest on Junior Subordinated Indebtedness if at
     the time of such payments and after giving effect thereto (i) the Debt
     Ratio shall not exceed 5.00 to 1, and (ii) at the time of such payments and
     after giving effect thereto no Default shall have occurred and be
     continuing."

          Section 2.06. Events of Default. Sections 9(m)(ii) and (iii) of the
                        -----------------
Credit Agreement shall be amended to read in their entirety as follows:

          "(ii)  at any time prior to the consummation of the 1999 Acquisition
     Transaction, either James Vaughn or John S. Koo shall, for any reason,
     cease to be actively involved in the day to day management and operation of
     the Company and its Subsidiaries (and Persons with equivalent knowledge and
     experience in the cable television industry reasonably acceptable to the
     Majority Lenders are not appointed to replace one or both of the them
     within 90 days thereof); or

          "(iii)  prior to a Qualified Public Offering, either (x) the Initial
     Equityholders shall cease to own, collectively, on a fully-diluted basis
     (in other words, giving effect to the exercise of any warrants, options and
     conversion and other rights), equity interests representing at least 51% of
     the aggregate fair market value (or, if greater, the aggregate liquidation
     value) of the equity interests of all classes of FrontierVision LP or (y)
     James Vaughn or John S. Koo shall sell, transfer, hypothecate or otherwise
     dispose of more than 50% of their direct or indirect economic interest in
     FrontierVision LP (other than any transfer to the spouse of either of such
     individuals, to his immediate family members, to trusts for the benefit of
     such spouse or immediate family members or pursuant to the 1999 Acquisition
     Transaction); or"

          Section 2.07. Exhibits. The Credit Agreement shall be amended by
                        --------
adding Exhibit J thereto to read in its entirety as Exhibit J hereto.

          Section 3. Miscellaneous. Except as herein provided, the Credit
                     -------------
Agreement shall remain unchanged and in full force and effect. This Amendment
No. 2 may be executed in any number of counterparts, all of which taken together
shall constitute one and the same amendatory instrument and any of the parties
hereto may execute this Amendment No. 2 by signing any such counterpart. This
Amendment No. 2 shall be governed by, and construed in accordance with, the law
of the State of New York.


                                Amendment No. 2
                                ---------------
<PAGE>
                                      -6-

  IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 2 to be
duly executed and delivered as of the day and year first above written.

                           FRONTIERVISION OPERATING
                           PARTNERS, L.P.

                        By:   FrontierVision Holdings, L.P., as general partner
                              of FrontierVision Operating Partners, L.P.

                              By:   FrontierVision Partners, L.P., as
                                    general partner of FrontierVision Holdings,
                                    L.P.

                                    By:      FVP GP, L.P., as general partner of
                                             FrontierVision Partners, L.P.

                                             By: FrontierVision Inc., as
                                                 general partner of FVP GP, L.P.



                                    By____________________________
                                      Title:


                             SUBSIDIARY GUARANTORS
                             ---------------------

          By its signature below each Subsidiary Guarantor (i) consents to the
foregoing Amendment No. 2 and confirms that the obligations of the Company under
the Credit Agreement as herein amended and under the Notes (if any) and in
respect of Pari Passu Obligations are entitled to the benefits of the Subsidiary
Guarantee Agreement executed by each Subsidiary Guarantor, respectively, (and
shall constitute "Guaranteed Obligations" (as defined in such Subsidiary
Guarantee Agreement) under and for all purposes of such Subsidiary Guarantee
Agreement) and (ii) together with the Administrative Agent (acting with the
consent of the Majority Lenders under the Credit Agreement) agrees that
references in such Subsidiary Guarantee Agreement to the "Credit Agreement"
shall be deemed to be references to the Credit Agreement as amended herein.

FRONTIERVISION CAPITAL           FRONTIERVISION CABLE NEW ENGLAND, INC.
 CORPORATION



By____________________________   By____________________________
  Title:                             Title:


                                Amendment No. 2
                                ---------------
<PAGE>
                                      -7-


                                      ADMINISTRATIVE AGENT
                                      --------------------

                                          THE CHASE MANHATTAN BANK,
                                             as Administrative Agent

                                          By____________________________
                                            Title:




                                Amendment No. 2
                                ---------------
<PAGE>

                                                                       EXHIBIT J

                       [Form of Junior Subordinated Note]


                          Junior Subordinated Note of
                    FrontierVision Operating Partners, L.P.
                            due ___________, _____



$____________                                               ____________, ____
                                                            New York, New York

  FrontierVision Operating Partners, L.P., a limited partnership duly organized
and validly existing under the laws of the State of Delaware (the "Maker"), for
                                                                   -----
value received, hereby promises to pay to [________] (the "Payee") the principal
                                                           -----
sum of [AMOUNT IN WORDS] ($___________) on ___________, ____ [not to be earlier
than one year after the latest maturity of all Loans under the Credit
Agreement], subject to the terms and conditions hereinafter set forth.


          Section 1.  Payment.  The Maker shall pay the principal of this Junior
                      -------
Subordinated Note in immediately available funds at the offices of the Payee at
________________________ or at such other place as the Payee shall designate in
writing.

          If any principal of this Junior Subordinated Note shall become due and
payable on any date other than a business day at the place of payment, the time
for the payment thereof shall be extended to the immediately succeeding business
day.

          Notwithstanding anything to the contrary herein contained, the Maker
may make payments of principal hereunder only at such times and to the extent
permitted under the Credit Agreement. As used herein, the term "Credit
                                                                ------
Agreement" shall mean, collectively, (i) the Second Amended and Restated Credit
---------
Agreement dated as of December 19, 1997, between the Maker, certain lenders, The
Chase Manhattan Bank as Administrative Agent, J.P. Morgan Securities Inc. as
syndication agent and CIBC Inc. as documentation agent, and (ii) any extension,
renewal, increase, modification or restatement thereof, or any agreement
refinancing any of the indebtedness thereunder, in each case as the same shall
from time to time be successively extended, renewed, increased, modified,
restated or refinanced.

          Section 2. Interest. This Junior Subordinated Note shall bear interest
                     --------
at the rate of [a market interest rate, subject to approval of the
Administrative Agent], which interest shall be payable only at such times and to
the extent permitted by the Credit Agreement.
<PAGE>
                                      -2-

          Section 3. Prepayment. The Maker may prepay this Junior Subordinated
                     ----------
Note, at any time in whole or in part without penalty or premium, provided that
no such prepayment shall be made to the extent not expressly permitted under the
Credit Agreement.

          Section 4. Subordination. Anything in this Junior Subordinated Note to
                     -------------
the contrary notwithstanding, the indebtedness evidenced by this Junior
Subordinated Note shall be subordinate and junior in right of payment, to the
extent and in the manner hereinafter set forth, to all indebtedness or other
liabilities of the Maker outstanding from time to time including, without
limitation, (x) the Maker's obligation to principal, interest and other amounts
under the Credit Agreement, and (y) any interest accruing after the commencement
of any proceedings referred to in clause (ii) below, whether or not such
interest is an allowed claim in such proceeding (all such indebtedness or other
liabilities and interest being herein called "Senior Indebtedness"):
                                              -------------------

               (i)  The holders of Senior Indebtedness shall be entitled to
     receive payment in full in cash of all amounts constituting Senior
     Indebtedness before the Payee is entitled to receive any payment on account
     of this Junior Subordinated Note, provided that the Maker may make, and the
     Payee shall be entitled to receive and retain from time to time, payments
     and prepayments in respect of the principal and interest of this Junior
     Subordinated Note to the extent permitted under Sections 1, 2 or 3 above.

               (ii)  In the event of any insolvency or bankruptcy proceedings,
     and any receivership, liquidation, reorganization or other similar
     proceedings in connection therewith, relative to the Maker or to its
     creditors, as such, or to its property, and in the event of any proceedings
     for voluntary liquidation, dissolution or other winding up of the Maker,
     whether or not involving insolvency or bankruptcy, then the holders of
     Senior Indebtedness shall be entitled to receive payment in full of all
     amounts constituting Senior Indebtedness before the Payee is entitled to
     receive, or make any demand for, any payment on account of this Junior
     Subordinated Note, and to that end the holders of Senior Indebtedness shall
     be entitled to receive for application in payment thereof any payment or
     distribution of any kind or character, whether in cash or property or
     securities.

               (iii)  If any payment or distribution of any character, whether
     in cash, securities or other property, in respect of this Junior
     Subordinated Note shall (despite these subordination provisions) be
     received by the Payee before all Senior Indebtedness shall have been paid
     in full in cash, such payment or distribution shall be held in trust for
     the benefit of, and shall be paid over or delivered to, the holders of
     Senior Indebtedness (or their representatives), ratably according to the
     respective aggregate amounts remaining unpaid thereon, to the extent
     necessary to pay all Senior Indebtedness in full.

          No present or future holder of Senior Indebtedness shall be prejudiced
in its right to enforce subordination of this Junior Subordinated Note by any
act or failure to act on the part of the Maker or by any act or failure to act,
in good faith on the part of such holder or any trustee or agent for such
holder. The foregoing provisions are solely for the purpose of defining the
relative rights of the holders of Senior Indebtedness on the one hand, and the
Payee on the other
<PAGE>
                                      -3-

hand, and nothing herein shall impair, as between the Maker and the Payee, the
obligation of the Maker, which is unconditional and absolute, to pay to the
Payee the principal hereof in accordance with the terms hereof, nor shall
anything herein prevent the Payee from exercising all remedies otherwise
permitted by applicable law in respect hereof, subject to the rights, if any,
under this Junior Subordinated Note of holders of Senior Indebtedness to receive
cash, property or securities otherwise payable or deliverable to the Payee.

          Section 5. Subrogation. The Payee shall be subrogated to the rights of
                     -----------
the holders of such Senior Indebtedness to receive payments and distributions of
cash, property and securities applicable to the Senior Indebtedness until the
principal of, interest on, and other amounts owing pursuant to, the Senior
Indebtedness shall be paid in full in cash. For purposes of such subrogation, no
payments or distributions to the holders of Senior Indebtedness of any cash,
property or securities to which the Payee would be entitled except for the
provisions of Section 4, and no payments over pursuant to the provisions of
Section 4 to the holders of Senior Indebtedness by the Payee, shall, as between
the Maker, its creditors other than holders of Senior Indebtedness, and the
Payee, be deemed to be a payment or distribution by the Maker to or on account
of the Senior Indebtedness.

          Section 6.  Defaults.  If after payment in full in cash of the Senior
                      --------
Indebtedness, any payment is not made when due hereunder, the Payee may declare
all amounts owing under this Junior Subordinated Note due and payable, provided
that if after repayment in full of the Senior Indebtedness, any payments of
Senior Indebtedness shall at any time be rescinded or otherwise must be returned
by the holder of any Senior Indebtedness, such demand, if made, shall be
automatically rescinded.

          THIS JUNIOR SUBORDINATED NOTE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAW OF THE STATE OF NEW YORK.


                              By:   FrontierVision Partners, L.P., as
                                    general partner of FrontierVision Holdings,
                                    L.P.

                                    By:   FVP GP, L.P., as general partner of
                                          FrontierVision Partners, L.P.

                                          By:  FrontierVision Inc., as general
                                               partner of FVP GP, L.P.


                                    By______________________
                                      Title: