<PAGE>
 
                                                                   EXHIBIT 10.29

             FOURTH AMENDMENT TO REVOLVING LINE OF CREDIT AGREEMENT
             ------------------------------------------------------
          THIS FOURTH AMENDMENT TO REVOLVING LINE OF CREDIT AGREEMENT (this
"Agreement") made as of this 31st day of December, 1997 between
RF POWER PRODUCTS, INC., a New Jersey corporation (the "Borrower"), and MELLON
BANK, N.A., a national banking association (the "Bank").


                             W I T N E S S E T H :


          WHEREAS, the Borrower and the Bank are parties to a Revolving Line of
Credit Agreement dated May 24, 1996 (the "Credit Agreement"), pursuant to which
the Bank agreed to extend to the Borrower a Four Million Dollar ($4,000,000)
revolving credit facility (capitalized terms used herein but not defined in this
Agreement shall have the meaning ascribed to them in the Credit Agreement);

          WHEREAS, the Credit Agreement was previously amended in order to
permit, among other things, equipment financing in the amount of $500,000
pursuant to that certain First Amendment to Revolving Credit Agreement dated as
of January 17, 1997;

          WHEREAS, the Credit Agreement was also amended in order to allow the
Borrower to term out and reduce the principal amount outstanding in connection
with the Credit Agreement by $500,000 (to $3,500,000) pursuant to that certain
Second Amendment to Revolving Line of Credit Agreement dated as of February 27,
1997;

          WHEREAS, the Credit Agreement was again amended in order to allow the
Borrower to term out and reduce the principal amount outstanding in connection
with the Credit Agreement by an additional $500,000 (to $3,000,000) pursuant to
that certain Third Amendment to Revolving Line of Credit Agreement dated as of
May 28, 1997;

          WHEREAS, the Borrower has requested that the Bank further amend
certain provisions of the Credit Agreement in order to, among other things,
increase the maximum principal amount available under the revolving credit
facility to $6,000,000; and

          WHEREAS, the Bank is willing to grant such request, subject to the
terms and conditions of this Agreement.

          NOW, THEREFORE, in consideration of the mutual promises contained
herein and intending to be legally bound, the Borrower and the Bank hereby
covenant and agree as follows:

          1.  Amendments.  Upon the execution and delivery by the Borrower and
              ----------                                                      
the Bank of this Agreement and the Fourth Amendment to Note and Security
Agreement dated of even date herewith (the "Note Amendment"), the Credit
Agreement shall be amended as follows:
<PAGE>
 
          (a)  The definition of Commitment Amount appearing on page 1 of the
Credit Agreement is hereby amended by deleting it in its entirety and replacing
it with the following:

COMMITMENT     The lesser of (i) $6,000,000,
AMOUNT:
               or (ii) 80% of Eligible Accounts (as hereinafter defined) less
                                                                         ----
               the amount of any outstanding principal which Borrower has
               elected to term out under the Alternative Principal Amortization
               provisions of the Note.

          (b)  The definition of Commitment Period appearing on page 1 of the
Credit Agreement is hereby amended by replacing the date May 30, 1998 with May
30, 1999.

          (c)  The definition of Cash Flow set forth in paragraph 5(a) of the
Supplement to Revolving Line of Credit Agreement (the "Supplement") shall be
deleted in its entirety and replaced with the following:

               (a)  "Cash Flow" means EBITDA minus capital expenditures, cash
                     ---------              ------                           
     taxes and dividends, all as measured on a rolling four-quarters basis and
     as determined in accordance with GAAP.

          (d)  The following definition of EBITDA shall be included as paragraph
5(d) to the Supplement:

               (d)  "EBITDA" means earnings before interest, taxes,
                     ------                                        
     depreciation, and amortization, all as measured on a rolling four-quarters
     basis and as determined in accordance with GAAP.

          (e)  The Credit Agreement shall be amended to reflect that references
in the Credit Agreement to the "Note" shall be references to the Note and
Security Agreement as amended by the Note Amendment and as may be still further
amended, modified or supplemented from time to time.

          2.  Representations and Warranties.  The Borrower hereby represents
              ------------------------------                                 
and warrants to the Bank that the Borrower is not in default under the Note, the
Credit Agreement or any other document executed in connection therewith.

          3.  Other Terms Confirmed.  All other terms and conditions of the
              ---------------------                                        
Credit Agreement, including, without limitation, the Supplement attached
thereto, are hereby confirmed and shall remain in full force and effect without
modification.  From and after the effectiveness of the amendments set forth in
Section 1 hereof, all references in any document or instrument to the Credit
Agreement shall mean the Credit Agreement as amended by this Agreement.

          4.  No New Indebtedness.  The Borrower specifically acknowledges and
              -------------------                                             
agrees that this Agreement shall not represent in any way the extension of any
new credit by the Bank to the Borrower, or the satisfaction of any indebtedness
evidenced by the Credit Agreement as amended hereby or the Note.

                                       2
<PAGE>
 
          5.  Counterparts.  This Agreement may be executed in any number of
              ------------                                                  
counterparts, each of which shall be deemed an original, but all such
counterparts together shall constitute but one and the same instrument.

          6.  Headings.  The descriptive headings which are used in this
              --------                                                  
Agreement are for convenience only and shall not affect the meaning of any
provision of this Agreement.

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be executed by their duly authorized officers as of the day and year first
written above.

Attest:                             RF POWER PRODUCTS, INC.



     /s/ Paul S. Zaun                      /s/ Domenic N. Golato
   ----------------------                ---------------------------
                                         By:  Domenic N. Golato
                                              Chief Financial Officer
[Corporate Seal]


                                    MELLON BANK, N.A.



                                    By: /s/ Marita McGough Carb
                                       --------------------------
                                       Marita McGough Carb
                                       Vice President

                                       3