<PAGE>
                                                                   EXHIBIT 10.50

                             THIRTY-SECOND AMENDMENT

                                       TO

                    CONVERTIBLE DEBENTURE PURCHASE AGREEMENT

THIS THIRTY-SECOND AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT (the
"Amendment") is entered into as of June 28, 2002, by and between DISC, Inc., a
California corporation (the "Company"), and MK GVD Fund (the "Purchaser").

R E C I T A L S:

A. WHEREAS on March 29, 1996 the Company and Purchaser entered into a
Convertible Debenture Purchase Agreement pursuant to which the Company agreed to
sell, and Purchaser agreed to purchase, an aggregate of $1,400,000 in principal
amount of Convertible Debentures, each convertible into shares of the Company's
Preferred Stock, which Agreement was amended as of December 31, 1996, April 11,
1997, December 31, 1997, March 27, 1998, June 30, 1998, September 25, 1998,
December 31, 1998, March 30, 1999, June 30, 1999, September 30, 1999, December
31, 1999, March 31, 2000, June 30,2000, September 30, 2000, December 29, 2000,
March 30, 2001, June 29, 2001, September 28, 2001, December 31, 2001, March 6,
2002, March 15, 2002, March 19, 2002, March 26, 2002, March 27, 2002, April 4,
2002, April 12, 2002, April 24, 2002, April 26, 2002, May 3, 2002, June 10, 2002
and June 27, 2002 to increase the aggregate amount of Convertible Debenture to
be purchased thereunder to $18,480,000.

B. The Company and Purchaser now seek to amend the Agreement to increase the
total amount of Convertible Debentures which Purchaser agrees to purchase
thereunder.

NOW, THEREFORE, for good and valuable consideration, receipt of which is hereby
acknowledged, and in consideration of the mutual covenants set forth herein, the
parties hereto agree as follows:

1. DEFINITIONS. Unless otherwise defined herein, capitalized terms used in the
Amendment shall have the same meanings ascribed to them in the Convertible
Debenture Purchase Agreement.

2. AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT. Section 1.1(a) of the
Convertible Debenture Purchase Agreement is hereby amended to provide that
Purchaser agrees to purchase, and the Company agrees to issue and sell, an
aggregate of $18,830,000 in principal amount of Convertible Debentures, each in
the form and having the terms and conditions set forth in the amended Exhibit B
attached hereto.

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

3. ENTIRE AGREEMENT; AMENDMENT. The Convertible Debenture Purchase Agreement, as
amended by this Amendment, constitutes the full and complete agreement and
understanding

<PAGE>

between the parties hereto regarding the subject matter of the Convertible
Debenture Purchase Agreement and shall supersede all prior communications,
representations, understandings or agreements, if any, whether oral or written,
concerning the subject matter contained in the Convertible Debenture Purchase
Agreement, as so amended, and that no provision of the Convertible Debenture
Purchase Agreement, as so amended, may be modified, amended, waived or
discharged, in whole or in part, except in accordance with its terms.

4. FORCE AND EFFECT. Except as modified by this Amendment, the terms and
provisions of the Convertible Debenture Purchase Agreement are hereby ratified
and confirmed and are and shall remain in full force and effect. Should any
inconsistency arise between this Amendment and the Convertible Debenture
Purchase Agreement as to the specific matters which are the subject of this
Amendment, the terms and conditions of this Amendment shall control. This
Amendment shall be construed to be part of the Convertible Debenture Purchase
Agreement and shall be deemed incorporated into the Convertible Debenture
Purchase Agreement by this reference.

5. COUNTERPARTS. This Amendment may be executed in one or more counterparts,
each of which shall be an original but all of which together shall constitute
one instrument.

IN WITNESS WHEREOF, the Company has caused this Amendment to be executed in
duplicate on its behalf by its duly authorized officer and Purchaser has also
executed this Amendment in duplicate, all as of the day and year indicated
above.

                                            DISC, INC.
                                            a California corporation

                                            By: /s/ Henry Madrid
                                            ------------------------------------
                                            Henry Madrid
                                            Chief Financial Officer


                                            PURCHASER:

                                            MK GVD FUND

                                        By: /s/ Michael D. Kaufman
                                            ------------------------------------
                                            Michael D. Kaufman, General Partner



                                       -2-