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                                                                    EXHIBIT 10.9

                      SENIOR SUBORDINATED CONVERTIBLE NOTE

$ 588,000.00                                                  SEPTEMBER 30, 2003

FOR VALUE RECEIVED, the undersigned, Eagle Test Systems, Inc., an Illinois
corporation (the "BORROWER"), HEREBY PROMISES TO PAY to the order of TA
Investors, LLC (the "NOTEHOLDER") the principal sum of $ 588,000.00, together
with interest on the unpaid principal amount from time to time outstanding at
the rate or rates and computed and payable at the times as described in the Note
Purchase Agreement (as hereinafter defined). Payments of the principal hereof
shall be made as provided in the Note Purchase Agreement. Notwithstanding any
other provision of this note, the entire balance of principal and accrued and
unpaid interest shall be paid in full on September 30, 2009.

      This note is one of the Convertible Notes referred to in the Note Purchase
Agreement dated as of September 30, 2003 (as the same may be amended, modified
or supplemented from time to time, the "NOTE PURCHASE AGREEMENT") among the
Borrower, the Noteholder and certain other Noteholders named therein.
Capitalized terms used herein and not otherwise defined shall have the meanings
set forth in the Note Purchase Agreement.

      The Noteholder may convert this note at any time pursuant to the terms of
the Note Purchase Agreement into (a) a Senior Subordinated Note due September
30, 2009 in the principal amount of $ 587,902.00 plus any interest owed in
arrears on this note on the Conversion Date, in the form attached hereto as
Exhibit A, and (b) a Common Stock Purchase Warrant of the Borrower for the
purchase of 4.116 shares of the Borrower's Common Stock (subject to adjustment
as provided for therein), exercisable at a price per share of $.01 (subject to
adjustment as provided for therein) in the form attached hereto as Exhibit B,
all as further described in the Note Purchase Agreement.

      Subject to, and at all times in accordance with, the provisions of the
Note Purchase Agreement (i) the Borrower shall have the right, at any time, to
voluntarily prepay all or any part of the outstanding principal amount of this
note, and (ii) the Noteholder shall have the right to require the Borrower to
repurchase this note upon the occurrence of a Mandatory Repurchase Event.

      In addition to the payment of interest as provided above, the Borrower
shall, on demand, pay interest on any overdue installments of principal and, to
the extent permitted by applicable law, on overdue installments of interest at
the rate set forth in, and in accordance with the provisions of, the Note
Purchase Agreement.

      The holder of this note is entitled to all the benefits and rights of a
Noteholder under the Note Purchase Agreement to which reference is hereby made
for a statement of the terms and conditions under which the entire unpaid
balance of this note, or any portion thereof, shall become immediately due and
payable. Notwithstanding anything in this note to the contrary, the terms and
provisions of this note shall at all times be governed by and subject to all of
the terms

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and provisions of the Note Purchase Agreement. To the extent that there is any
conflict with, or inconsistency between, the terms and provisions of this note
and the terms and provisions of the Note Purchase Agreement, the terms and
provisions of the Note Purchase Agreement shall at all times govern and control.

      The Borrower hereby waives presentment, demand, notice, protest and other
demands and notices in connection with the delivery, acceptance or enforcement
of this note.

      No delay or omission on the part of the holder of this note in exercising
any right hereunder shall operate as a waiver of such right or of any other
right under this note, and a waiver, delay or omission on any one occasion shall
not be construed as a bar to or waiver of any such right on any future occasion.

      The terms and provisions of this note are subject to the dispute
resolution provisions contained in Sections 9.11 and 9.12 of the Note Purchase
Agreement.

      This note shall be deemed to be under seal, and all rights and obligations
hereunder shall be governed by the laws of the State of New York (without giving
effect to any conflicts of law provisions contained therein).

                                        EAGLE TEST SYSTEMS, INC.

                                        By: Leonard Foxman
                                            ---------------------------------
                                            Name:  Leonard Foxman
                                            Title: President