Thermo Fisher Scientific Inc. (NYSE:TMO) continues to call in blocs of old debt, announcing plans to redeem nearly $175 million in 30-year, senior convertible debentures due in 2033.

Coinciding with the upcoming tender offer, the Waltham, Mass.-based laboratory instruments maker said it plans to buy back up to $750 million worth of its own stock, filing registration documents indicating its plans to fund the buyback via the sale of two new series of bonds, set to mature in 2015 and 2020, respectively.

According to a news release, $1,000 worth of the 2033 convertible notes can be turned into 33.84 shares of Thermo Fisher common stock. That works out to a conversion price of $29.55 per share, or nearly half Thermo Fisher’s current share price on the open market.

The proposed tender bid would keep the company and its existing shareholders from seeing their respective positions diluted by to 1.4 percent if the note-holders opted to fully convert the securities into stock, based on about 409.5 million shares of Thermo Fisher stock now outstanding.

The notes are a holdover from the 2006 merger between Thermo Electron Corp. and Fisher Scientific International Inc. Fisher acquired the notes in August 2004 as part of its buyout of Apogent Technologies, which originally used the cash raised through their sale to buy back stock and pay down a term loan.

Since acquiring responsibility for the issue, Thermo Fisher has been paying a floating yield equal to the 3-month LIBOR rate, minus 125 basis points (now a near-negligible 0.18 percent, but previously climbing as high as 5.3 percent in late 2007 before the recent economic crisis prompted a sharp decline in interest rates).

The new notes Thermo Fisher intends to issue likely will carry a fixed interest rate of around 3 percent.