((This March 2 story removes incorrect reference to Sprint as a Vision Fund investment))

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NEW YORK (Reuters) - SoftBank Group Corp Chief Executive Masayoshi Son, under pressure from hedge fund Elliott Management to rein in his mercurial investment style, turned on the charm in a meeting with U.S. investors on Monday, but offered few concrete concessions.

“I promise you I’ll start to be more careful and listen. My view doesn’t change, but my behavior becomes a little more careful,” sources quoted Son as telling investors who attended his presentation at the Lotte New York Palace hotel in Manhattan.

Son, who built SoftBank into a technology investment powerhouse, is now having to defend his track record after several of its expensive bets on startups, including office space-sharing firm WeWork, soured.

Elliott, which oversees $40 billion in assets, has held discussions with SoftBank’s management and is calling on the company to buy back some $20 billion of its stock, improve its governance by increasing the independence and diversity of its board and improving transparency, sources said last month.

Son said on Monday he had not given enough weight to the opinions of investors and the company’s independent board members, according to three sources who attended the meeting which was closed to media and provided details on condition of anonymity.

Son pointed to SoftBank’s stock trading at a big discount to the value of its assets as an opportunity for investors to buy in.

His approach to investing heavily in companies with disruptive technology potential and giving carte blanche to the founders of startups he backs has helped make him one of the world’s wealthiest investors. Yet it has also led to blunders that have blemished SoftBank’s performance.

Chief among SoftBank’s misses is WeWork, in which SoftBank invested billions of dollars to back CEO Adam Neumann before stepping in to bail the company out and replace him.

The WeWork investment was made out of SoftBank’s $100 billion Vision Fund, which has also invested in or acquired the likes of Uber Technologies Inc and British chip technology firm Arm Holdings.

SoftBank was aiming to raise $108 billion in funds for a second Vision Fund, and has committed $38 billion of its own money toward that goal. However, Son said last month its launch had been delayed due to investor concerns about the performance of the first Vision Fund.

As a result, Son has said SoftBank may spend up to two years investing its own money in a bridge fund, to build a portfolio that will give investors enough confidence to participate in a second Vision Fund.

In his remarks on Monday, Son said WeWork new CEO Sandeep Mathrani will turn SoftBank’s investment around, but cautioned this could take years.

Son also said that up to 15% of the Vision Fund’s portfolio may flop, 15% could be large successes, and the remainder are “passable” investments, a performance mix typically seen in Silicon Valley’s venture capital funds. He singled out deals for Arm and Sprint as signs of SoftBank’s success, the sources said.

Son told the investors in New York that the pending all-stock merger of its U.S. wireless carrier Sprint Corp with Deutsche Telekom AG’s T-Mobile US Inc would generate an internal rate of return, a closely followed measure of investment performance, of 25% for SoftBank.