Although, during Brian McMaster's reign, the Edinburgh Festival excluded national youth orchestras - even the admirable and ambitious National Youth Orchestra of Scotland - from its programme, there is evidence that his successor, Jonathan Mills, has lifted the ban. This year, the celebrated Simon Bolivar Youth Orchestra of Venezuela is appearing.

If Mills remains open-minded, it will be good news for Richard Chester, NYOS's dedicated director for the past 20 years, who is already engaged in talks for an NYOS appearance in 2009.

Chester has fought constantly for his orchestra to be restored to an event in which it last performed in 1988, when Frank Dunlop was in charge and James Loughran conducted Thea Musgrave's sensational Concerto for Orchestra and Berio's Boccherini-inspired Night Music of Madrid. For sense of adventure, NYOS has never sold the Edinburgh Festival short. Five years earlier, it triumphed over the complexities of Webern's Six Orchestral Pieces, Nielsen's Inextinguishable Symphony and Ravel's Sheherazade in a single programme conducted by Sir Alexander Gibson.

McMaster, as Chester admits, made his policy about youth orchestras "extremely clear" during his Edinburgh tenure. International orchestras were acceptable if they were of the calibre of the Gustav Mahler Jugend with Claudio Abbado as conductor. National ones were not, even if Sir Simon Rattle (one of Chester's high hopes) was potentially in charge.

But whatever the future holds, it is not something in which Chester himself will participate. In September he departs - "very quietly," according to this congenially reserved man of music - from his Glasgow appointment for a new international career.

Born 64 years ago in a north Yorkshire village, Chester first established himself in Scotland as the RSNO's principal flute during the Gibson era, before taking over as director of the then recently-formed NYOS. The original orchestra, which had made its debut playing Dvorak's New World symphony in a tent in Falkirk, was a singular organisation in the sense that there was only one of it.

With Chester at the helm, however, it has expanded in so many directions - it has its own children's orchestra, jazz band and modern music group, as well as a pre-professional chamber orchestra named Camerata Scotland - that it is now definitely plural. Today the initials stand for the National Youth Orchestras of Scotland, though the big symphony orchestra, with its winter and summer concerts, international tours and a vast tally of commissioned works at its disposal, remains the flagship.

At this significant point in its progress, does Chester regret his decision to leave so protean an organisation, which now has a staff of 13 in its office instead of the original two? Obviously yes, but he believes it to be the moment to go - just as he felt he left the RSNO at the right time, when its stupendous link with Scottish Opera was over, its glorious vanguard programmes of Musica Nova had ground tragically to a halt, and even the proms were not what they were.

In fact, having brought NYOS to its flourishing state, Chester needed a new challenge, and he has found it in the career he will develop as an international consultant.

Once again, youth orchestras will be a focus of his activity, but this time with special emphasis on people who want to participate in amateur rather than professional music-making. By 1997, he says, a survey disclosed that 32 per cent of members of NYOS went into music professionally, most of them as orchestral players. Today, in his travels around the world, he has recognised that more and more young players want to continue performing music while pursuing other careers.

Britain, of course, has always been a land of part-time music-making, with its numerous operatic societies and amateur orchestras. But Chester has been finding the same enthusiasm elsewhere, not least in the amateur orchestral festivals that are springing up in Japan and China, with professional musicians closely involved in providing advice.

It is in this visionary idea that Chester, as a specialist activist, sees his future. He will, he says, "play things quietly" until the end of 2007, then start developing his new career next year. Last month he made a productive visit to Shanghai, noting that the level of musical knowledge and enthusiasm in China has excitingly reached the state of Japan's in the 1990s.

Having already travelled the world with NYOS, which he took on a five-city tour of Australia as well as to Japan, he certainly knows the scene and what the prospects are.

As for Europe, it was NYOS - not the RSNO or the BBC SSO - which was the first Scottish orchestra to play in Amsterdam's great Concertgebouw.

Already his advice is being sought for a new National Youth Orchestra of Denmark. It looks as if Chester, at an age when other people are retiring, is going to be busier than ever.