THE 1970's aren't over yet.
Not, at least, in record stores or on radio stations, where Boston's ''Third Stage'' has become one of this year's blockbuster albums, rising almost effortlessly to No. 1. ''Third Stage'' has all the hallmarks of a 1970's hard-pop album - the stately tempos, the grandiosely meticulous arrangements, the histrionic lead vocals and clarion high harmonies, and lyrics along the lines of ''I'm gonna say it like a man/And make you understand/Amanda/I love you.''
These add up to what FM radio's ''album rock'' programmers call anthems, songs that use rock instrumentation to beef up ballads. On stations that play primarily the recent oldies known as ''classic rock'' - big hits from the 1970's - ''Third Stage'' fits right in. The album has received saturation exposure since its release.
Technically, ''Third Stage'' is also a 1970's-style album. It is arranged for guitars, drums, pianos and organs -no synthesizers - and recorded on analog (not digital) equipment. For that matter, it doesn't bother with 1980's-style visual marketing; there's no video clip for the single ''Amanda,'' and not even an album-cover photograph of the band.
Most of all, what links ''Third Stage'' to late-1970's methodology is its leisurely schedule. Boston was last heard from in 1978 with its second album, ''Don't Look Back''; its debut LP, the 1976 ''Boston,'' sold more than six million copies. Since then, the band's songwriter-guitarist-producer Tom Scholz has been ensconced in his laboratory and recording studio, inventing (and patenting) the Rockman, a sandwich-sized guitar effects generator playable through earphones. He has been working on ''Third Stage'' since 1980 with Boston's longtime singer, Brad Delp, and a new drummer, Jim Masdea. The 39-minute ''Third Stage'' took about as long to create - six years - as the Wagner opera ''Parsifal.''
''If I knew how to do it any faster, I would have,'' Mr. Scholz said yesterday by telephone from Boston. ''It wasn't like I was wasting a lot of time - at the beginning, I was putting in 50 or 60 hours a week in the studio. And it wasn't for lack of trying. I guess I make a lot of mistakes.''
For much of ''Third Stage,'' Mr. Scholz worked virtually as a one-man band as well as his own engineer. ''I don't like having to work with other people, to tell them, 'Play this, play that,' '' he said. ''I can't listen to them closely while I'm playing, so I'd rather do it myself. Every time you add another instrument by overdubbing, it adds time. And it adds another possibility of things working completely wrong.''
''I never thought of myself as a perfectionist,'' Mr. Scholz continued. ''If you saw my car or what I wear or my house or anything, you'd know I'm not a perfectionist. But when I get involved in ebony porn something I'm making or building, I get picky about it, and the details suddenly become very important. It's not only in music, it's in anything I'm making - but it ends up costing me an inordinate amount of time to do it.''
Record-company pressure to complete an album - including a battery of lawsuits and countersuits among Epic Records, Boston's former label, the band, and MCA Records, its current label - didn't speed things up. ''There were threats and attacks by people who were looking to exploit Boston for money,'' Mr. Scholz said. ''They were looking for a mediocre piece of junk put out every year to make more money. But that wouldn't be true to Boston's music.
''I dropped everything for six years,'' Mr. Scholz said. ''Everything except basketball, because I knew that once I stopped playing basketball I might never slam-dunk again. But that was literally my only outside activity. It gets pretty serious when you don't go to see your mother for Christmas - you know that you've gotten heavily involved in something. Now, I'm looking forward to doing a lot of things. I need a vacation.
By JON PARELES
Published: November 5, 1986