This receiver is exceptionally complex, and I would not trust anybody to work on it, except master old school technicians. The service manual is not like a manual at all, more like a book or encyclopedia. It has a ton of protection circuits built in that will prevent it from fully powering up if some circuit values are not in line with expectations. I am presuming it never comes out of protection mode? When it does, there will usually be a click after anywhere from 5 to 20 second delay. Adjustments to bias and DC offset are critical to getting it to come out of protection. Furthermore, there are some semiconductors that are basically unobtanium and some creative circuit replacement is often necessary to get these working again. This includes a varistor in the protection circuit as well as the output transistors. If and when replacements are found, the replacements are treated with kids glove to ensure that these rare and valuable replacement pieces are not toasted upon powerup because other related issues were bit addressed before repalcement.
I'll tell you this, most service centers that are not intimately familiar with the idiosyncracies of this model will not be able to successfully repair this model if they don't by somehow already have these rare parts on hand if they are required. You will then be given the bad news and probably a hefty bill to retrieve your receiver.
If you want a recommendation, there are a few guys on Audiokarma such as echowars or markthefixer who knows this model very well although they are typically very very busy. I do have a guy in mind who I would trust with this receiver and if you want a referral, pm me for his contact.
Make no mistake, unless the only thing wrong with this receiver is the need to adjust bias and DC offset to get it going, and presuming that it will even adjust to within specs, you should be prepared to pay some $$'s to get it running properly. But to be fair, this receiver is itself a very pricey and valuable model especially if fully working with service documentation to back it up, and cosmetically nice. So while it may cost to fix it up, the end result will be a very valuable receiver which truly deserves to be fixed up properly.
To my eyes, this is possiblly the most beautiful receiver in the world, ever. Pictures don't really do it justice because when viewed in real life, in all it's sparkling glory and humungous size, one can't resist comparing it to fine jewelry, artistically designed in the form of audio gear.
Good luck with the fix and congrats on the purchase. They are exceptionally rare. In 3 decades, I was able to collect 3 Marantz 2500 examples (still got them) half a dozen Sansui G receivers (another beautiful receiver) but only found and was able to afford 1 of the SX-1980's which I still have. It also, like yourse, needed service to get up and running but that was 20 years ago when parts were far more accessible. Mine is minty but come to think of it, who knows if it will still power up today? Even sitting, it's possible that some values have drifted off rendering it no longer able to power up. Hopefully I'll be able to drag it out and enjoy it for a bit before time runs out for me. Literally.