Pioneer sx-636 vintage reciever?

MIGHT not matter, but I had the SX-727, same vintage and while it was OK in most regards, the FM tuner would not stay stable on frequency. As soon as it warmed up, it'd drift off frequency. This model had both a center and signal strength meter. Several trips to the fix it guy were NO help.

If its in good working order, its a nice find at $40.

Your NAD is probably better sounding overall though.

Pioneer always made reliable receivers and the SX series was no exception (I've serviced hundreds of them). However you have to understand that the unit is not 'permanent', in that the filter capacitors in the power supply need to be replaced, and likely a good number of the electrolytic coupling caps as well.

IMO it will not be worth it to rebuild, but for $40, if it is working, you might get a few years out of it before it dies completely, although the unit will not perform the way it is supposed to.

It might be worth it to rebuild if you are a collector of old receivers, speakers and such. I myself have several old Sansui, Marantz, and Pioneer receivers that I like to swap in and out of my basement vintage system, along with various old speakers such as Pioneer, Dahlquist, EPI, University, Yamaha, Wharfedale, Infinity, Bose, etc. It's fun to listen to different setups while I work on my latest project - and all of the above have been (or will be) projects at one time or another. The most I paid for any of them was $50 (plus whatever it took to fix them up).
I don't fool myself that I will be able to reap huge profits by selling them, though. I always feel a little guilty selling any of them, actually - it's like selling my kids.

The 636SX was my first receiver (along with EPI speakers and a Dual turntable) back in 1979.

$40 for a 40 year old receiver that is working ok is fine, just don't expect the world from it.  It was the lower mid end receiver in the Pioneer back then, so its performance was acceptable for the price I paid, but once I upgraded to Adcom equipment a number of years later, the comparison between the two wasn't even close.

Think of it as a hobby piece.  If I had one today, it would use it in a bedroom system.  

Rich