Great find! I'll mention that there are two different NC-183's One is the NC-183 very similar to the NC-173 except it has p/p 6v6's in the audio output. Your's is a NC-183D it's dual conversion set. So when looking for info stuff for the '183 won't be much good. A common problem with those set is a burned up power transformer. It's caused by a paper cap tied across the hv winding. It shorts and will take out the transformer. Please remove it! If it hasn't been already. Manual for it is here:
http://bama.edebris.com/manuals/national/nc183d/
Gud lk
Terry