National NC-120 / Navy RAO

The before picture shows an example of the type of condition some of this heavy metal comes in. This is the National NC-120 which wears a WW II Navy uniform as the RAO. (And this one looked like it went through the war. :-)   I have not yet absolutely determined which of the RAO models this one is, but believe it to be the RAO-4. This version has the S-meter but no connector for a Panadaptor. Based upon the civilian NC-100XA, the National NC-120 was primarily manufactured for sale to the military. Whether it was ever offered for sale to non-military users is an open question. It is believed to exist only as the RAO.

This NC-120 covers 540 KHz through 30 MHz for general coverage in five bands. It and all other NC-100 series sets are sliding coil-tray receivers. The entire bottom section consists of a cast aluminum tray of tuning circuits, each for a different band. The entire tray moves so as to select the proper set of coils for each band. The tray is attached to a rack (a flat piece of metal with teeth to match a gear) which is moved by a small gear (pinion) in the same manner as rack-and-pinion steering for an automobile. The large lower knob moves that pinion. (See also the National NC-200). In designing the sliding coil tray sets, National engineers wanted to avoid the use of bandswitches. The sliding coil tray design gave them the advantages of the HRO-style plug-in coil sets in an all-in-one design. The NC-100/100XA was a compromise since its coil tray design size allowed for only one RF stage. The HRO had two. The NC-120 has two tandem sliding coil trays and two RF stages and more closely emulates the HRO. With obvious superb mechanical and electronic engineering, all of the sliding coil tray sets and especially the NC-120 show their heritage as descendants of the early HRO.

Why did National build the NC-120/ RAO with two RF stages when the NC-100XA only needed one? The Navy wanted a very quiet receiver that could not be detected by an enemy. Maximum radiation had to be less than 400 picowatts. This required an added input tuned RF amplifier in a separately shielded compartment. The separate section helped to prevent RF energy from the receiver oscillator and mixer sections from reaching the antenna and enemy direction finders.

According to NAVSHIPS 900,351-1B and the National RAO-2 manual available for download from BAMA
"The equipment is designed for use at installations where radiation from the high frequency oscillator, located within the Radio Receiver, must be less than 400 micro-micro-watts as measured at the receiver antenna input terminals."
(Thanks to Sherry Guttery of Sherry's Boatanchors for this information.
See also the Scott SLRM for info on another low radiation receiver and the Navy's use of RME preselectors for isolating other radios from antennas.

Both the NC-100XA and the NC-120 have 11 tubes. The NC-120 has single ended audio while the NC-100XA has push-pull. The NC-120 has two RF amp tubes while the NC-100XA has only one.

Testing and repair notes
Used Caig Deoxit on all controls and contacts including those on the variable cap. Did a thorough safety check including testing for electrical leakage from the line cord to the chassis. Replaced the wrong oversize fuse with the proper size. Removed the rectifier tube and carefully used a variac to do an initial power up for pilot lights and filaments only. I carefully monitored the current required by the receiver and checked for power transformer heating. Checked for B+ line shorts and leakage using a separate monitored power supply (A Heathkit PS-4 regulated high voltage supply.). This particular set uses non-electrolytic power supply filter caps so there was no need to reform the filter caps.

After I was satisfied with the results of all of these initial tests, I re-inserted the rectifier and carefully powered up the set using a variac while monitoring B+ voltage and current consumption. A 600 ohm to voice coil transformer was used for the audio line to the speaker. With final power up, the receiver came alive and worked very well on the broadcast band with just a hank of wire for antenna. I found that the pins on the coil tray need to be cleaned further since shortwave bands C and D kicked in at either side of the detent but opened when the coil tray is on the detent itself (Follow-up; determined which set of contacts were the problem and did careful mechanical cleaning and adjustment to solve the problem.) The AVC switch needed some troubleshooting. (Spraying a bit more Deoxit directly on the switch while working the switch solved the problem.) Replaced several of the capacitors, including the critical capacitor at the input grid of the 6K6 output tube. (It is critical because leakage can upset the bias and cause the output stage to draw too much current resulting in excess heating of the power transformer. Replacing this cap reduced the AC power draw by several watts. ) This set does not have any capacitors from the power line to chassis. If it had such capacitors, I would typically replace those with modern equivalents rated for AC since leakage can be hazardous.

The IF was correctly aligned on the crystal frequency. The NC-100 alignment instructions call for adjusting the RF and mixer caps in the coil trays for maximum noise while at the high end of each band. A set with 2 tuned RF and 2 IF stages is very sensitive indeed when aligned properly and working well. This set is a tribute to National's design skills, a fine broadcast and short-wave band cruiser.