National NC-100 Receiver

As received, this National NC-100 was in a very sorry state. Several knobs were missing, as were most of the valves (and those that were there were not the correct ones).

The set had received a few modifications, one of which had involved removal of some of the tuning capacitor vanes.

The National NC-100 is a prewar communications receiver using 12 Valves.

Valve	Type	Description	Function
V1	6K7	Pentode – Vari-Mu	Radio Frequency Amplifier
V2	6J7	Straight Pentode	1st Detector (Mixer)
V3	6K7	Pentode – Vari-Mu	Local Oscillator
V4	6K7	Pentode – Vari-Mu	First Intermediate Frequency Amplifier
V5	6K7	Pentode – Vari-Mu	Second Intermediate Frequency Amplifier
V6	6C5	Triode	2nd Detector (Anode Bend Detector)
V7	6E5	Magic Eye	Tuning Indicator
V8	6J7	Straight Pentode	Beat Frequency Oscillator (BFO)
V9	6J7	Straight Pentode	AVC Amplifier (Needeed because there’s no diode detector)
V10, V11	6F6	Power Pentodes	Push-Pull Output
V12	80	Rectifier	Provides High Tension DC for Valves

The push-pull interstage transformer was missing and an attempt had been made to get the output valves operating in parallel. The magic-eye was long gone and the set looked in rather poor shape.

I thought that the best plan might be to do a partial redesign using the parts that were left. Fortunately, one of my valve suppliers suggested that I get in touch with a radio collector, who assisted me with parts and a circuit diagram.

When getting radios in this condition to work, I usually start from the power supply, then the audio, and so on working backwards to the RF stage and aerial. The power supply was straightforward – it needed to be rebuilt. The audio stage also needed to be rebuilt, but it was lacking the push-pull driver transformer. This, according to Rider’s needed something like 8000 turns on the secondary.

As luck would have it, the company I worked for had an unused coil winder, so I made them an offer for it. In due course it was put out to tender, and I won the bid. The other fortunate thing was that it was a push-pull interstage transformer – very straightforward. The coil winder was loaded with hair-fine wire and bobbins placed on the mandrel. The pre-set turns counter was set and the winding began. And ended.

With a winding speed of 11000 rpm, my transformer was ready in a few minutes. (This probably proves that it is wise to study the manual for any new piece of equipment – especially a coil winder, otherwise you might end up knee deep in wire.) The transformer was placed in the radio and worked, but several decoupling capacitors promptly blew. These were replaced and all was well.

The detector stage, which had puzzled me so much was an anode bend detector. I had sort of expected it would be a double-diode triode, like the World War 2 military radios I was used to.

At this point the set would receive stations but the sensitivity was poor because of the changes made to the tuning capacitor. The sound quality was excellent, in spite of one of the 6F6 output valves having a huge ding in its metal envelope.

The original circuit of the NC100 used a DC energised loudspeaker, with the speaker windings acting as a further choke coil in the power supply. The output transformer in the original was perched on the back of the speaker and was quite small. The DC energising coil was replaced with a home-wound choke, and the output transformer used was a rather large item from my junk box. These were all placed in the speaker cabinet.

The magic eye was plugged in, but didn’t do anything. This turned out to be because a vital resistor had gone missing.

The final job was to fix the tuning capacitor. These use aluminium vanes bonded to a brass bushing. The offending section was removed and new vanes were cut from thin aluminium sheet. The vanes were soldered to the bushing with aluminium solder and the capacitor was reassembled. Some of the noisy controls were replaced with new ones.

The cabinet was then stripped of all paint (except round the trade mark) and resprayed using wrinkle finish paint, however, I did very little to the front panel.

The set performs as well as it should do for a set of that era. As expected, sensitivity is not as good as later radios, such as the RCA AR88 and resolving a single-sideband signal is somewhat challenging.

The excellent sound quality makes it a good receiver for broadcast AM stations compared to some rather more modern radios that have a narrow intermediate frequency bandwidth.

In common with other National radios of the time, you have to tune the radio in by relating the value on the planetary tuning dial to its value on a graph of frequency vs dial reading. Perhaps this is the only reason I don’t use the radio very much!