The Columbus model 75 was manufactured for Columbus Radio Centre Ltd by Radio Corporation of New Zealand Ltd in 1940.

Other radios based on the same chassis:

The Columbus model 75, 6 valves plus magic eye tuning.

This model was the worlds first* production domestic bandspread radio.

It solved the problem of tuning in short-wave stations which were notoriously finiky to get (and keep) on station in domestic receivers.  The solution was to spread out sections of the short wave spectrum to make tuning easier.  While communication receivers had used 'band-spreading' for a while, until now it had been too costly to implement and beyond the budget of a home receiver.

The model 75 was said to have been developed from a receiver that a lab engineer at RNZ had modified for William Marks (The MD) so his wife could listen to the news back home in Russia.  The initial proof of concept was made on an existing radio by Kem Collett, and finished by others in the lab after Kem was called up in WWII.  The four are remembered in the Columbus booklet advertising the model 75.

Columbus model 75

Various cabinet styles were produced, most of which were carried over to the successor model (model 90).  Visually both models are similar, but the model 75 has the magic eye on the right, while its located in the center of the dial on a model 90.

There was also a battery version, the model 86.

1940 Columbus  model 75 1940 Columbus  model 75 1940 Columbus  model 75 1940 Columbus  model 75 1940 Columbus  model 75
1940 Columbus Promotional Booklet

1940 Columbus model 75

*World first, if you don't take into account RCA's band spread radio from at least a year earlier - although the RCNZ set was certainly more refined and a 'better' all-round solution by all accounts

John W Stokes wrote of the model 75 in the NZVRS Bulletin:

Although mains-operated domestic radios incorporating one or more shortwave bands had been available since 1931, they nearly all used standard size tuning condensers on all bands, thus making tuning on the higher frequencies overly critical. It was this drawback, coupled with an inherent tendency to drift in the oscillator section which made serious shortwave listening of limited interest to the average set owner of the day. Thus it was that by the late 1930s attempts were being made by some manufacturers to provide receivers which could be more easily tuned on the shortwave bands and which were also more stable in operation. In 1938 RCA became the first manufacturer in the world to produce such a set, their model HF2/HF4. 
To obtain easier shortwave tuning it was obviously necessary to reduce the size of the tuning capacitors, but the need to cover the broadcast hand in one sweep placed a limitation on the minimum size which could be used; a suitable compromise could not be reached. So what could be done? 
One solution was to use a separate smaller capacitance condenser solely for tuning the shortwave bands, but this approach was largely impractical. Another approach was to use a split-stator tuning capacitor, as was done in Radio Ltd's model CNU of 1939, but the degree of bandspreading obtainable by this method was limited. A third approach was to artificially reduce the capacitance of the tuning condenser by inserting a small fixed capacitor in series with each section when switched to shortwave. It could easily be arranged so that a fixed capacitor of suitable size could be switched into circuit on each band, thus providing the desired degree of 'bandspreading', as it was called. This, then, was the heart of bandspread tuning. 
As used by Radio Corp, this system was modified using an additional fixed capacitor in parallel with each shortwave tuned circuit thus enabling the .use of only one shortwave coil in each stage resulting in a considerable saving in cost, not to mention the saving in space. 
It must now be recorded that New Zealand was in the forefront of this new development, for with the production by RCNZ of their model 75 early in 1940 they became not only the first N.Z. manufacturer to market a full bandspread receiver, but seemingly the first in the world after RCA. Although it was claimed in a brochure describing the model 75 that it was "....the first domestic radio of its type in the world.", the accuracy of this claim hinges on the words "of its type". Whether there were sufficient differences in the circuitry of the two makes to justify Radio Corp's claim is a matter of opinion, but certain it is that RCA were, by a considerable margin, the first in the world to produce a domestic bandspread receiver, even though it lacked the refinement of having variable tuning on the shortwave ranges of the RF and mixer stages! 
Another claim made by RCNZ was that the model 75 was put into production ...a full 17 months before any others, in October 1939," yet no receivers have. ever been sighted whose date coded serial numbers bear out this statement. Nowadays it is hard to believe that RCNZ were unaware of RCA's work in this area, though this does not alter the fact that the production of the model 75 was very much of a pioneer effort which gave the firm a clear lead lead in this part of the world. 
It was over a year after the arrival of the 75 before other firms in this country had bandspred models on the market. Even in Australia. AWA's first bandspread set did not appear until 1941, although one firm, Howard, had produced a semi-bandspread model as early as 1938. 
During its production lifespan, a matter of nearly three years, the model 75 underwent several modifications, the first of which was the replacement of the original permeability-tuned IF transformers with the (then) more conventional compression trimmer tuned types. At this time the first two valves, types 6K7G and 6K8G, were changed to Osram types KTW61 and X65 respectively. Incidentally, when Osram valves later became unavailable due to wartime restrictions, Radio Corp had to revert to using American valves. 
Final, and major, changes occurred when the three large diameter shielded aerial, RF and oscillator coils were replaced by more modern smaller types mounted in a compact metal box. The BC band coils were now litz wound, with the aerial and RF coils having non-adjustable iron cores in addition. Not only were the new coils more efficient but they also saved space. At this time also the simple continuously variable tone control was replaced by an 11-point switch together with improved circuitry providing a wider range of control. Meanwhile, the development of a new model had been going ahead and in 1942 came the model 90 to replace the the 75, though judging by observed serial numbers there was some overlap in their production. Outwardly the two chassis appeared to be identical, the only visible difference being that the magic eye tuning indicator had been repositioned directly behind the upper central part of the glass dial scale with the result that it was now slightly obscured. The altered position had been brought about by the need to provide space on the dial face for a tone indicator which took the form a moving white dot matching the band indicator at the other end of the dial. Although the tone switch still had eleven positions its associated circuitry differed from that used on the 75, being rather more complex in that as well as including variable inverse feedback from the secondary of the oversize output transformer, an iron-cored inductor formed part of a frequency selective circuit in some of the switch positions. Another externally visible change was that the 16 metre bandspread band had been replaced by a 16 to 49 metre general coverage band, a change which became standard practice in future bandspread models. 
Production of the model 90 extended from 1942 to 1949, but it must be remembered that there was a space of about three years when none were made due to wartime restrictions which prevented the manufacture of radios for civilian use. Even so, the 90 had the longest production lifespan of any model for after the war Radio Corp recommenced production by using pre-war designs. 
Following existing policy, the 75 and 90 were sold under both the Columbus and Courtenay brand names and, as usual, the cabinets differed sufficiently to make one brand distinguishable from the other. Variations in cabinet style also occurred within each brand. A distinguishing feature of the model 75 was the use of wooden knobs, each brand using a different style. By comparison the model 90 used bakelite knobs of Radio Corp's own make, again with different styles for each brand. 


Technical Information

Valves (6 + eye):
first version: 6K7G, 6K8G, 6K7G, 6B8G, 6F6G, 5Z4G and 6U5 Magic Eye
second version: KTW61, X65, 6K7G, 6B8G, 6F6G, U50 and Y63 Magic Eye

Intermediate Frequency: 455kc/s

Frequency Bands: 5

Chassis Notes(most schematics can be clicked to download a full size version) Broadcast band:
Shortwave Bands:

1940 Courtenay and Columbus model 75 Schematic

1940 Courtenay and Columbus model 75 Schematic Supplement

General Construction Notes for Radio Corporation of New Zealand Ltd:

The first digit of the serial number typically indicates the year of manufacture of RCNZ chassis' (although not the decade - that requires a little knowledge of the valves, construction, etc). Sets from around 1934 onwards were often (but not always) constructed in a distinctive pressed 'baking pan' style chassis, seemingly unique to RCNZ.

Model codes beginning with a 0, for example the model 051, are Osram valve versions of the model without the leading 0. Technically the 0 should be an O (for Osram), however the digit 0 was used throughout the site before this fact was discovered.

The E suffix indicates a magic eye option is fitted (in models which were available with or without, such as the model 25).

A and B suffixes appear to be simply updates to the current model, R also appears to be simply an updated model ('R'edesign, perhaps?)

P indicates either a permanent magnet speaker version of a model which also came with an electromagnet speaker (the model 26 for example), or a portable model (like the model 694P).

N and M indicated miniature valve versions of a model which started with all (or a mix, ie: model 5) of larger valves. One of these two codes may indicate a transitional mixture of octal and miniature - clarification is required.

S often indicates a stereo model.  It can also indicate 'self-biased' in the transition period between back-biased and self biased sets where there were models with both methods employed (53S for example)

Finally, other suffixes and prefixes make occasional appearances in the RCNZ lineup - like the 66W (a variant of the long-running model 66) and the 75XA (a 10-valve version of the model 75 with a separate amplifier chassis).

Model nicknames are often sourced from either newspaper advertising, company literature or the NZ Radio Traders Federation official trade-in price books (Particularly Courtenay models from this publication)

Other documented models using this chassis (2 in total)

1940 Courtenay model 75 'Super Defiant'