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

The Columbus model 75 was a world first* radio that 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, it was costly to implement and beyond the budget of a home receiver.

*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

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

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

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.

All documented models using this chassis (2 in total)

1940Columbus model 75 - this set
1940Courtenay model 75