The Columbus model 38 was manufactured for Columbus Radio Centre Ltd by Radio Corporation of New Zealand Ltd in 1937.

The Columbus model 38 used a diamond-shaped point of light to indicate the frequency in a system they called "Spotlight Spiral Dial" tuning. None of the cabinet designs have a known nickname, although the slatted front model was advertised as being scientifically designed to spread all sound frequencies evenly.  The console seen here was sighted for sale in Australia and the cabinet may be local to that market.

Battery model of this set is the model 77, and there is a 9-valve (all metal valves) version, the model 43.

The diamond shaped point of light is generated by two aluminium discs with opposing spiral-shaped slots cut in them. They rotate independently of each other and in sync with the tuning gang in order to produce a hole where the two spirals meet that indicates the position of the tuning gang.


1937 Columbus model 38

1937 Columbus model 38

Valves (6 + eye): 6D6, 6A7, 6D6, 6B7, 42, 80 and 6E5 Magic Eye

Intermediate Frequency: 456kc/s

Frequency Bands: 3

Chassis Notes(most schematics can be clicked to download a full size version)

3-band (all-wave) chassis covering broadcast (550 - 1500kHz), Intermediate SW (2.1 - 6MHz) and SW (6 - 18MHz).

The chassis uses a unique spiral dial tuning system with an effective tuning distance of over 2 feet and fast / slow tuning to provide fine control of the frequency. The dial assembly consists of either a red-tinted spiral line of light or a red dot (depending on brand) that indicates the frequency on a novel trio of spirals for the three bands. The band selector slightly rotates the system so that the dot or the end of the line aligns with the correct band.

The spiral tuning system is complex, and unless its been restored it always seems to be broken on sets found today.  Repairing and setting the dial up takes some patience and trial & error, and while there is indication in the model 38 service documentation that there was to be a service bulletin from RCNZ for the dial assy, it has never been sighted.

Repair of the dial plates often requires the plates to be separated (on a lathe is easiest), then the old and normally torn / unglued paper to be removed and a new piece made.  The red cellophane is often damaged as well and will most likely need replacing.

Radio Corp NZ model 38 spiral dial assembly disassembled
Dial assembly split - new hand-cut black paper template shown top left

RCNZ model 38 spiral dial assembly repair
New paper and cellophane glued in place ready to reassemble

RCNZ model 38 spiral dial assembly repaired
Dial plates rebuilt, showing operation

It appears like the original dials used chain, possibly to eliminate any chance of stretch and to ensure accuracy of the pointer - however nylon dial cord has been used successfully in recent repairs.  The dial cord runs from the tab at the outside edge of the back of the plate around the pulley then around the central bakelite spiral pully (which should be free to spin on the shaft) 2½ times then to the arm on the wave-change switch which should be roughly 45º above horizontal from the shaft when switched to the middle band to give correct actuation - this will require experimentation. Ensure the dial cord sits in the right place on the central pully so it doesn't run off either end, and watch the spike that it seems to have - purpose unknown?

RCNZ model 38 spiral dial dial string on dial plates
Dial cord and return spring shown in position

RCNZ model 38 dial assembly fitted and dial cord strung
Dial assembly remounted and strung

RCNZ model 38 dial assembly restring process
Note use of an aligator clip to hold the cord in place while getting the length and position of the lever correct

Restoration of the dial assembly is a lengthy process, but well worth the effort.

RCNZ Model 38 Spiral Dial set - schematic diagram

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 (4 in total)

1937Columbus model 38 - this set
1937Stella model 38
1937Pacific model 38
1937Courtenay model 38