During the early 1900s and through World War 1, the US Navy was on the bleeding edge of radio technology. At that time radio was called wireless and was almost exclusively morse code. The SE-1420 was one of the most successful medium-wave receivers of the period, to the extent that these radios were in use until WW2 forced the military to ban regenerative receivers as their RF oscillations could be detected by the enemy.
The SE-1420 was Designed by the brilliant engineer Louis Alan Hazeltine while working as a consultant for the Washington Navy Yard’s radio laboratory. The mechanical design was rugged and reliable, built to the Navy’s bulletproof requirements.
The SE-1420 is a one tube regenerative receiver, able to tune from 235 TO 7500 meters (40 kHz to 1.275 MHz). It was one of the first receivers to be fully shielded, reducing the effects of hand capacity, and could be set to use either the internal vacuum tube or an external crystal as the detector.
There are a large number of resources on the web for the SE-1420, but the most useful document for this project has been INSTRUCTIONS FOR USE OF U. S. NAVY MEDIUM WAVE RADIO RECEIVER TYPE SE 1420 Written at the Radio test shop of the U.S. Navy Yard in Washington, D.C. The schematics and wiring diagrams are correct and the operating procedures are clearly explained. Some websites that were invaluable include the Western Historic Radio Museum and Navy Radio.
In 2018 Brown Cow was able to purchase a pair of derelict Wireless Specialty Apparatus manufactured 1420s on ebay. Neither would have been restorable on its own, but between the two there was enough to contemplate assembling a complete radio. Due to other commitments, this project remained on the shelf until the 2020 pandemic redefined the meaning of “spare time”.
Evaluation
Once both 1420s were on the bench it was possible to perform an evaluation and determine what parts were present, missing or duplicated.
Set #1 (on the left) was in better physical condition but had the following faults:
- The Ebonite front panel had several holes that were drilled after the radio was manufactured.
- The original buzzer, used for adjusting a crystal detector, was missing and an incorrect replacement had been installed.
- All interior wiring had been removed along with the ground condenser, variable phone condenser, and iron-cored, lattice-wound R.F. choke.
- The original tube socket, along with its complex shock-mounting was missing.
- While the radio came with a mahogany enclosure, it was determined to be from an Amrad set. A correct dovetailed oak enclosure would need to be constructed.
Set #2 could supply the tube socket and mounting, both in need of repair. Fortunately this set did have the variable phone condenser, a part that would have been very difficult to reproduce. the buzzer, R.F. choke and ground condenser would need to be reproduced since replacement parts are unobtanable.
Rebuilding the Tube Socket
The tube socket assembly consists of 38 separate parts, not including fasteners. The socket looked very sad when it was removed from the donor receiver but it was mostly dirt and corrosion. The assembly was torn down which required removing solder from every screw and nut. Locktite wasn’t available in 1918 so every fastner in Navy receivers of this period had solder applied to threaded components.
The socket itself is mounted on a hard rubber plate, supported by three springs that are attached to a metal plate. The plate is supported by six springs which are connected to the front panel with long rods.These springs are constrained by a hard rubber ring that was broken in several pieces. This ring also serves as a junction point where chassis buss wiring joins the fine flexible wire going to the tube contacts. Since the ring is stressed when the tube moves it was necessary to make a replacement. The replacement was made from bakelite sheet, cut to shape on the lathe and drilled using the original ring as a template. After sanding it with 800 grit wet-or-dry sandpaper the new ring looks very much in place.
Replicating the choke
The receiver’s design incorporates an iron cored choke to keep RF out of the headphones. High frequency current in the plate circuit is bypassedto ground by the variable mica phone condenser mentioned above. Re-creating this choke was the most time consuming job of the entire restoration.
First it was necessary to construct the laminated steel core. This necessitated locating a small quantity of silicon steel sheet, and while buying a ton of this material is easy but expensive, getting a couple of square feet is almost impossible. Fortunately the owner of a custom transformer shop was willing to send some cutoffs and after making a steel stamping die we had the core ready for its winding.
The winding is lattice wound. The wire guide moves side to side across the coil surface as the coil is wound and since the coil is self-supporting the whole thing is very finicky. It was necessary to build a coil winding machine, based on Dave Gingery’s design. 4 or 5 coils were almost completed before an acceptable one was produced.
Building a condenser
The 0.017uF mica grounding condenser was missing so a new one would need to be constructed. Searching eBay surfaced a Ukraine-based seller who had boxes of the right thickness mica from the Soviet period. 30 alternating copper sheets and mica dielectric produced the required value. It would have been nearly impossible to assemble the layers without some sort of fixture, so we built one out of aluminum.
Panel, Hardware and Wiring
Holes that had been drilled in the panel, to install a headphone jack, retrofit an incorrect tube socket and other mysterious purposes had to be filled. Epoxy resin tinted to match the panel color as best as possible. Vacuum degassing the epoxy helps to ensure that air bubbles won’t be a problem. After the epoxy had thoroughly cured, the patches were carefully filed and sanded flush with the panel surface.
Most of the panel hardware was heavily oxidized, scratched and otherwise damaged. As a restoration, this radio deserved a front panel that looks like the day it was built. All the hardware was thoroughly cleaned and he old plating stripped. Any damage to the brass parts was repaired and the parts were polished and replated. Nickel plating isn’t difficult and there are many references on the WWW.
Since this SE-1420 was missing all of the original bus bar wiring we needed to recreate the wiring and deduce the routing based on photos from an unmodified set and from the original schematic. The wiring was mocked-up using inexpensive 14 gauge house wiring. Once all of the wires were identified and modeled the mock-ups were used as patterns for 12 gauge tinned bus bar wire. The insulating tubing was made from loomed cotton cord with the internal strings removed. The loom was stained and coated with multiple coats of orange shellac in order to simulate the original “spaghetti”.
