This is the first Three Dialer I got. It has a huge teac cabinet, 72 cm x 23 cm x 24 cm, the front panel is made from ebonit and is black and shiny. Being build in 1924 (or really in 1925, the patent tag is indicating April 14th 1925 as the last date), it is a Neutrodyne receiver for 6 tubes. Looking on the schematic I was surprised by the lot of voltages it needs:

A first look and measurements have shown  the 2nd interstage transformer was gone, broken on the primary winding. Gerd of Reinhoefer Electronic agreed to rebuild it. Gerd was working a lot on this issue and wrote a documentation in German about it:

Restoring the Interstage Transformer

 

Finally the new wounded transformer (the left one) is installed and working.  The interstage transformers are coupling the detector stage to the 1st A.F. stage and the 1st A.F. stage to the 2nd A.F. stage. The receiver has no output transformer so it needs a high impedance speaker or horn (or headphone).

Some spare parts can also be found at  Play things of past . Gary is specialized on pre-WWII radios and has a huge collection of radio parts.

Beside this interstage transformer issue the NR-7 was almost ready to run. The receiver is build to work for the eternity, no comparison e.g. to an Atwater Kent #20. In 1924 it costs $ 110 without tubes, speaker, antenna and batteries.

To supply the receiver with the needed voltages I've build a stand-alone power supply in a mahogany chassis. You can find it on my Antique Radio page. 

This NR-7 unit has the series number 043B. Surprise: all caps in the unit are close to their nominal value, just one of the 1 µF bypass caps is now in the range of 0.7 µF. The one and only resistor in the unit was about 3.3 MOhm, should have 2 MOhm, so I replaced it.

The teac cabinet was sanded and got a new painting. Also I got a 8 wire cable from Antique Electronic Supply to build a new battery cable for the receiver. If you have also to provide a new battery cable for an American receiver, I'll give you the right color code:

 

A+	red
A-	black
B+ (maximum)	blue
B+ (intermediate)	white
B-	yellow
C+	brown
C- (maximum)	green
C- (intermediate)	orange

This works not for every receiver, but it is a good guideline. For the NR-7 C+ and A- is the same connection, so I used the black wire for it, and A+ and B- are also one connection, which is now red. I used yellow for the low C+, and the brown wire was obsolete.

With a good antenna ground connection the receiver can get in the dark Radio Stations from the UK and France. Reception is concentrated at about 650 MHz, when the variable air capacitors are turned fully out. With the 71A in the power stage NR-7 plays really loud, even on a full floating speaker.

 

The connections for the battery cable under the "mainboard"	The 1µF capacitors, originally "Made in Germany"

 

The "mainboard":  right the detector tube C-301A, in the mid the 1st A.F. stage RX-201A, in front the 2 sockets of the 2nd A.F. stage.  The receiver can use 2 * 201A/301A tubes in parallel for the 2nd A.F. or 1 * 12A or 1 * 71A. In my setup I'm using 1 * 71A (left) und leave the 2nd socket empty. Right in front are the binding posts for the speaker.

The R.F. stage: 2 * CX-301A.  In front of the tube sockets are the neutralisation capacitors: small metall tubes soldered over an interrupted peace of wire. In the upper right corner are the binding posts for the antenna: ground, short antenna, and long antenna decoupled by a small capacitor.