Hallicrafters S-72 Battery Box Construction

In a previous blog on radioboatanchor.com, I restored a Hallicrafters S-72 portable receiver. Here is a before and after picture of my S-72.

Click on the button below to visit my blog about rebuilding my Hallicrafters S-72.

Hallicrafters S-72 Portable Receiver Rebuild

What made the S-72 "portable", besides the carry handle, is that it could be operated under battery power. I am not talking about a 9 Volt or two AA batteries, this radio had a compliment of eight miniature tubes which required a 90Volt "B" battery for the tube's anodes and a 7.5Volt "A" battery for their filaments. A battery pack was designed to combine the A and B batteries into one convenient package. In the picture below, the empty area behind the chassis is where the battery pack was installed, a black cloth strap held it in place. 

Below is a list of compatible battery packs for the S-72, straight out of the manual. Your local hardware or DIY store will not have them as these battery packs have not been produced since the 1960s! 

Below is a picture of the completed S-72 battery box, which will replace the original battery pack. I will be wiring 9 Volt batteries in series to simulate the 90 Volt DC "B" battery. In addition, I will be wiring several D batteries in series to simulate the 7.5 Volt DC "A" battery. 

Battery Box Contruction

I started out with this 15 Inch mini wood crate I purchased from a local crafts store.

Mini wooden crates on Amazon!

I partially disassembled the mini wood crate. With some carefully placed cuts I was able to reduce the crate's overall dimensions to 2.75 in x 11in x 5.25in. This was approximately the size of the original S-72 battery pack. I used finishing nails and wood glue to hold the new structure together.  

Time for a quick check to ensure the new battery box fits into the S-72. Looks like it is a perfect fit!

I was able to reuse a scrap wood piece I cut off the original mini wood crate to fabricate a hinged top piece. This hinge piece is required to ensure the structural integrity of the battery box as it is being strapped down to hold it in place inside the cabinet. It is hinged so that you can easily access the batteries inside. The tiny brass hinges, screws, and hasp were all acquired at a local crafts store.  

The S-72 chassis connected to the original battery pack using a single connection. I found that a standard octal tube socket mates perfectly with the S-72's connection. Here I used two pieces of scrap wood from the crate to fabricate a mount for the octal tube socket. The wood pieces are held in place with finishing nails and wood glue. 

Tube sockets on Amazon!

A leftover hinge and brass screw reinforce the octal tube mount from the inside of the battery box.

I will be connecting ten 9 Volt batteries in series for the 90 Volts DC "B" battery. The batteries will be aligned facing up for easy access to their terminals.

I am using a wooden paint stirrer, cut to the proper length, to hold the 9 Volt batteries in place. At each end, I bent a solder lug in order to form a make-shift right-angle bracket. The right-angle bracket can be adjusted slightly then tightened down to form a tight fit against the 9 Volt battery cases.

We only need five "D" 1.5 Volt batteries wired in series to achieve the required 7.5 Volts for the "A" battery. I have mounted three twin D battery holders, one battery slot will not be used.  I also added a 9 Volt battery holder, circled in red. This will hold an extra 9 Volt battery connected in series to boost the B battery voltage back up to 90 Volts DC as the 9 Volt batteries start to drain.

The nail-gun countersunk the finishing nails I used to assemble the battery box. I used wood putty to conceal their holes. 

I used an orbital sander with fine (150 grit) sandpaper to sand the wood putty flush with the wood and to smooth out the rough cut wood used in the original mini wood crate.

I also used the wood sander to sand smooth the hinged tops and to remove the labeling on the wooden paint stirrer used to hold the 9 Volts batteries in place.

Here is a picture of the battery box parts after sanding.

I then treated all wooden battery box parts to two coats of water based polyurethane, followed by light sanding and then a final coat.  

As mentioned, the 1.5 Volt D cells must be connected in series in order to achieve the required 7.5 Volts DC for the "A" or filament power supply. I wire the D cell battery holders in series except for the one slot, circled in the picture below, that will not be used.

The eleven 9 Volt battery clips are connected in series. While ten fully charged 9 Volt batteries in series will achieve 90 Volts DC, I included an extra battery clip to introduce another battery in the circuit to boost the voltage back to around 90 Volts as the 9 Volts battery's voltage drops as they discharge. This eleventh battery clip is jumpered when fresh batteries are used.     

I used a gold paint pen to label each 9 Volt battery clip.

I attached rubber feet to the side of the wooden paint stirrer that comes in contact with the 9 Volt batteries. 

This will prevent the batteries from shifting sideways in their mount.

Time to wire the octal tube socket I am using as a female connector. I plugged the male connector from the S-72 chassis into the tube socket in order to determine the proper connections for 90 Volts DC, 7.5 Volts DC and Ground. For reference, here is how the the male connector from the S-72 chassis is wired. 

Time to mount the octal tube socket used as a connector to the battery box.

Here is a picture of the battery box with 9 Volt and 1.5 Volt D cells installed. Eleven 9 Volt batteries are installed as they are older batteries and their Voltage is dropping under load. Five D cells are installed with one battery slot left open.

Batteries on Amazon!

Time to test before installing the battery box into the S-72. I first test for the "B" battery voltage, which should be around 90 Volts DC.

Then I test for the "A" battery voltage, which should be around 7.5 Volts DC.

Time to do some initial testing, I already had my S-72 chassis out of the case for cleaning. This made it easy to connect the chassis to the battery box. After a brief warmup, the S-72 sprung to life and I could immediately hear a local sports talk station in the standard AM broadcast band. 

Note, the plug on the line cord must be plugged into the chassis, in order to run on battery power. ​See area circled in the picture below.

After the S-72 chassis was once again safely installed in the cabinet, it was time to install the battery box. Once the battery box is in place, you secure it with the black cloth strap built into the cabinet. You must also connect the chassis to the battery box. The connection point is circled in the picture below:  

​A final test is to see if the S-72 cabinet side door closes properly with the custom battery box installed. Looks like it is a perfect fit as the side door closed and latched properly.

​Finally, here is a picture of the finished product!

Here is a video of my battery powered S-72 in action!