Below are before and after pictures of the chassis of my Hallicrafters S-72 Portable Receiver. I think you'll agree that my rebuild was a complete success!
About the Hallicrafters S-72
The S-72 is a Superheterodyne radio receiver built by Hallicrafters and sold from 1949 to 1953. This model covered the standard AM broadcast band. In addition, it covered three shortwave bands with continuous coverage from 540Khz all the way up to 30Mhz.
An internal loop antenna was used for standard AM broadcast band reception while a telescopic whip antenna is used for shortwave reception.
This radio employs eight miniature tubes and one Selium rectifier.
Here is the miniature tube compliment:
1T4, 1U4, 1R5, 1U4, 1U4, 1U5, 1U5, 3V4
The S-72 has an RF Amplifier, Mixer, Two IF (Intermediate Frequency) Amplifiers, Detector, and Audio Output stages. These stages are common in most AM radio receivers. In addition, it has a BFO (Beat Frequency Oscillator) for CW (Continuous Wave) or Code reception. Band change is accomplished by a four position selector switch that connects in different LC resonant tank circuits in the RF Stage.
Controls of the S-72 are fairly common, it has a Tuning, Band Spread, Band Selector, and On-Off Volume controls. The Voice-Code controls wether the BFO is on for Code reception and the frequency of the oscillator. There is also a "phones" jack for listening in noisy areas.
Powered could be derived from a dry cell battery pack. You have to insert the line cord plug into a receptacle located on the chassis to run on battery power.
The S-72 could also be powered from 105 to 125 Volts AC or DC line power as apparently direct current was still used in some areas.
The S-72 electronics were housed in a portable luggage style cabinet with a carry handle on top. I am not sure if "portable" is the proper term given that it weighs a whopping 16 pounds without the battery pack!
Access to the controls and dial is achieved by opening of the top cover. Access to the battery, line cord, and electronics is done through a side cover.
The S-72 retailed for $109.95 the first year of sale (1949).
The inside of my S-72 was in pretty good shape, although very dusty. All the vacuum tubes were in place and the two air capacitors, one for tuning the other for band spread, looked like they didn't have any bent plates. The empty area in the bottom is where the battery pack is housed. The black strap in cinched down and keeps the battery pack from moving around during transport. A single plug connects the battery pack to the chassis.
As you can see from the picture below, the faceplate, knobs, and whip antenna are missing from my S-72. The chassis was hastily put in place inside of the case and the two nuts that hold it in place were missing.
The external phone jack was just floating freely inside the case. See area circled in the picture below.
The manual, including schematic and alignment instructions, for the S-72 can be obtained from radiomuseum.org at no cost. Click on the button below to go to the web page.
"Old Time Radios! Restoration and Repair" book on Amazon
I consult this book often during radio restoration. I grew up in the transistor and diode era and this book taught me a lot about vacuum tube and selium rectifier technology.
Step 1 Chassis Removal
As mentioned, just two bolts hold the chassis in place. The nuts that secured the chassis to the inside of the cabinet were missing from my S-72.
Here is a picture of the bottom of the chassis. The discrete parts (resistors, capacitors, transformer, coils) are dirty but in fairly good condition. Notice the blackened area in the lower left area of the chassis. It looks like there might have been one or more component failures in this area. I will need to investigate the components carefully in the blackened area before ever applying power.
Step 2 Chassis Cleaning and Lubrication
Contact cleaner was applied to all of the Band selector's contacts. I then moved the selector to all four positions in order to clean the contacts.
I applied Labelle 107 Oil to the shafts of all dial indicator pulleys. I have this oil on hand due to my model train hobby. This oil will will not harm plastic components.
In addition, I applied this oil to the shafts of the Tuning, Band Spread, Band Selector, On-Off Volume controls and Voice-Code controls.
Labelle 107 Oil was also applied to the shafts of the Tuning and Band Spread Variable Capacitors.
Labelle 106 Grease was applied to the bearing located to the front of the Tuning and Band Spread Variable Capacitors.
Labelle lubricants on Amazon
I use Labelle oil and grease for my radio restoration projects. I already have these products around as I am also a model railroad enthusiast. These products will not harm plastic.
The exterior of the chassis was cleaned with paper towels moistened with Windex glass cleaner. I use Q-Tips moistened with Windex to get into hard to reach places.
Step 3 Check Vacuum Tubes
Unlike semiconductor transistors that have a near infinite lifespan, vacuum tubes age and become less efficient. In addition, their filaments burn out rendering them inoperable. It is always a good idea to test all vacuum tubes in a radio you are servicing before doing any more troubleshooting.
In addition, make sure that that the right tube is in the right socket. This will save you troubleshooting time later.
Pictured below is my Eico Model 635 Vacuum Tube Tester testing a tube from the S-72.
Step 4 Electrolytic Capacitor Replacement
As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace 65 year old electrolytic capacitors like the ones in the S-72!
Step 4a - Restoring the Multi-Section Capacitor
Multi-section capacitors, which were aluminum cans containing several discrete capacitors all connected to a common ground, were popular in the 1950s. There were used mostly in the power supply sections of vintage electronic devices. You can purchase replacement multi-section capacitors but they are expensive. I typically rebuild them by replacing their guts with inexpensive discrete capacitors of the same or slightly greater capacitance and working voltage. The multi-section capacitor in the S-72 is circled in the picture.
The multi-section capacitor in the S-72 contains the following discrete capacitors:
Capacitor A 60uF @ 150Volts
Capacitor B 20uF @ 150Volts
Capacitor C 20uF @ 150Volts
Capacitor D 2000uF @ 15Volts
The components connected to it's base terminals, circled below, must be desoldered before removing it from the chassis. I labeled each connection during the desoldering process.
The phenolic base is held in place with two rivets, they must be drilled out. Later, they will be replaced with machine screws, lock washers, and nuts.
Here is a picture of the multi-section capacitor removed from the chassis. The four solder lugs in the center connect to the positive side of the internal electrolytic capacitors. All internal capacitors share a forth common solder lugs at the rim for their negative connection.
I use wire cutters to uncrimped the bottom of the multi-section capacitor. Then I use needle nose pliers to pull the crimped sections away from the base. This allows me to pull the guts of the multi-section capacitor out of the aluminum can.
The anatomy of a multi-section capacitor. The four internal electrolytic capacitors are rolled up into one assembly.
I was able to fit all four replacement electrolytic capacitors inside the aluminum can. A dab of hot glue holds the components in place. I use tiny brass screws I get from a local craft store as solder anchor points.
Pictured below is the refurbished multi-section capacitor installed back into the chassis.
All of the components attached to the multi-section capacitor have been soldered to its bottom terminals.
I use a Dremel with cutoff tool to remove the area from the base of the multi-section capacitor's aluminum can that I bent with needle-nose pliers.
I then use the Dremel with wire brush tool to remove burrs.
I spray the external cardboard wrapper of the multi-section capacitor's aluminum can with some flat black paint to make it look as good as new.
Hot glue holds the aluminum cover in place over the multi-section capacitor's base. You will never know it was refurbished once the aluminum cover is installed!
There is one other 100uF capacitor capacitor, circled below, that also needs to be replaced.
The top electrolytic capacitor in the picture below is the original 100uF 25Volt capacitor, the bottom one is its modern equivalent. Notice how the new modern equivalent is in a much smaller package?
Circled below is the newly installed 100uF 25Volt capacitor.
Step 5 Replacing Paper Capacitors
Paper capacitors, like the ones circled below, become very unreliable with age and should be replaced with one of similar capacitance and equal or above voltage rating. Paper capacitors are not polarized although they may have a black band on one side indicating which lead is connected to the outside foil.
I replace paper capacitors with new Polypropylene type. Circled in the picture below.
I almost forget to replace the paper capacitor that is on the top side of the chassis. This capacitor is part of the antenna circuit.
Here is a pic of the new Polypropylene capacitor soldered in place.
Step 6 Selenium Rectifier Replacement
The S-72 employs a Selenium rectifier, circled below, to convert household AC (Alternating current) current to DC (Direct current) required by the receiver. As Selenium rectifiers age, their forward resistance increases to the point where it causes the power supply voltage to drop. Initial testing while powered by household current showed that the B Voltage, measured at capacitor C38c, was below the specified 90 Volts DC.
Below is a comparison in size of a Selenium rectifier compared to its modern replacement, a 1N4005 Silicon rectifier. The 1N4005 has a forward current rating of up to 1 Amp where the original Selenium rectifier only have a forward current rating up to 150 Milliamps.
When replacing the Selenium rectifier with a modern 1N4005 Silicon rectifier, I had to install a 100 Ohm tapped wire-wound resistor in series with it as a Silicon rectifier is much more efficient. I then adjusted the tap on the wire-wound resistor until I measured the desired "B" Voltage of 90 Volts DC at capacitor terminal C38c.
I was having issues with filament or "A" Voltage being below the specified 7.5 Volts DC. This voltage is measured at capacitor terminal C38a. With the power off, I performed some resistance checks and determined that part of the tapped wire-wound resistor, R29 in the schematic, had increased in resistance and was out of spec. I connected a 75 Ohm 25 Watt wire-wound resistor in parallel to decrease its equivalent resistance. The filament Voltage once again rose to the required 7.5 Volts DC.
Step 7 Marking the Hot side of the AC plug
Depending on how the non-polarized line cord plug of the S-72 is plugged into a power outlet, an high voltage potential can exist on the metal chassis. Below my Multimeter is reading 120 Volts AC between the chassis and ground, in this case ground is the metal enclosure of the household sockets. This poses a potential shock hazard!
Now, if I reverse the line cord plug, I get a very low voltage (less than one Volt) AC between the chassis and ground.
Normally, I would install a polarized plug on the end of the radio's line cord, ensuring that the chassis was at or near ground potential. I can't do that with the S-72 as you have to plug the line cord plug into the slots on the top of the chassis in order to operate on battery power.
Since I could not install a polarized plug, I marked an "H" for Hot on the side of the plug that should go into the smaller slot on the AC socket. If properly wired, the smaller slot on the household AC socket is "Hot" or 120 Volts AC with reference to ground. The larger slot on the household AC socket is "Neutral" and it should be less than 1 Volt AC with reference to ground.
Step 8 Alignment
btuIf is a perfect time to perform the alignment procedure, as the S-72 chassis is already removed from the cabinet. Alignment involves injecting a AM modulated signal, of frequency specified in the manual, then watching for maximum AC Voltage measured at the speaker terminals. This is done while tweaking variable capacitors if the input frequency is at the high end of the band or ferrite slugs in RF coils if on the low side of the band. You must perform this process for each position on the Band switch. Page 6 of the S-72 manual explains the process. There is button called "Hallicrafters S-72 Manual" in the beginning of my blog that will take you to a web page where you can obtain the manual.
Step 9 Cabinet Cleaning and Restoration
The S-72's cabinet is covered in a leatherette material with brass accents. I sprayed the exterior with Windex then wiped it down with a paper towel. I repeated this process, discarding the paper towel and getting a new one, with each cleaning cycle. The process was repeated until the paper towel showed very little evidence of dirt. Below is a picture of my paper towel on my first cleaning pass.
Meguiar's Motorcycle All Metal Polish was used to remove years of oxidation from S-72's brass plated feet.
Meguiar's Motorcycle All Metal Polish was also used to shine up the brass plated air vents in the side access door and the pull handle.
Meguiar's Chrome Polish on Amazon!
Meguiar's Metal Polish works great on the brass and chrome fixtures on vintage radios.
The air vents and pull handle on the side access door cleaned up pretty good. Unfortunately, this was not the case for the brass plated hinges, they were badly rusted with most of the brass plating gone.
I decided it was best to paint the hinges on the side access door. First, I applied masking tape around the hinges to ensure that I would not mar the leatherette exterior. I then used my Dremel with wire brush attachment to remove as much rust and corrosion as possible in order to prep for paint.
I wiped down the hinges with Prep-All, a degreaser and de-waxer used for automotive paint prep. I then masked off the side access door, so that only the hinges were exposed. The hinges were treated with a coat of brass colored Acrylic Enamel spray paint.
The hinges turned out really great! The brass colored Acrylic Enamel spray paint adhered really well to the old hinges.
I also wire brushed the heads of the screws that attach the side access door to the cabinet, degreased with Prep-All, then treated them to a coat of brass colored Acrylic Enamel spray paint.
The leatherette material that covers the cabinet was coming off in places. I used Elmer's School Glue to adhere it back to the cabinet.
I cleaned the inside of the cabinet with a lemon-oil based furniture polish.
I sprayed then wiped down the outside of the cabinet with an "Armor All" like protectant that I purchased from a local auto parts store.
The lid was also sprayed and wiped down with an "Armor All" like protectant.
Step 10 Front Panel Replacement
The front panel was missing from my S-72, I believe a previous owner was using this as a "parts" radio.
Luckily, I have a father-in-law with a CNC router setup in his garage. He was more than happy to fabricate a new front panel for me. First, I needed to plot out the dimensions for the panel to give him. I put my drafting skills to work.
Next, I created a paper template from my dimensions to double-check for fit.
I then scanned to PDF my front panel drawing with dimensions then e-mailed it to my father-in-law who lives on the California. A few weeks later I received the finish faceplate in the mail. It was beautiful! My father-in-law machined it out of a composite material that consist of a layer of plastic sandwiched between to sheets of thin aluminum. I checked the new faceplate for fit, by temporarily installing the chassis into the cabinet and installing the knobs.
The new front panel fit perfectly. I decided it was a good idea to install the dial lens and to glue the new speaker grill cloth in place before installing the front panel.
The front panel is going to be glued to the top of the S-72 cabinet. I am going to use large fender washers secured with screws and bolts to hold the face plate in position while the glue dries. I put electrical tape on the side of the fender washer that is to comes in contact with the front panel so that is does not mar the surface.
I put a bead of E6000 adhesive on the back of the front panel.
Once the front panel was aligned properly on the cabinet, I used the fender washers with nuts and bolts to secure it in place while the adhesive dries. I used the phone jack to secure the upper right-hand corner.
Here's how the front panel looks once the fender washers, nuts, and bolts have been removed. Notice that the holes in the front panel are slightly out of alignment with cabinet holes. The knobs will conceal this alignment problem.
E6000 Adhesive on Amazon
This is my main go-to adhesive for use around the shop. This adhesive is strong, flexible, dries clear, and works great for bonding wood, metal, fiberglass, and ceramics.
Step 11 Assembly
The original antenna mounting bracket was missing from my S-72. I fabricated a new one from an aluminum heat sink I had in my junk drawer. This heat sink already had a solder lug attached to it which made it easy work to solder it to the external antenna connection on the chassis.
In order to make handling the chassis easier, I removed the phone jack. Now, I had to connect it back in before the chassis was inserted into the cabinet.
The chassis slips in through the side panel and is held in place by two bolts built into the inside of the cabinet. I had to supply my own lock washers and nuts as the originals were missing.
I found a suitable replacement telescopic whip antenna on Amazon. My fabricated antenna bracket is held on the side of the cabinet with tiny brass screws I purchased from a local crafts store.
When mounting the knobs, I place a slim piece of cardboard underneath them while tightening the set screw so that each knob is a uniform distance from the front panel.
Time to attach the side panel. It is held in place with four screws. These screws are the ones I painted earlier in my blog.
With the side panel in place you must solder the two connections between the chassis and the built-in loop antenna (circled below).
I built a battery box that contains 9 Volt batteries connected in series to supply the required 90 Volts DC and 1.5 Volt D Cell batteries connected in series to provide the required 7.5 Volts DC. A tube socket is mounted to the side of the battery box. The battery connection from the chassis plugs into the tube socket. I have a separate blog that covers the battery box construction.
You must plug the line cord into the chassis in order to operate from battery power.
Hallicrafters provided these handy leather straps in order to tame the line cord.
Here is a picture of my S-71 with custom battery box installed.
A final test is to see if the side door closes properly with the custom battery box installed. Looks like it is a perfect fit as the side door closed and latched propertly.
Finally, here is a picture of the finished product!
Video of my newly rebuilt Hallicrafters S-72 in action!
Who Writes This Blog?
John is an IT professional from Cleveland, OH who enjoys amateur radio, ham radio, metal detecting,
Copyright © 2017
Radio Boat Anchor
This page and all the pages on Radio Boat Anchor generate income based on an affiliate relationship with our partners including Zazzle, Amazon, and Google. Prices listed are subject to change without notice.