The YAESU Model FT-720R, another great find for my father-in-law, or "The old man from California" as he likes to refer to himself. In 1981 they wanted over $400 for this unit and my shrewd  father-in-law was able to gain possession of this jewel of Ham Radio history for a mere $30 at a swap meet.

This Model FT-720R was recently shipped back East to me so that I could bring it back to original spec. My father-in-law enjoys the "hunt" but cannot keep all of his treasures due to limited space.

The FT-720R Series is a compact VHF/UHF mobile, microprocessor controlled, transceiver that offered modular flexibility. Starting with the FT-720R Control Head, you could either connect a modular FT-720RU 10Watt RF Deck for 70cm (440.00-449.975 Mhz) operation or the FT-720RVH RF Deck for 25Watt operation on the 2M (144.00-147.99 Mhz)  band.

But wait, there's more, with the addition of the S-72 Switch Box, you could tether both the FT-720RVH and FT-720RU RF Decks to the FT-720R Control Head for dual band 2M/70cm operation!  Dual-band operation is taken for granted these days but was pretty revolutionary back in the 1980s. 

I possess a YAESU Model FT-720R with the attached FT-720RU RF Deck, which means it is setup for 70cm operation.

The FT-720R series is built like a tank and is very sturdy with an all metal case for the Control Head, Switch Box, and RF Deck modules. The metal knobs and buttons give you confidence that this YAESU was built to last!

The Control Head sports a LED (Light Emitting Diode) readout but many Ham Radio enthusiasts complain that the vintage LEDs are not visible in bright daylight, which could be a problem with in vehicle operation. 

I can't wait to dive into this project. It will start with fashioning an adapter cable allowing me to connect one of my other YAESU microphones to the Control Head. Followed by a "re-cap", which is the replacement of all of the Electrolytic Capacitors, as they have a finite life span.

My wife got me the ELENCO Model M-2666K Multimeter Kit for Christmas as she knows how I love to toil in my basement workshop, restoring vintage electronic equipment and building new.

The Model M-2666K Multimeter Kit I am building is a full function Digital multimeter that includes such advanced functions as a diode/transistor checker and the ability to test capacitance up to 200uF! ELENCO is able to provide these advanced functions in this easy to build kit by embedding the processor to the main printed circuit board and assembling the function switch for you.

ELENCO offers both Digital and Analog Multimeter kits. In addition, they offer DC Power Supply kits, Radio kits, and Electronic Educational kits including their Snap-Circuit kits for young electronic enthusiasts.
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The multimeter kit also comes with a detailed assembly manual, shown below, which includes circuit theory, testing, and troubleshooting sections. The manual rivals what you would expect from a vintage "Heathkit" manual! ​​ I consider Heathkit manuals the "Gold Standard" of assembly instructions.

In addition, the manual comes with a section on how to properly solder components to a printed circuit board and identify electronic components, including determining the values of resistors and capacitors. 

The Model M-2666K Multimeter Kit manual can be used in a classroom environment, it includes a resistor reading quiz and identifies approval points so that the instructor can inspect work done before the student continues on the project. This is to insure that students adhere to the instructions so that there multimeter project is a success! 

Certain components, like the microprocessor, (round black circle to the left) and the selector switch come pre-assembled on the printed circuit board. ELENCO is able to provide advanced functions in this easy to build kit by embedding the processor to the main printed circuit board and assembling the function switch for you. Make no mistake, there is still plenty of soldering and assembly work to be done in this kit!

ELENCO separates parts into bags with section letters. The manual identifies which bag of parts is required for each section of the book. This allows the assembler to quickly get to the parts he/she needs and reduces the risk of losing other parts needed further in the assembly process.

Components, such as resistors and diodes, are attached to a cardboard identification strip. This ensures that the right component is picked and soldered to the printed circuit board.

You assemble the multimeter kit in sections, at the end of each section you test a certain functionality of the meter. The first test is to see if you get the LCD display to work. When testing, you use the front sticker, with backing still attached, to identify the multimeter's settings.  

Soldering is fairly easy even for the novice, although, I wasn't too happy with the solder included with the kit and switched to a Kester Rosin Core 60/40 Solder after soldering the first couple components.  I didn't like how the solder included in the kit flowed. It could be that the solder included with the kit was lead free to meet some sort of mandated requirement.

Most sections require a bit of testing and calibration at the end. You do need a solid electronics foundation to perform testing and calibration as the manual does not go deeply into these topics.

Calibration is typically achieved by comparing measurements with a "known" good reference. For DC Volts calibration, I am had both the ELENCO kit and my Radio Shack multimeter read the same 9 Volt battery. I adjust a potentiometer on the printed circuit board until the ELENCO multimeter reads the same as the reference Radio Shack multimeter.

Calibration of the low Amp (under 200ma) setting is achieved by connected the ELENCO multimeter in series with a "known" good multimeter, a battery, and a load which in this case a 47K 1/4 Watt resistor. The ELENCO multimeter should read the same as the reference multimeter when properly calibrated.

Calibration of the high Amp (20Amp) is a little more difficult and requires the adjustment of the distance of the shunt, circled in red, from the printer circuit board. It took me several try of desoldering/soldering the shunt to get the distance right and the Amp reading accurate.

I relied on my old Heathkit HP-23 Power Supply to provide a 6 Volts AC Reference Voltage, to be used for the AC Voltage and AC Current calibration steps.

This ELENCO multimeter employs a Piezoelectric speaker to indicate continuity. I used the alligator clip from a test lead to hold it in place while soldering it to the printed circuit board. In addition, you have to solder a lead from the center of the Piezoelectric speaker disk to the printed board. The connection on the speaker should be made as quickly as possible so as not to damage it. 

Calibration of the Capacitance meter built into the ELENCO multimeter is also done by adjusting the reading using a potentiometer to be the same as a reference meter. Make sure you use the SAME capacitor on both the ELENCO and reference multimeter during calibration.  The picture below shows two different capacitors used, which is a mistake.

The soldering process leaves a lot of brown flaky flux residue on the printed circuit board. While it probably will not affect the operation of the ELENCO multimeter, I think it looks unsightly. 

This is not a step in the assembly manual, I use a flux remover spray to eradicate all of the unsightly flux residue from the printed circuit board. I spray it on the board then work it into the areas where there is a lot of flux residue with a toothbrush then dry the printed circuit board with compressed air. Make sure you wear gloves and work in the well ventilated area as most flux remover can irritate skin and the fumes are noxious!

The ELENCO multimeter printer circuit board, devoid of flux residue!

The ELENCO multimeter comes with an easy to assemble case. It is held in place with just two screws on the back of the case.

Here is the final product, the completed ELENCO Model M-2666K Digital Multimeter. It is a pretty impressive for being kit-built!

This YouTube Video shows some basic DC Voltage, DC Current, Resistance, Continuity, Capacitance,  and Diode tests using my newly constructed ELENCO Model M-2666K Multimeter.

I sing the following praises of the ELENCO Model M-2666K Digital Multimeter kit:

• The M-2666K Digital Multimeter kit is a great value for the money. For roughly 35$, you get a high end multimeter you build yourself with advanced functions like Capacitance and a built-in Transistor checker. 
• Detailed "Heathkit" like assembly instructions.
• The kit also excels in parts identification and placement on the printed circuit board.
• Would make a great kit for an instructor led electronics class.
• Assembly time, about 10 hours in my case, was long enough to keep me engaged but not too long as to get bored with the project.

There are some negative, perhaps nit-picky, aspects of the M-2666K Digital Multimeter kit:

• I had issues getting the solder that came with the kit to flow correctly around the solder joints. This was remedied by using a Kester 60/40 Rosin Solder which provided shiny clean connections.
• It was tricky calibrating the high Amperage 20Amp setting, I spent about an hour desoldering/soldering the shunt components at different distances from the printed circuit board to get a correct reading.
• The Piezoelectric speaker used as the continuity buzzer is not very loud, probably because it cannot oscillate freely with the base being solder directly to the printed circuit board. I believe it would be louder if affixed to the printed circuit board using a foam "donut" mount. 
• The rubber protective holster feels "cheap" and does not grip the case of the multimeter tightly. 

Overall, the positive aspects of the ELENCO Model M-2666K Digital Multimeter kit far outweigh the negatives and the kit building process was a positive one.

This multimeter will grace my workbench for many years to come!

I picked up a new Trav-Ler style radio at auction. I say "Trav-Ler style" because is it setup similar to a 1947 Trav-Ler Model 5020 I recently restored.  Trav-Ler radios were unique at the time as they were setup to work equally well on 120 Volts AC (Alternating) Household Line Current or DC Battery Power from A and B batteries. Their transformer-less design also allowed them to be powered from 120 Volts DC or Direct Line Current that was still available to households in areas were electric trolleys were still in use.  

Typically in this style radio, the left knob is volume, the right knob is tuning while the center knob allowed you to choose between Line Power-Off-Battery.

This radio could be a newer model of a portable Trav-Ler radio or another manufacturer's facsimile of it trying to cash in on the business. I won't know until I open it up. Hopefully the chassis will have tell-tale make and model information. Anyway, I can't wait to open this little beauty up and see what mysteries it conceals!

Back in the late 1980's, I was in a vocational electronics program at my high school and I was also an avid electronics hobbyist. One of the first projects I built was an audio frequency generator from plans published in the now defunct Radio-Electronics magazine.

The audio frequency generator was a simple circuit that consisted of a single LM386 integrated circuit. It's output was a triangle wave from 0 to 25Khz. A three position switch was used to put the circuit in standby (no output), low, or high frequency output. It also had a volume control to control the amplitude. The power switch was integrated into the volume control. This circuit used a small incandescent light bulb pointed at a cadmium sulfide light sensor as feedback for frequency stabilization. As you can see, my craftsmanship on this first project leaves much to be desired. I didn't even bother to install a knob for the variable frequency control!

I made the printed circuit board myself using a printed circuit board making kit I purchased from Olson's electronics. We had a local Radio Shack but I found that our local Olson's store was a little cheaper to purchase discrete parts and the printed circuit board making kit. As you can see, I had to modify the printed circuit board dimensions to fit in the project case. The project case was recycled from a high voltage tester I built in vocation electronics class. I fried the components of the high voltage tester by setting the front switch to the wrong voltage range. Most of the components (resistors, capacitors, potentiometers) were recycled from other electronic projects or salvaged televisions and radios I trash picked. Even the 9 Volt battery clip was recycled. I tore apart a dead 9 Volt battery and salvaged the terminals. Looks like something from MacGyver would build! Time for a Ty Pennington style "Extreme Makeover" of my Audio Frequency Generator project!

Remove audio output connection points from the front panel. See areas circled in red.

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Here is the back side of the printed circuit board I made using the Olsen's Printed Circuit Board kit.

​All of the electronics components removed from the old project case.

Here is a picture of the "sad" old project case.

Picture of my home made 9 Volt battery clip.

Forrest M. Mims is a great author of books for the electronic hobbyist. I highly recommend the books below.

The original resistors were salvaged from an old AM radio. They took a beating during the "recycle" process so I decided to replace them.

Picture of the old removed resistors.

Picture of the printed circuit board with brand new resistors installed.

​Many of the original capacitors were salvaged from a TV and had high voltage ratings which is why they are physically large.

 I replaced the capacitors with low voltage ones that have a working voltage or only 15 Volts. This is why they are much smaller.

To aid in troubleshooting and replacement, I unsoldered the LM386 Integrated Circuit and installed a DIP (Dual Inline Pin) socket in its place (see area circled in red). I then re-installed the LM386 Integrated Circuit in the DIP socket.

​I used a wooden craft box purchased from Amazon as a project case. I simply removed all hinges, clasps, and other hardware.

​I turned the craft box base upside down to use as a chassis. I use the top or the craft box as the front panel. I did a "dry" of of all parts then I treated the craft box to two coats of water-based Polyurethane lightly sanding between coats. 

I used a Darice wooden craft box for my Audio Frequency Generator Makeover project.

​A much neater wiring of the printed circuit board. Notice I installed a "real" 9 Volt battery clip!

Back view of my updated Audio Generator Project.

Front view of my updated Audio Generator Project.

Picture of all of the parts I replaced in the Audio Generator Project.

With tips from this blog, you too can breath new life in an old project and give it a Ty Pennington style "Extreme Makeover".

A Crystal Radio is a form of AM (Amplitude Modulation) receiver, invented before the dawn of vacuum tubes or semiconductor transistors. Actually, a Crystal Radio employs a primitive semiconductor diode called a "Detector" formed at the junction of where a brass wire, called a "Cats Whisker", comes on contact with the surface of a crystalline mineral called Galena. The most basic crystal radio consists of following components;

• Antenna - converts electromagnetic waves into alternating RF (Radio Frequency) current.
• Coil - Tuned circuit consisting of a coil and capacitance of the antenna to select a specific Radio Frequency to listen on.
• Detector - A device that only allows current to flow in one director, used to strip the RF envelope from the embedded audio signal. 
• Reproducer - A device that will turn alternating AF (Audio Frequency) current into sound. I use a high impedance piezoelectric crystal earphone or audio amplifier for this purpose.
• Ground - Offers a low resistance return path for the RF current in the turned circuit. In my case it is a connection to cold water pipe that is buried in the earth.

Below is a picture of my Crystal Radio made from modern components. I used the instructions from a Poof Slinky crystal radio I purchased to determine the number of windings and diameter of the coil. 

Circled below is my Detector that consists of a brass wire, call a "Whisker", touching a piece of Galena crystal. I use alligator clips to hold the assembly together as you have to be able to adjust where the brass wire contacts the Galena for optimal audio volume.
I obtained the Galena from our local Natural History museum gift shop for 25 cents! Their  gift shop contained all sorts of interesting mineral samples including Galena.

In the early days amateur radio enthusiasts used to key or switch on their own "Spark Gap" transmitter and use that signal to adjust the Galena detector for maximum audio volume.

I use a simple audio amplifier, based on the LM386 audio amplifier integrated circuit, to listen to the output of my Crystal Radio. I could have just as easily used a piezoelectric crystal earphone.

I see a lot of negative reviews of Crystal Radio kits on Amazon. I believe most Crystal Radio kit builds are epic failures in picking up radio stations is that builders don't appreciate the importance of a good antenna and ground. I get satisfactory  reception from my Crystal Radio because I connect it to a 102 Ft long G5RV dipole Amateur Radio antenna that I have hanging outside. In addition, the Crystal Radio is grounded to the main water line in my house, which is conveniently located in my Radio Shack. Unlike modern radios, no amplification is employed in a Crystal Radio and all power is derived from the incoming electromagnetic waves! While my antenna and ground setup may be overkill for a Crystal Radio, you still need a good antenna and ground to be able to hear radio stations.

The YouTube video below is my Crystal Radio in action. You can hear a gospel choir from a religious radio station singing, if you listen closely. 

Here is an example of a crystal radio I built, originally the parts were from a Proof Slinky Crystal Radio kit I purchased from Amazon. I made it more nostalgic by adding a wooden base and Fastenal clips for connections. In addition I replaced the 1N914 Silicon diode that came with the kit for a more sensitive 1N35 Germanium diode. A crystal radio uses the RF (Radio Frequency) energy rectified by the diode detector to drive a high impedance ear phone.

You need the proper A and B voltages before you can power a vacuum tube radio. The A battery is used to power the filament or heater in the tube. It is usually low voltage but requires a pretty hefty amount of current to keep the filament glowing. The B battery is used for the plate voltage, is it usually high voltage but the current draw is very low. For my projects I decided that I would need between 1.5 and 3 volts for the A voltages and 22.5 or 45 volts for my B voltages. Now, you could build an expensive AC power supply to provide these voltages but I thought it would be simpler and safer to run the tube radio off of batteries. Also, I decided to make the battery power supply modular so that I can use it with different vacuum tube projects I constructed so I created "The Battery Box". The wood box I used was an unfinished jewelry box available at a local craft store. I us two D cell batteries with a tap between them to provide the 1.5 and 3 volt A voltages. But how do I get the B voltages of 22.5 and 45 volts from a normal battery? Simple, I take five 9 volt batteries and wire them in series with a tap after the third 9 volt battery for 22.5 volts. I also include an SPST switch in line with the negative connection for both A and B connections so as be able to cut power when needed.

The crystal and tube regenerative radios can only drive a high impedance ear phone. In order to better hear I created an audio amplifier using a 386 audio amplifier integrated circuit.

 chose to build a regenerative receiver as my first vacuum tube project. A regenerative receiver or "autodyne" was invented in 1914 by Edwin Armstrong. It consists of an amplifying vacuum tube with its output connected back to its input through a positive feedback loop thereby amplifying the incoming radio signal by a large factor. The high gain factor also increases the radio's selectivity or the ability to reject interfering signals. This type of receiver made a great first project as it requires very few components. The Regeneration knob controls the amount of positive feedback fed back into the amplifier and needs to be adjusted after you tune in a new broadcast. The receiver employs a common 1T4 Pentode tube that is still cheap and easily obtainable.

Here is a picture of the “guts” of my first regenerative receiver. As mentioned the radio consists of very few parts.

Bipolar transistors work equally well in a regenerative receiver. Because the regenerative circuit is driving a 386 integrated circuit audio amplifier I was able to connect this circuit to a low impedance speaker.

As you can see from the backside of my first transistor regenerative receiver, only a 9v battery is required to power this circuit. I used a printed circuit board purchased from Radio Shack to mount all of the components.

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The only external connections to the transistor regenerative receiver are ground and antenna.

Here is a picture of the front of my second tube regenerative receiver. This one has the ability to receive both standard AM broadcast band and a part of the shortwave spectrum depending on the position of the band select switch. The receiver employs a common 1T4 Pentode tube that is still cheap and easily obtainable.

From the back side you will see some differences from my first tube regenerative receiver, instead of a variable resistor to control regeneration I employ a vario coupler. The outside coil is part of the tuned LC circuit used to receive a signal at a specific frequency. The internal "tickler coil" is movable and connected to a dowel rod attached to a knob at the front of the radio. It generates the positive feedback needed to increase the gain of the tube circuit. By changing the ticker coil's orientation relative to the coil that is part of the LC circuit, the amount of positive feedback is changed.

Here is a back view of my Tube Regenerative Receiver With Vario Coupler.

The third regenerative receiver I built uses a vintage Type 30 Triode tube and a vario couple to provide positive to increase amplification. I don't know when the triode tube I used was made, but I understand from online research that tubes with two digit designation extended back to the 1920s.

From the back you can clearly see the Type 30 tube in the porcelain base. This regenerative radio has the fewest discrete components of any regenerative radio I built.

Here is a top view of the vario couple I built for positive feedback to increase signal amplification.

As you can see my work is never done. Here is a picture of a new regenerative tube radio I am building that employs one 1T4 Pentode tube for RF application and another 1T4 Pentode tube for AF application.

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By far, Dave's Homemade Radios site has the most extensive collection of Crystal and Regenerative radio schematics.

The Xtal Set Society, in my opinion, is the leading authority on the art of building crystal radios.

E-bay is a good source to find vintage vacuum tubes. Most sellers offer “buy it now” options if you don’t want to wait for the bidding war. There are many companies on the web that sell vacuum tubes as well. Remember, musicians and audiophiles still cherish the mellow sound given off from an amplifying vacuum tube so there are still plenty of sources for said device. In addition I have had good luck with the vendors below:

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Building Crystal and Regenerative radios has really been a satisfying hobby for me. If you have an interest and have basic electronic, soldering, and construction skills I recommend trying it yourself.

In the early days of electronics, vacuum tube devices were powered by batteries as many homes had not been wired for electricity. The batteries used for powering these early electronic vacuum tube devices were classified as "A", "B", and "C" batteries.

The "A" battery was used to provide power to the filament of a vacuum tubes in the device. Typically this battery was low voltage (usually between 2 to 7.5 volts) but required a high amperage to power the vacuum tube filaments. Lead-acid types, like classic car batteries, were used initially until better high amp-hour Zinc-Carbon batteries were developed.

The "B" battery was used to provide the plate voltage of a vacuum tube. These were typically of the Zinc-Carbon variety with many cells wired in series to provide the low amperage and high voltage (usually 22.5, 45 or 90 Volts) required.

The "C" battery was used to provide bias to the control grid of a vacuum tubes. This was typically a low voltage and low amperage (9 Volts  with several 1.5 Volt taps) Zinc-Carbon battery. The C battery was largely done away with as it was soon discovered that the needed voltage/current bias could be derived from the "A" battery using a grid leak resistor or voltage divider biasing.  

Pictured above are examples of an "A" and "B" battery I just pulled out of a 1947 TRAV-LER Model 5020 portable vacuum tube radio that I am in the process of restoring. Two "A" batteries were connected in series to provide the required 9 Volts for the tube filaments. In addition, two "B" batteries are connected in series to provide the required 90 Volt plate voltage. No "C" battery is required as the Model 5020 uses grid leak resistors to provide required voltage.

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.

I wanted to be able to power my vintage radios, such as my Trav-Ler Model 5020, from both the AC line and battery power so as to test all circuitry. In addition, I wanted to also be able to power any radios I might purchase that require battery power only. 

Below is a picture of one of the A Batteries I am going to rebuild. It has a nominal Voltage of 4.5 Volts DC. My Trav-Ler Model 5020 required two of these batteries in series to provide the high Amperage 9 Volts required to power the filaments of the vacuum tubes.

I carefully used an X-Acto knife to open the bottom of the cardboard box that houses the battery cells so as not to rip or tear it. I want to install the replacement batteries in the cardboard shell so as to give it a "vintage" look. As you can see below three 1.5 Volt cells are contained in each "A" battery. They are wired in series to achieve the 4.5 Volts DC. The middle cell is insulated from the two outer cells by a cardboard divider. This is to prevent shorting out of internal battery cells as the outer metal case of each cell serves as the negative connection.

I wear disposable Nitrile gloves during the "A" and "B" Battery disassembly process so as not to expose my hands to harsh chemicals. In addition, I disassemble the batteries on disposable paper plates so as not to get the dried chemicals all over my workbench.

Yuck! Here is the contents of the "A" battery. Notice the chemical leakages on the outside of each battery cell. The dry chemicals have impregnated the outer cardboard case of the "A" battery and internal cardboard spacers. The only thing that I can salvage is the top connector of the battery as it is made of Phenolic resin that can be easily cleaned. I used a pencil to mark a plus sign next to the positive terminal of the battery connector. Disassembly of the second "A" battery was the same.

Here is the picture of one of the Burgess M30 "B" batteries. Its nominal Voltage is 45 Volts DC. Two are wired in series to achieve the 90 Volts needed for the plate voltage of the vacuum tubes in the Trav-Ler Model 5020. 

Once again, chemicals have impregnated the outer cardboard housing of the "B" batteries so I am only going to salvage the Phenolic resin connector at the top. I used a pencil to mark a plus sign next to the positive terminal. It will make wiring the new batteries in the proper polarity much easier later. 

Here is the inside of a Burgess M30 "B" battery. Each circular black object is an individual 1.5 Volt cell that are wired in series to achieve 45 Volts DC. If you count them you will find 30 individual 1.5 Volt cells as 30 x 1.5 Volts = 45 Volts!

Below is the damage that the chemicals that leaked out of the "A" battery did to the chassis of my Trav-Ler Model 5020 radio. I don't what to chance having the chemicals in the cardboard housing of the "A" and "B" batteries doing further damage, which is why I am abandoning everything but the top connectors.

My new strategy is to create battery boxes out of 1/8 plywood I obtained from a local crafts store. The battery boxes will have the same dimensions as the "A" and "B" batteries they will be replacing. Below is a picture of the plywood after I marked out all of the pieces I require for the two "A" and two "B" battery boxes. I made sure that I left room between each piece for "kerf" or the width of the saw blade cut.  

Time to build the "A" and "B" battery boxes, I carefully sanded the edges of each plywood part with fine (200) grit sandpaper then dry fit the parts together. Once satisfied with the fit, I glued the pieces of the battery boxes together using Elmer's wood glue. I placed a piece a kitchen wax paper under the glued pieces so as to prevent the them from sticking to my work bench if some of the glue oozed out of the joints. Gluing the battery boxes together takes some patience, you cannot glue all of the pieces together at one time. I first glued the sides first then the top and bottom and finally glued reenforcement pieces in place as needed.

It is time to mount the battery connector at the top of the "A" battery boxes.  I use two small brass screws acquired from a local crafts store to hold the connector in place.

Time for fitment of the battery connector at the top of the "B" battery boxes.  Two small brass screws also hold this connector in place.

I choose to install three AA batteries in the "A" battery boxes to achieve the nominal 4.5 Volts DC required. I found two inexpensive 3 x 1.5 Volt AA battery holders on Amazon for a couple of dollars. The AA battery holders have an on/off switch protruding from the bottom side. I simply mounted the battery holder 1/16 inch off the bottom of the battery box to allow for clearance of the switch. The battery holder is fastened to the side of the battery box with two brass screws.

Mounting the 9 Volts batteries in the "B" battery box was a little more difficult. I decided to use the bottom and side of the battery box to hold the batteries in place. I then cut a 9 Volt metal battery clip in half to use as the third mount point. The tension of the "springy" 9 Volt metal clip does a good job of holding the 9 Volt batteries in place. Five 9 Volts batteries are required in each "B" battery box to achieve the 45 Volts DC. 

Time to disassemble all part from the battery boxes and apply water based Polyurethane to them. I apply two coats of Polyurethane then sand with a fine grit (200) once dry. I then apply a final finishing coat. I use water based  Polyurethane because it is easy to clean the paint brush and wash off of my hands.  

Time for final assembly of the "A" battery boxes. I soldered the wires from the AA battery holder to the battery connector. I wrote in pencil the proper polarity of the connector in an earlier step. 

Here is a picture of one of the completed "A" battery boxes.

It was easier to perform all the wiring of the "B" battery box outside of it. The five 9 Volt battery clips are wired positive to negative in a loop that extends to the terminals of the  battery connector.  I wrote in pencil the proper polarity of the connector in an earlier step. ​

Next, I mounted the battery connector, wiring, and 9 Volt Battery clip assembly to the "B" battery boxes. 

Here is a picture one of the completed "B" battery boxes.

Testing time, I connected the finished "A" and "B" battery boxes to my vintage 1947 Trav-Ler Model 5020 radio, then placed the power switch in the Battery position. Success! after a brief warmup I started to hear AM radio stations from the speaker.  

Notice how nice my "A" and "B" battery boxes fit in the battery slots of my vintage radio.

Here is a picture of my "A" and "B" battery boxes in place and ready for the back cover to be installed on my Trav-Ler radio.

oops, I still need to tuck the AC cord into the back of the radio, It is not needed as I will be running on it on battery power!

Here is video of my "A" and "B" battery boxes in action powering my Trav-Ler Model 5020 radio!