,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.
There was a void left in the electronic kit building market with the the Heath company shuddered its Heathkit division in 1992. Faced with increasing foreign competition, and the demand of providing increasingly complex designs that are easy to construct in kit form, while maintaining a competitive price was too overwhelming for the Benton Harbor company. Some other manufacturers, such as ELENCO, Vellman, and CanaKit partially filled the void with basic electronic kits. Ramsey Electronics offers more sophisticated kits. 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. The Model M-2666K Multimeter Kit I an 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. Make no mistake, there is still plenty of soldering and assembly work to be done on this kit. The multimeter kit also comes with a detailed assembly manual, show below, which includes circuit theory, testing, and troubleshooting sections. The manual rivals what you would expect from a Heathkit manual. 
I will give a full breakdown and review of the Model M-2666K Multimeter Kit in a future post once I have finished the build. ELENCO Multimeter Kits on Amazon!
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Both my father-in-law and I share in one passion, Ham Radio. He lives in California and enjoys perusing flea markets and Hamfests on the weekends. If he finds something really cool, he'll throw it in a box and ship it back East to me. I was delighted to find a vintage Heathkit HW-8 in a recently received package and can't wait to dig in and restore it to original specifications.
What is a HW-8?
The Heathkit HW-8 is a solid state low powered (QRP) CW transceiver kit offered by the Heath company from 1976 to 1983. The HW-8 kit had a $140 price tag the first year of its offering.
The HW-8 was the second QRP CW Transceiver kit offered by the Heath company. It had a much improved direct conversion receiver, which made up for the poor sensitivity issues in it predecessor, the HW-7. Heathkit HW line of transceivers are sometimes affectionately nick-named "Hot-Water", for example "Hot-Water-8", by Ham Radio enthusiasts. That's right the HW-8 is CW, or Continuous Wave only, so you can only send and receive Morse code. The HW-8 covers 15m, 20m, 40m and part of the 80m amateur radio bands. Transmitter output power is between 1.5 to 2 Watts. RF output is though an unbalanced RCA jack with a 50 Ohm impedance. There is no internal speaker on the HW-8. You have to plug in external headphones or an amplifier to hear incoming transmission. Audio output impedance is 1000 Ohms. The HW-8 requires an external 13.4 Volt DC Power Supply that can source at least 1/2 Amp. Some nice features of the HW-8 is its simple front panel layout with push button band switching and a power meter. Most HW-8 owners conclude that having a back-lit meter dial would have been a great plus but this is easy enough to rectify. There are many modifications published for the HW-8 on the Internet. One of the most popular is the RIT (Receiver Incremental Tuning) modification with allows you to slightly vary the receiver frequency while transmitting.
Here is picture of the back of my HW-8, notice the jacks for a Morse code key and headphones. In addition, there is a jack for 13.4 Volts DC Input. There is a Dummy load made up of two 1 Watt 100 Ohm Carbon resistors in parallel attached to the RCA antenna jack.
Get your Amateur Radio License!Don't just sit on the sidelines listening to Amateur Radio broadcasts, become an active participant! These ARRL publications below contain all you need to pass the exams. I studied the books, took the practice tests that came with it, and passed each exam the very first time. Most Amateur Radio clubs sponsor the testing and conduct it at a public place like a local library.
 My old Radio Shack PRO-2052 is back!
Years ago, I gave dad my Radio Shack PRO-2052 Scanner because I could not pickup broadcasts despite programming the proper frequencies used by our local Police and Fire Departments. I knew dad still lived in an area where the local municipalities still used analog. At the time I visited our local Radio Shack and they confirmed that I would need a new expensive scanner to receive digital Police and Fire broadcasts in our area that they were only too happy to sell me.
My dad passed a couple years ago and I reclaimed the PRO-2052 scanner while helping mom sort through dads things. For the heck of it, I decided to once again program the proper frequencies used by our local Police and Fire before committing the scanner to my pile of obsolete electronic equipment. After a few seconds it came to life with a Fire broadcast! Apparently many of our local Fire departments still use analog while the Police departments moved on to digital. While if would be nice to receive both Police and Fire broadcasts, I am still pretty happy with Fire broadcasts only. My Radio Shack PRO-2052 Scanner proves its usefulness again! The Dreaded CC&R
More neighborhoods these days come with home owner associations with restrictive CC&Rs. A CC&R (Covenants, Conditions, and Restrictions) are limitations and rules placed on a group of homes by a builder, developer, neighborhood association and / or homeowner association. A CC&R could restrict the color pallet of the exterior of the houseor the mounting of an external Amateur Radio or Scanner Antenna. My neighborhood does not have a home owners association or CC&Rs in place but sometimes it is a good idea to conceal antennas to keep nosy neighbors from blaming everything from television interference to global warming on you antenna installation! Hellloooo....a Police and Fire Scanner Antenna only receives radio signals, it does not transmit them! Naturally, this Hub would be a good read for someone that does live in a neighborhood with restrictive CC&Rs The Antenna
I chose the AntennaCraft ST2 Scanner Antenna with 15 elements and a Frequency Range from 30Mhz to 1300Mhz. I matched the Frequency range to that listed in the my Scanner Specifications. It comes with a matching transformer, 50 feet of RG-6 Coaxial Cable, and a F to BNC adapter so it will fit the antenna connection of most Police/Fire Scanners. The ST2 antenna does not indicate if this antenna would work well for Digital Police/Fire signals although I believe it would not matter as long as those signals were in the Frequency Response of the antenna. I am using an older analog scanner with this antenna. The ST2 retails for about $50 on Amazon, since I am subscribed to Amazon Prime it arrived on my doorstep two days later. Unfortunately, when it arrived the rectangular box it was shipped in was slightly V shaped bending several elements. Fear not, the ST2 antenna elements are made of soft aluminum and can be bent back into position with care. Once the ST2 is unboxed, you must expand and snap the elements in place in their plastic clips. You may want to wait to do this until you drag the antenna into your attic as you want it as compact as possible. The ST2 construction is reminiscent of television antennas I remember helping my dad put on our house in the 1970s. It is all aluminum construction to make it lightweight an corrosion resistant. The plastic clips that hold the elements seemed to be made over sized most likely to bolster their toughness. The ST2 antenna is made in the USA! Scanner Antennas on AmazonAttic Mounting the ST2 Antenna The ST2 Scanner Antenna mounted in the attic.
My vintage house has ample space in the attic for installing the ST2 antenna. We never store anything in the attic as it is difficult to carry things up the folding attic stairs. I collapsed all of the ST2 elements, like it was configured for shipping, so that it was easy to transports up the attic stairs. The ST2 instructions specify that for best reception that the Center Main Boom Elements be perfectly vertical in orientation. Upon expanding all of the elements to the proper configuration I determined that it was not possible to mount it with the Center Main Boom Elements perfectly vertical. I reached out to AntennaCraft support via e-mail and they specified that a good compromise would be to slightly angle the antenna up with the Center Main Booms as close to vertical as possible. I used a block of wood to attach the antenna to one of the roof trusses. I only used a couple dry wall screws to hold it in place as I do not have to worry about wind. A small matching transformer connects to the front main boom elements with wing nuts. I secured the matching transformer to the boom with a tie strap. ST2 Antenna Wiring Routing the RG-6 cable through the Linen closet.
All of my amateur radio equipment is located in the basement. This is where I am going to place my PRO-2052 Scanner as well. My house is a ranch so it was pretty easy to route the RG-6 wiring from the attic to the basement. I simply drilled holes in the ceiling and floor of a linen closet and fished the cabling down to the basement. The 50 feet of RG-6 coaxial cable included with the ST2 antenna has F connectors installed on each end. You connect one side of the RG-6 cable to the matching transformer on the ST2 antenna and the other side connects to your Scanner. The RG-6 coaxial cable was just enough to reach the back of my scanner located in my basement Amateur Radio Shack. Connecting the ST2 Antenna to a Scanner F to BNC adapter connection to the back of my Scanner.
The ST2 Antenna comes with a F to BNC adapter. This worked perfectly as my analog scanner has a BNC connector for an external antenna. Low Profile (Stealth) Antenna Books on AmazonConclusion My Amateur Radio Shack located in the basement.
My new ST2 Antenna works great! I am picking up Fire broadcasts on my scanner that I was not picking up with the built-in telescoping antenna.The ST2 Antenna was very easy to assemble and install. Maybe my Hub will give you ideas to mounting your own stealth antenna?
Still, he proudly displayed his Heathkit Mohican Receiver on his dresser in my parents bedroom. This receiver was perched on my dad's dresser from the time of my early childhood until his death several years ago, when it came into my possession.
The last time I remember the Heathkit Mohican Receiver worked was when Ronald Reagan was still president! As a tribute to my father, I spent a month restoring his Heathkit Mohican GC-1A Receiver to a condition close to when it was originally built in 1965. Old Time Radios! Restoration and Repair 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.
Heathkit Mohican Manual Download****Polarity Alert****
Electrolytic Capacitor Kits on Amazon
Amazon has a great selection of electrolytic capacitor kits, perfect for the electronic hobbyist!
XP-2 AC Power Supply Recap
The XP-2 AC Power Supply is installed in the back of the Mohican Receiver. It is held in place by two snap fittings. Use the pull ring and pull it straight back. Disconnect the internal power supply cable. Remove the rectangular aluminum cover.
The old nasty electrolytic capacitors.
The replacement electrolytic capacitors installed. Notice they are much smaller in physical size than the originals.
The nasty old yellowed original AC cord.
The new replacement AC cord installed.
Look at the cool way I attached the .1Mfd capacitors between the Input and Ground and Output and Ground pins. These capacitors will prevent the 7812 from oscillating during high loads.
After the electrolytic capacitor replacement, the output voltage shot up to 15 Volts DC. I installed a 7812 Voltage Regulator to keep the Voltage at around 12 Volts. Do NOT secure the regulator tab to the metal base. Remember, positive ground.
Below is a schematic containing my modification to the XP-2 Power Supply.
Get your Amateur Radio License!
Don't just sit on the sidelines listening to Amateur Radio broadcasts, become an active participant! This book contains all you need to pass the Technicians Exam. I studied the book, took the practice tests that came with it, and passed the exam the very first time. Most Amateur Radio Clubs sponsor the testing and conduct it at a public place like a local library.
Disassembly
Time to remove the metal cabinet from the chassis so that you can get at the electronics!
Remove the antenna by unscrewing and pulling up and out. 
Remove the two screws from the back for the receiver on the lower right-hand side.
Remove the two screws from the back for the receiver on the lower left-hand side then slide the chassis forward until you can access the speaker.
Remove the four nuts that hold the internal speaker to the top of the cabinet. The chassis is now free and clear of the cabinet.
Bottom view of the chassis.
Cleaning and LubricationThe Mohican Receiver's chassis has not seen the light of day since the 1980s when my dad tried to re-align the RF and IF amplifier sections. It had many layers of dust and dirt inside. Contact Cleaner on Amazon
Most radio restorations require the use of contact cleaner to remove years of oxidation from switch contacts.
Look at all the dust and dirt on the IF Board!
The rest of the chassis had a layer of dust and dirt on it as well.
I used an air compressor to blow all of the dust and dirt off of the chassis. Damp Q-Tips helped clean the less accessible areas of the chassis and IF Board.
I used electrical contact cleaner to clean all the potentiometers connected to the front panel. I would spray the cleaner into the small cutout by the terminals then work the potentiometer back and forth through its full motion.
The dial lights did not work even though my ohmmeter indicated that the bulb filaments were still intact.
Corrosion in the dial light bulb sockets prevented the light bulbs from making good contact.
The dial lights worked once again after I cleaned the bulb sockets with a Q-Tip dipped in denatured alcohol.
I used Labelle 107 Oil to lubricate all rotating parts including the shafts of potentiometers and variable capacitors. This oil will not harm plastic parts.
I used Labelle 106 Grease to lubricate the gearing connected to the Main Tuning and Band Spread capacitors. This grease will not harm plastic parts.
I used a touch of Super Glue to secure the Main tuning dial pointer to the dial string. The pointer was loose and would not track properly along the dial.
Labelle Lubrication Kit 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.
Recap
As mentioned, "recap" is the replacement of old electrolytic capacitors in vintage equipment. Electrolytic capacitors have a finite lifetime. As they age the electrolyte dries up causing their capacitance to fall in value. This section describes the replacement of electrolytic capacitors in the Mohican receiver.
C56, C57, and C58 are all housed in the same case. Each has a value of 100Mfd with 15Volt Max Rating. 
Triple capacitors are hard to find these days. I replaced C56, C57, and C58 with three discrete electrolytic capacitors of the same rating.
The next capacitor to be replaced was C59. It has a value of 50Mfd and a 15Volt Max Rating.
I replaced C59 with a 47Mfd electrolytic capacitor with a 15Volt Max Rating. Most capacitors have capacitance tolerance of 20%, so this value is close enough.
C55 is a wax covered capacitor rated 150Mfd at 15Volt Max Voltage Rating.
I replaced C55 with a 100Mfd and a 47Mfd electrolytic capacitors in parallel, which combines for a total of 147Mfd capacitance. This is close enough to the original value.
C27 is a 10Mfd capacitor with a 10Volt Max Rating.
C27 was replaced with a new electrolytic capacitor of spec similar to the original one.
C54 is located on the underside of the printed circuit board and is connected to a terminal strip. It has a value of 50Mfd with a 15Volt Max Rating.
I replaced C54 with a 47Mfd electrolytic capacitor with the same voltage rating.
Volume Control Repair
For some reason the 5K Potentiometer used as the volume control suddenly froze. I could not turn it.
Access to the volume potentiometer can be achieved from the underside of the chassis. Remove the knob then the nut that attaches it to the front panel. It takes some finessing to get it out so that you can unsolder it. 
Turns out the spring that contacts the resistive material somehow got folded over causing the extreme increase in turning resistance. Once repaired, I did a continuity check to make sure the potentiometer was functioning properly.
The process of soldering all of the wires back on the volume control potentiometer was a daunting task as the wires are very short. With some patience, you will be able to solder all wires to the proper terminals of the potentiometer.
Receiver Alignment
It is important to go through the Receiver Alignment procedures in the Heath Kit Assembly manual in order to get peak performance out of your Mohican Receiver.
I would definitely recommend going through the Receiver Alignment procedures if you were not the original builder of the Mohican Receiver as kit builders range in all skill levels. A lot of kit builders may have never went through the Receiver Alignment procedures, as they may not have had access to an rf Generator, which is required for the alignment process. I would recommend going through the Receiver Alignment process even if you were the builder of the Mohican Receiver. It is possible that the receiver could have gotten out of alignment after 50 years as components slightly change value due to age. IF Amplifier Alignment
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Believe it or not, you can still find TV Alignment tools to purchase. These tools are rarely needed these days with Digital TV receivers and SDR (Software Defined Radio) but were once a must for any serious electronic technician.
Dial Pointer Adjustment
Band Alignment
Band Alignment - Initial Setup
Band Alignment - Process
Testing
Time to see the fruits of my "Band Alignment" labor and connect the Mohican receiver to my G5RV external antenna. I was amazed at all the stations I was picking up of each band!
Cabinet Restoration
Years of tobacco smoke have taken their toll on the Mohican's enclosure. You can't just wipe the tar off, it is embedded in the paint.
Even the aluminum carry handle has a brown hue due to tobacco tar.
Be very careful what you use to clean the Mohican's enclosure! I used a mild polish that was no more abrasive than toothepaste and it left these strange watermarks. Still the watermarks beat the brown tar coating that originally coated the enclosure.
Another picture of the strange watermarks that appeared after I lightly polished the enclosure.
I used motorcycle chrome polish and an old toothbrush to clean the Mohican receiver's knobs, antenna, and handle.
Look how pretty the knobs and antenna look after polishing!
Mohican Receiver Assembly
My dad's newly restored Mohican Receiver in action!Great VHF/UHF FM Transceivers for under 40 Dollars!
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The "All American Five"From the 1930s until the 1960s, electronic manufacturers in the United States adopted superheterodyne radio designs that eliminated the power transformer in order to keep the cost of each unit low. These radios were nicknamed the "All American Five" because of the manufacturers adoption of a standard complement of five vacuums in that their filament voltages added up to line voltage when in series, which is between 110 and 117 Volts. The B+ voltage, was derived from rectifying and filtering the line voltage, which amounted to about 160-170 Volts DC (Direct Current). A tell-tale sign of an "All American Five" style radio, is a lack of power transformer installed on the chassis. One of the advantages of "All American Five" radios is their ability to run on either a line voltage of AC (Alternating Current) or DC (Direct Current). In some areas of the United States, DC line current was still in use. A major disadvantage of "All American Five" designs is that it had a metal chassis connected to one side of the power line, which was a dangerous electric shock hazard and required an insulated cabinet and knobs. Caution, Hot Chassis, Risk of Electrocution!My Trav-Ler Model 5020, shown below, is an example of an "All American Five" style radio, one wire of the line cord connects directly to the metal chassis when the power switch is in AC mode. This can mean that line voltage potential (110-117 Volts AC) can be on the metal chassis, which can pose an electrocution risk, depending on how the non-polarized AC cord plug is plugged into the socket. You need to be mindful of this when operating "All American Five" style radios with the chassis outside of its cabinet!  The Original Non-Polarized PlugAs mentioned earlier, the line voltage potential (110-117 Volts AC) can be on the metal chassis, which can pose an electrocution risk, depending on how the non-polarized line cord plug is plugged into the power outlet. Below is a picture of the original non-polarized plug connected to the Trav-Ler Model 5020.  Polarized Plug InstallI replaced the plug with a polarized with one that most closely matched the original. The wider blade should connect to the chassis as this is the Neutral connection. The thinner blade connects to Hot. I replace the plugs with a polarized type for all vintage radios of "All American Five" design as it adds a margin of safety when servicing and enjoying the radio.  Performing Safety CheckAfter the polarized plug is installed, when you plug the "All American Five" radio into an electrical socket, you should detect only a very low voltage (less than one Volt) AC between the ground connection on the wall socket and the metal chassis. 
FM Stereo Theory
In the late 1950s, several systems to add stereo to FM radio were considered by the FCC.
The GE and Zenith systems, so similar that they were considered theoretically identical, were formally approved by the FCC in 1961 as the standard stereo FM broadcasting method in the United States. It is important that stereo broadcasts be compatible with mono receivers. For this reason, the left (L) and right (R) channels are encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver or FM Tuner with a Multiplexer will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel. The (L+R) Main channel signal is transmitted as baseband audio in the range of 30 Hz to 15 kHz. The (L−R) signal is modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal occupying the baseband range of 23 to 53 kHz. A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it, is also generated. A 8 to 10 percent modulation level is used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase. The final multiplex signal from the FM stereo broadcast contain the Main Channel (L+R), the pilot tone, and the sub-channel (L−R). This composite signal, along with any other sub-carriers, modulates the FM transmitter. Radio Restoration Book On Amazon
Heathkit AC-11 Circuit Theory
An FM Receiver, such as the Heathkit AJ-10, recovers the audio and multiplex information from the FM Stereo Broadcast. The multiplex information is used to decode the left and right stereo channels. The audio plus multiplex information is fed into the AC-11 using a shielded cable.
Basically, this input signal to the AC-11 contains the L + R and L - R information, plus the pilot tone. Tube V1A amplifies the signal to compensate for any losses. Tube V1B lowers the impedance of the signal since it is a cathode follower stage. Coil L3 and capacitor C4 act as a low-pass filter which passes only the L + R signal. This signal can then be adjusted to the proper level by the Separation Control for insertion into the matrix network. The output of V1A is also applied to the grid of tube V2A, which is another amplifier. From tube V2A, only the L - R signal passes through the bandpass filter, consisting of C5, C6, and L1, to the tuned circuit formed by C16 and L2. The network consisting of R17, C16, and L2, in conjunction with the bandpass filter, will also attenuate any "storecast" signals which may be transmitted by the radio station. "Storecast" is a commercial music channel, transmitted on the same FM carrier as the stereo program. The output from V2A is fed also through C8 to a keyed oscillator, which accepts only the pilot tone (19,000 Hz). This tone keys the oscillator so that it produces a 38,000 Hz signal. The output of the oscillator is then coupled through T1 and R18 to the tuned circuit consisting of C16 and L2. The L - R and 38,000 Hz signals are mixed together across this tuned circuit. AC-11 SchematicHealhkit AC-11 Restore
The steps below described how I restored my Heathkit AC-11 Multiplexer.
Step 1 Disassemble AC-11
Remove three sheet metal screws from the back of the AC-11.
Remove the four sheet metal screws from the bottom of the AC-11. Slide the bottom cover towards the back of the unit to remove. Slide the chassis towards the back to remove the top cover.
AC-11 with top and bottom covers removed.
Step 2 Vacuum Tube Testing/Replacement
Unlike solid state transistors and integrated circuits, vacuum tubes have a finite life span. As they age, fewer electronics are released from the cathode making them less efficient. In addition, the tube filaments burn out much like the filament in a light bulb.
All of the vacuum tubes in the AC-11 Multiplexer were original. I can tell because they are Heathkit branded. 
Test each vacuum tube in the AC-11 Multiplexer. Replace any tubes that are defective or weak.
Step 3 Electrolytic Capacitor ReplacementAs 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 50 year old electrolytic capacitors like the ones in the AC-11 Multiplexer! Step 3a - 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 1960s. 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.
C13, C14, and C15, all 20uF 250Volt Capacitors, are all contained within the same multi-section capacitor (underside circled in picture). You must desolder the components connected to it's terminals in order to remove. 
Here is a picture of the multi-section capacitor removed.
The inside insulated terminals connect to the positive plates of the internal capacitors. All internal capacitors negative plate share a common ground with are connected to the outside terminals.
I use wire cutters to cut the crimped 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.
The anatomy of a multi-section capacitor. All three internal capacitors are rolled up into one assembly.
Three replacement 22uF 250Volt electrolyte capacitors will not fit inside the aluminum case of the mulit-section capacitor. I wire one of the capacitors to the base during assembly.
Circled in red is the remaining capacitor that cannot fit inside the aluminum case of the multi-section capacitor.
The refurbished multi-section capacitor installed back in the chassis. You will never know it was refurbished once the aluminum cover is installed.
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. Hot glue holds the aluminum cover in place.
Time to replace C3, a 2uF 150Volt capacitor (circled in red)
C3 was replace with a 2.2uF 150Volt modern electrolytic capacitor that is much smaller. Make sure you observe the capacitor's polarity when installing.
Time to replace C12, a 20uF 250Volt electrolytic capacitor (circled in red).
I used a 22uF 250Volt electrolytic capacitor for C12's replacement.
Pictures of the two discrete capacitors replaced.
Step 4 Cleaning and Lubrication
It is important to clean mechanical switch contacts and lubricate moving parts. Cleaning the chassis is not required but adds aesthetics to the AC-11 Multiplexer.
I spray contact cleaner into all switches, rheostats, and potentiometers then work the control back and forth to clean the internal contacts. 
A toothbrush works great for cleaning up the knobs. Plus the AC-11 Multiplexer will not get cavities ;-)
I use Meguiar's Motorcycle Wax to bring back the luster of the AC-11's case.
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The first step is to check the power supply voltages against the schematic in the manual to make sure they are within spec. I also check the voltages on each pin of the vacuum tubes to make sure they are the same as on the schematic.
Perform the Installation and Operation steps at the end Heathkit AC-11 Multiplexer Manual. This is done with the AC-11 connected to the Multiplex output of an FM Tuner and the right and left outputs connected to the inputs of a stereo amplifier.
Step 6 AssemblyFollow "Step 1 Disassemble AC-11" section in reverse order to reinstall the top and bottom covers. Conclusion
The restoration of this Heathkit AC-11 Multiplexer was most gratifying. This project took my mind off of the stresses of my daily job and reminded me of a simpler time when people had the time to sit around and listen to the stereo for enjoyment.
 HistoryPaper capacitors, sometimes known as paper/wax capacitors, were widely used in electronic equipment in from 1930 until the late 1950's due the fact that could be manufactured easily and at a low cost. A paper capacitor consists of two long pieces of tin foil sandwiched between wax paper tightly rolled up so that the tin foil was exposed at the end of the roll. Wires were soldered to tin foil exposed at the end of the roll. The assembly was stuffed in a cardboard tube and sealed and protected with a coat of wax. Many paper capacitors have a black band closest to one of the wire leads. This is to indicate the foil closest to the outside of the foil/wax paper roll. Paper capacitors are not polarized and can be put into the circuit in either direction, although, radio manufactures of the day did specify which connection the lead closest the black band should be connected, which was typically closet to ground potential. For some reason the prevailing consensus of the day was that paper capacitors wired this way would last longer if wired in this fashion. Some paper capacitors, called "molded" capacitors, are housed in plastic case. These are just paper capacitors with just a prettier facade. Paper capacitors range in capacitance from 0.001 uF and 1.0 uF and can have a working Voltage up to 500 Volts, depending on their construction. Should they be replaced?The consensus of hobbyists that restore vintage radios and Amateur Radio enthusiasts that restore "Boat Anchor" or vintage receiver/transmitter equipment is a resounding "YES". They site the following reason to replace paper capacitors: - Old paper capacitors are inherently unreliable. - Paper capacitor's capacitance change with age. - As paper capacitors age, their Effective Series Resistance (ESR) increases and their leakage resistance decreases. These are not good trends for these parasitic resistances in a capacitor. - Corrosion at the lead ends will cause the ESR to increase to a point that it effectively takes the internal capacitor out of the circuit. - Aged paper capacitors with their yellowing cardboard body and dripping wax, due to heating by vacuum tubes, become unsightly. - Paper capacitors deteriorate with age due to acid in the paper. What is a good paper capacitor replacement?The general consensus of antique radio and Amateur Radio enthusiasts is to use plastic/polyester film capacitors due to their small size, low cost, and stability. I use metalized film polypropylene capacitors to replace paper ones in my restore projects as they are more stable at higher frequencies than polyester capacitors but cost about the same. When replacing paper capacitors:
Below is a picture of my 1947 TRAV-LER Model 2050 AM Radio with its original paper capacitors, circled in red.  Here is a picture of the my 1947 TRAV-LER Model 2050 AM Radio with newly installed metalized polypropylene capacitors, circled in red. I believe replacement of the paper capacitors really improved the appearance under the chassis. I know, who looks under the chassis? I would know these ugly paper capacitors were there and it would bug me!  Background
Batteries
Pictured below are examples of an "A" and "B" battery I just pulled out of my 1947 TRAV-LER Model 5020 portable vacuum tube radio that I am in the process of restoring. Two "A" 4.5 Volt batteries were connected in series to provide the required 9 Volts for the tube filaments. In addition, two "B" 45 Volt 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.
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 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. I plan on gutting these vintage "A" and "B" batteries, then installing modern batteries wired in the proper fashion to achieve the proper voltage/current required. Step 1 Chassis Removal
Here is the back view of the Model 5020, with the back cover and batteries removed.
As in most vintage radios, all of the electronic components are attached to a metal enclosure called a "chassis". In the Model 5020, the chassis is behind the internal antenna mounted on a cardboard form. That chassis is attached to a wooden shelf in the Model 5020's enclosure. The wooden self allows the 45 Volt batteries to be housed under the chassis for operation under battery power. 
In order to remove the shelf that the Model 5020's chassis is attached to, you have to remove one screw from each side of the cabinet.
You must remove the knobs located on the front of the Model 5020 enclosure before you can slide the chassis out the back. The tuner and the power selector switch pull off while the volume knob is held in place with a set screw.
Once the knobs have been removed, you can slid the shelf with chassis attached, out the back. Here is a picture of the chassis and shelf removed from the Model 5020's cabinet.
You will need to remove the wooden shelf in order to get at the components installed on the underside of the chassis. Two screws, circled in red, connect the shelf to the chassis.
Here is a picture of the bottom of the chassis with the wooden shelf removed.
"Old Time Radios! Restoration and Repair" book on Amazon
I consult this book often during radio restoration. I grew up in the transistor and rectifier era and this book taught me a lot about vacuum tube and selium rectifier technology.
Step 2 Test Vacuum Tubes
I use my trusty Eico 635 Tube Tester to check each vacuum tube in the Model 5020. Any defective or deficient vacuum tubes should be replaced before moving on to the next step.
Caution, Hot Chassis, Risk of Electrocution!
The Model 5020 connects one wire of the AC cord directly to the metal chassis when the power switch is in AC mode. This can mean that 120 Volt AC line voltage potential can be on the metal chassis, which can pose an electrocution risk, depending on how the non-polarized AC cord plug is plugged into the socket. You need to be mindful of this when operating the Model 5020 chassis outside of its cabinet! We will address this safety issue later in this blog with the installation of a polarized plug on the AC cord.
Step 3 Troubleshooting
You will need a schematic diagram of the Model 5020 to perform any more troubleshooting. I have provided a link below to said schematic.
There was no sound output when I initially plugged in the Model 5020 to 120 Volt AC line. I determined that part of the Multitap Candohm resistor that fed the tube filaments had an open. The resistance between the taps that was open was 1050 Ohms, according to the schematic. I used test leads with alligator connectors to wire in series four 250 Ohm wire- wound resistors then attached this resistor network across the taps with the broken resistance. SUCCESS! I could hear local AM Radio stations once the filaments heated up.
It was time for a more permanent fix. I soldered a 1000 Ohm wire-wound resistor across the two taps with the broken resistance. See area circled in the picture below.
Step 4 Electrolytic Capacitor Replacment
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 electrolytic capacitors that are over 60 years old!
All electrolytic capacitors should be replaced with one of the same or slightly greater capacitance and working voltage rating. Here is the original 70uF 25 Volt capacitor.
I replaced it with a 100uF 25 Volt electrolytic capacitor. I was able to piggy back the new electrolytic capacitor across a resistor due to its small size.
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 1940s. They 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 Model 5020 employs a multi-section capacitor that contains three 40uF 150 Volt capacitors. The multi-section capacitor is circled in the picture below. 
You must first un-solder all of the connections at the bottom of the multi-section capacitor before you can remove it from the chassis. You also need to drill out two rivets that hold the multi-section capacitor in place.
The multi-section capacitor removed from the chassis. The side of the cardboard cylinder indicates capacitance and working voltage of the internal capacitors.
The guts of a multi-section capacitor. I use needle nose pliers to un-crimp the bottom of the aluminum can exposing the innards. I always wear nitrile gloves when working with the guts of capacitors as the electrolyte may cause skin irritation.
The new replacement capacitors soldered to the multi-section capacitor base. I use a dab of hot glue to secure them in place. Note: I am only able to hide two of the new 47uF 250 Volt capacitors in the "can", the third will be located under the chassis.
I use two machine screws to hold the phenolic wafer in place,
You must re-solder all of the connections to the base of the rebuilt multi-section capacitor after it is once again mounted to the chassis. The third 47uF 250Volt Electrolytic capacitor installed is circled in red below.
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.
Here is a picture of the installed newly reconstructed multi-section capacitor. Not very pretty at the moment.
I then slip the aluminum can of the multi-section capacitor over the new assembly. It is held in place with a couple dabs of hot glue at the bottom.
I then slide the cardboard cover over the aluminum can. No need for glue as the cover is a snug fit.
Step 4b Replacing the other Capacitors
Paper/Wax capacitors, circled in red below, deteriorate with age due to acid in the paper.
I decided to replace them with modern Metallized Polypropylene Capacitor of equivalent capacity and working voltage. These are the yellow and grey capacitors circled below.
Step 5 Cleaning and Lubrication
It is important to clean mechanical switch contacts and lubricate moving parts. Cleaning the chassis is not required but adds aesthetics to the Model 5020 restore.
I spray contact cleaner into all switches then work the switch back and forth to clean the internal contacts.
I also spray contact cleaner into the innards of potentiometers and rheostats then work them through their full motion of movement.
Electrical contact cleaner on Amazon
I use Labelle 107 model train oil to lubricate pulley shafts and other mechanisms. This oil will not harm plastics.
I use Labelle 106 model train grease to lubricate the bearings of the rotary switch. This grease will not harm plastics.
I also use Labelle 106 model train grease to lubricate the bearings of the variable capacitor.
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.
Step 6 Dealing with Chassis Rust
The Trav-ler Model 5020 I had recently acquired had a fair amount of rust on the edge of the chassis.
The rust was caused by a leaky 4.5 Volt Zinc-Carbon "A" battery left in the case by the previous owner.
The chemicals in this vintage Ray-O-Vac "A" battery had leaked through the cardboard case then attacked the steel chassis where it made contact.
The first step to chassis rust repair is to give yourself some room to work. I removed the vacuum tubes in the near proximity. I later decided to unsolder the red and blue wire that connects a AF (Audio Frequency) amplifier tube socket to the audio transformer then pull the wires out of the grommet. Then the grommet was removed as there was rust around the feed-through hole.
I started with a Dremel with a grinding wheel installed to remove as much of rust as possible. I then sanded the rusted area starting with coarse sandpaper finally finishing with a fine sandpaper. 
Below is a picture of the area masked off ready for paint. I used denatured alcohol to degrease the area before paint.
I use silver Dupli-Color automotive rim paint as it is the closet color I can find that matches the natural color of most chassis. Apply in light coats and feather it out so as to make a natural transition from the newly painted area to the chassis color.
Repair job complete! Time to install a new grommet in the hole then thread the audio transformer through the installed grommet and solder to the AF (Audio Frequency) amplifier vacuum tube socket. The final step is to install the vacuum tubes into their respective sockets.
Duplicolor Rim Paint on Amazon
I use this paint to touch up places on the chassis that have rusted.
Step 7 Testing Under Battery Power
The Model 5020 was designed to run on 120 Volts AC Line Voltage or battery power which consists of Two "A" 4.5 Volt batteries connected in series to provide the required 9 Volts for the tube filaments. In addition, two "B" 45 Volt batteries are connected in series to provide the required 90 Volt plate voltage. I used my "battery box", the woodgrain case below, which contains ten 9 Volt batteries in series, to provide the "B" 90 Volts. In addition, I used a single 9 Volt battery for short duration to provide the "A" filament voltage. The model 5020 functioned as expected and I was able to detect several AM stations upon warm-up.
Step 8 Install Polarized Plug
As mentioned earlier, an 120 Volt AC line voltage potential can be on the metal chassis, which can pose an electrocution risk, depending on how the non-polarized AC cord plug is plugged into the power outlet. Below is a picture of the original non-polarized plug.
I replaced the plug with a polarized one that most closely matched the original. The wider blade should connect to the Model 5020 chassis as this is the Neutral connection. The thinner blade connects to Hot.
With the model 5020 plugged into an electrical socket, you should detect only a very low voltage (less than one Volt) AC between the ground connection on the wall socket and the chassis.
Step 9 Cabinet Restoration
I use Meguiar's PlastX and a toothbrush to clean the knobs.
Here are the knobs after my cleaning job. Look ma, no cavities!
Meguiar's PlastX on Amazon
Meguiar's PlastX works great for cleaning up vintage knobs or to remove the "yellowing" of clear plastic lenses used over the tuning indicator. I also use other Meguire's automotive detailing products to bring back the luster of old radio cabinets.
I used Windex and a paper towel to do the initial cleaning. Look at the dirt on my paper towel just from the initial rub down!
There are certain marks or stains that you just simply can't remove, like the dark black smudges in the picture below.
I use Elmer's wood glue to reattached the external cloth that has come loose during cleaning.
Elmer's wood putty also comes in handy to fix a spot, where the wood screw that holds the shelf in place, broke through the wood. Elmer's wood putty is "machinable" meaning you can drill a pilot hole in it in order to re-thread the screw.
An automotive vinyl/leather protectant, similar to ArmorAll, really made the cabinet shine!
I use Old English furniture polish to clean/protect all internal wood surfaces.
Step 10 Chassis Install
Perform a final test of the chassis under 120 Volt AC power. Make sure you can still pickup AM radio stations. Follow the section "Step 1 Chassis Removal" in reverse order to reinstall the chassis back into the cabinet.
My newly restored Model 5020 in action!Conclusion
I spent many hours restoring this vintage 1947 Trav-ler Model 5020 back to its original specifications. It was a great project.
I wonder if the original owner of the Model 5020 ever thought that his/her radio would still be functioning well into the 21st Century? 
What is an antenna Tuner?
An Antenna Tuner is a device that is connected between a radio transmitter's (or transceiver) antenna output and the antenna. Its job is to provide efficient power transfer between radio transmitter and the antenna. The Antenna Tuner achieves efficient power transfer by matching the impedance (resistance to alternating current flow) of the antenna to that of the radio transmitter. In most modern transmitters, the antenna output impedance is 50 ohms.
Most Antenna Tuners have a SWR (Standing Wave Ratio) meter which measures forward and reflected power. You use this meter to correctly adjust the dials and switches until the SWR meter indicates a ratio of close or equal to 1:1. This is when the most efficient power transfer occurs. An antenna that is not properly impedance matched to the transmitter antenna output could cause the transmission line to heat up, or the reflected power could cause damage to your transmitter! When do I need a Antenna Tuner?
If an antenna's impedance is perfectly matched to the band of frequencies the transmitter is to operate, then a Antenna Tuner is not required. For example, I have a Yaesu FT-1900R 2M FM Transceiver connected to an Arrow J-Pole Antenna in the attic. I do not need an Antenna Tuner as the antenna is impedance matched for the 2M band.
Typically, an Antenna Tuner is required when using a multi-band transceiver to drive a multi-band antenna. A mutli-band antenna typically cannot be perfectly impedance matched for all the bands of frequencies it is intended to be used on. The Antenna Tuner is used to compensate for the compromises made to the antenna in order to cover multiple bands of frequencies. What's inside the MFJ-949C Antenna Tuner?
The MFJ-949C is a manual Antenna Tuner that is composed of an T Network consisting of two air insulated variable capacitors in series between the Antenna Input and Antenna Output SO-239 coax connectors. There is also a mutli-tapped air wound inductor between the capacitors which connects to ground. Impedance match is obtained by changing the capacitance and inductance of the T Network using the front knobs until a SWR meter reading equal or close to 1:1 is obtained.
A tapped torrid transformer on the input is first rectified then attenuated in order to drive the forward and reflected power meter movements. The MFJ-949C also has a built in 50ohm dummy load that can be switched in and can handle 300 Watts of transmitter power for short durations. A 1:4 Balun is also installed in the enclosure to allow the MFJ-949C to drive a balanced line instead of coax. In addition, the SWR meter has a jack on the back so the back light can be powered by a 12 Volt source. The tough life of my MFJ-949C Antenna Tuner
My MFJ-949C has been riden hard and put away wet! It is the first and only Antenna Tuner I ever owned since I started my Amateur Radio hobby. As a "newbie", my poor Antenna Tuner had to bear the brunt of my inexperience. More then once I did not have it set correctly an internal arcing has occurred. In addition, I have "pegged" the SWR meters by having the switch on the front set to the 30 Watt instead of the 300 Watt range. Now it is time to assess the damage done.
Keep things organized during the dis-assembly process!
I keep all of the nuts, bolts, screws, knobs, and washers I remove during dis-assembly in a disposable salad bowl so I don't lose anything!
Step 1 Cover Removal
There are three screws on each side of the MFJ-949C that need to be removed. See areas circled in red.
In addition, two screws on the top of the cabinet need to be removed as well. See areas circled in red below.
Step 2 Examine the inside of MFJ-949C to look for problems
The first thing I noticed was carbon on the printed circuit board around the connection to the inductor (circled in red). I found that this connection was loose, probably where the arcing occured.
I also found a carbon resistor, circled in red, with a piece of the external coating missing. Could have been a defect in the resistor or over heating.
Step 3 Circuit Board Removal
De-solder the wires from the circuit board to the connectors on the back of the chassis. See areas circled in red in the picture.
De-solder the wire from the circuit board to the Transmitter Matching variable air capacitor. See area circled in red in the picture.
Disconnect the coupler that connects to the Mode Switch knob on the front of the Antenna Tuner. See area circled in red in the picture.
Disconnect the tapped air wound Inductor from the base of the Antenna Tuner, it is held in place by two screws. See areas circled in red in the picture.
Remove the knobs, then unbolt the Transmitter Matching, Inductor Selector, and Antenna Matching shafts from the front of the Antenna Tuner.
Three screws, circled in red, attach the printed circuit board to the base of the Antenna Tuner. They will need to be removed.
Picture of T Network removed from Chassis!
Move Evidence of Arcing!
Circled in red is more evidence of arcing between the printed circuit board and the chassis, most likely due to an error in on my part when adjusting the Antenna Tuner's settings.
Step 4 Look for damage on the trace side of the printed circuit board
The trace side of the circuit board is where the discrete components are joined electrically. Circled in red in the picture is where arcing occurred between the printed circuit board and the metal chassis. Examine all traces closely for other damage.
Step 5 Clean Electrical Contacts
The electrical contacts of the Inductor Selector switch can be cleaned by spraying contact cleaner at the spots electrical contact is made then rotating the switch to all positions. Repeat this process until the contact points look shiny and clean.
The wafer switch connected to the Antenna Selector can be cleaned by spraying contact cleaner in the holes, then rotating through all positions.
Step 6 Replacing Compromised Resistor
It is important to find a replacement resistor as close as possible to the value of the original resistor, otherwise, it could affect the calibration of the SWR meters. De-solder then measure the resistance of the original compromised resistor.
Resistors usually come in multiples of 10, 20, 50 or 100, all of the same value. Due to tolerance, a resistor's value can be slightly off. Choose a replacement resistor that most closely matches the resistance of the original.
Insert replacement resistor through the appropriate holes in the printed circuit board then solder in place. Clip the excess leads.
Step 7 Re-solder the Inductor connection to the printed circuit board
Insert the Inductor connection through the appropriate hole in the printed circuit board then re-solder in place. This is where the arcing occurred so make sure you scrap off the carbon from around this connection as is could be conductive.
Step 8 Re-mount printed circuit board to chassis
The circuit board is held in place by three screws and standoffs. Make sure the 30/300 Watt Selector button is properly protruding out of the front of the enclosure.
Step 9 Re-mount the T Network
Place the T Network components that are wired together back in the enclosure. See area outlined in red in the picture.
The shafts to the Transmitter Matching, Inductor Selector, and Antenna Matching controls are to be insulated from the chassis, make sure you re-install the fiber washers!
Install the metal washer and nut onto Transmitter Matching, Inductor Selector, and Antenna Matching shafts, carefully tighten.
Use multimeter to check for continuity between nut on Transmitter Matching shaft and chassis. There should be no continuity.
Use multimeter to check for continuity between nut on Inductor Selector shaft and chassis. There should be no continuity.
Use multimeter to check for continuity between nut on Antenna Matching shaft and chassis. THERE SHOULD BE CONTINUITY
Re-mount Air Inductor to chassis, you will need to use needle nose pliers to hold the nut in place while you thread the screw in from under the chassis.
Step 10 Re-solder connections to Printed Circuit Board
Re-solder the connection between the printed circuit board and the Transmitter Matching variable capacitor.
Re-solder the connections between the printed circuit board and the Twin Lead, Antenna, Coax1, and Coax2 connectors located on the back of the chassis, see areas circled in red in the picture.
Use a multimeter to check for continuity between the center pin of the Antenna connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.
Use a multimeter to check for continuity between the center pin of the Coax1 connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.
Use a multimeter to check for continuity between the center pin of the Coax2 connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.
Step 11 Re-install Knobs
It is important to re-install the knobs so that they point to the proper position.
I made sure I clicked though all positions to check alignment once the Antenna Selector knob was installed.
The Transmitter Matching and Antenna Matching Knobs are properly installed if the air capacitor's plates are fully meshed when at position 6.
Picture of air capacitor's plates fully meshed. See areas circled in red.
The Inductor Selector knob should be pointed to A on the front of the Antenna Tuner when the center ring is touching the contacts to right of the bottom bolt that holds the rotary switch together.
Step 12 Cover Install
There are three screws on each side of the MFJ-949C that need to be installed.
In addition, two screws on the top of the cabinet need to be installed as well.
Conclusion
Manual Antenna Tuners, such as the MFJ Industries MFJ-949C Versa Tuner II, are pretty simple to service as long as you keep organized and don't loose any parts during the process!
The Trav-ler Model 5020 I had recently acquired had a fair amount of rust on the edge of the chassis.
Here is a picture of the rust from a side view.
The rust was caused by a leaky 4.5Volt Zinc-Carbon "A" battery left in the case by the previous owner.
The chemicals in this vintage Ray-O-Vac "A" battery had leaked through the cardboard case then attacked the steel chassis where it made contact.
The first step to chassis rust repair is to give yourself some room to work. I removed the vacuum tubes in the near proximity. I later decided to unsolder the red and blue wire that connects a AF (Audio Frequency) amplifier tube socket the audio transformer then pull the wires out of the grommet. Then the grommet was removed as there was rust around the feedthrough hole.
I started with a Dremel with a grinding wheel installed to remove as much of rust as possible. I then sanded the rusted area starting with coarse sandpaper finally finishing with a fine sandpaper. 
Here is a closer look of the sanded edge from bottom view.
Below is a picture of the area masked off ready for paint. I used denatured alcohol to degrease area before paint.
I use silver Dupli-Color automotive rim paint as it is the closet color I can find that matches the natural color of most chassis. Apply in light coats and feather it out so as to make a natural transition from the newly painted area to the chassis color.
Automotive Rim Paint on Amazon!
Repair job complete! Time to install a new grommet in the hole then thread the audio transformer through the grommet and solder to the AF (Audio Frequency) amplifier vacuum tube socket. The final step is to install the vacuum tubes into their respective sockets.
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Who Writes This Blog?John is an IT professional from Cleveland, OH who enjoys amateur radio, ham radio, metal detecting, Archives
March 2021
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