The Story
In 1986 I was in my first year of a two year T and I (Trade and Industry) Electronics vocational program at my high school. The T and I Electronics program was offered to Junior and Senior high school students and took a sizable chunk of the school day. At the end of the program you received a certificate indicting you completed X number of hours of vocation electronic training. The first year of T and I Electronics was centered around making you a competent electronic technician. As with many electronic vocational programs, the building of an Graymark Model 536 AM Radio kit was one of the tools used to teach you soldering, troubleshooting, and alignment skills. As there was a lot of material to cover the first year of T and I Electronics program. We were only required to get the Graymark AM Radio working in breadboard format pictured above. You will notice initials on the breadboard, these are of the instructors as he had to qualify the build of each section of the AM radio, including measurements and conclusions before allowing the student to continue with the project.
Printed Circuit Board and Cabinet Assembly
My Graymark AM Radio kit has been in breadboard form for a little over 30 years! I have chosen to complete the build by installing the components on the provided printed circuit board then mount it in the cabinet.
 Graymark Manual
The Graymark Manual was very well written. It is more than just an assembly manual and provides students with information about proper soldering techniques, information about radio waves and the evolution from crystal set to that of that of a Superheterodyne radio that is to be built in this kit. In addition, it covers proper testing and troubleshooting techniques. The manual is broken down into sections corresponding to a block diagram of the AM radio. You start with the build of the Audio Amplifier, then build the Audio Preamp, Detector, IF Amplifiers, and Mixer/Local Oscillator sections. Each section starts with soldering the required components to the breadboard. The breadboard indicates proper electronic component placement and interconnectivity with other components. Next the manual provides a procedure on how to test the section just built including fields to record important data. Finally there is a quiz section to reenforce what was learned with the answers in the back of the manual.
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Printed Circuit Board Prep
After 30 years, a tarnish had built up on the copper pads of the printed circuit board making it impossible to get solder to flow on the pads. The solder would just ball up and a thin layer of flux would separate the solder from the copper pad. I used a pencil eraser to remove all of the tarnish then I removed the pencil eraser residue with Windex glass cleaner. Below is a picture of the printed circuit board ready to accept solder.
 Electrolytic Capacitor Replacement
As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace electrolytic capacitors that are over 30 years old!
Electrolytic capacitors should be replaced with one of the same or slightly greater capacitance and working voltage rating. Electrolytic Capacitor Kits on Amazon!
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Audio Amplifier Section
One of the first steps in the Audio Amplifier section is to mount the speaker inside the front cabinet. I had to improvise and use a modified solder lug as a speaker holder due to some small parts of this kit had been lost over the years.
Picture below are the components of the Audio Amplifier section mounted to the printed circuit board and soldered on the copper foil side. I had to dig in my junk box to find the screws and nuts used to mount the volume control potentiometer.
Testing of the Audio Amplifier section was pretty straight forward, the first thing that needed to be done is to solder the speaker wires and 9 Volt battery clip to the proper copper pads on the printed circuit board. You then connect a 9 Volt battery and power the Audio Amplifier section on by rotating the volume control to the ON position. The manual had you introduce AM hum into the input of the Audio Amplifier circuit using a screwdriver and your body as a natural antenna then listen for the sound from the speaker. I chose to use an my home brew Audio Frequency Generator to provide the input signal. Upon a successful test of the Audio Amplifier, you unsolder the speaker leads and move on the the Audio Preamplifier Section.
 Audio Preamplifier Section
Pictured below is the printed circuit board with the addition of the components for the Audio Preamplifier section. Each component was soldered to pads on the copper foil side.
Testing of the Audio Preamplifier section was similar to that of the Audio Amplifier section.
Once again I used my home brew Audio Frequency Generator to provide the input signal but this time to the Audio Preamplifier section. This time the audio was significantly louder at the same volume control setting as a new amplifier section was added to the circuit.  Local Oscillator/Mixer/Detector Sections
The epoxy that held one of the windings on the loopstick antenna became brittle and fell off causing a winding to come off of the cardboard form. I rewound the winding and held it in place with a dab of hot glue.
In order to do any more testing, components for the Mixer, Oscillator, and Detector sections need to be installed and soldered to the printed circuit board. We can now test as a full AM Radio with these sections in place.
Testing of the Mixer, Oscillator, and Detector sections of the AM Radio is accomplished by adding a .02 mfd coupling capacitor from the output of the Mixer/Oscillator circuit to the input of the Detector circuit. The Mixer/Oscillator circuit takes a AM radio station signal and converts it to a 455Khz AM (Amplitude Modulated) signal. For testing we are feeding this 455Khz AM signal into the Detector circuit that converts it from a Amplitude Modulated RF(Radio Frequency) signal into a weak audio signal which is then amplified by the Audio Preamplifier and Audio Amplifier circuits. As you rotate the tuning dial you should hear at least one strong radio station from the speaker.
 IF Amplifier Sections
The Graymark Model 536 AM Radio incorporates two IF Amplifier sections. The manual has you assemble each IF Amplifier section on the printed circuit board then test. Below is the finished printed circuit board with Mixer, Oscillator, IF Amplifiers, Detector, Audio Preamplifier, and Audio Amplifier sections installed.
The IF Amplifiers are special RF amplifiers designed to amplify only 455Khz signals coming from the Mixer/Oscillator section. The amplified IF signal is fed into the Detector section. Testing of the First IF Amplifier is accomplished by using a .02mfd capacitor to couple the IF output of the First IF Amplifier to the input of the Detector circuit. You should hear a couple AM stations out of the speaker when the radio is powered on.
 The Problem
I installed the components for the Second IF Amplifier onto the printed circuit board and then soldered them to the copper pads. At this point this should be a fully working AM Radio! I connected the battery then turned the volume control to the ON position. I got a strange squealing sound from the speaker instead of getting the hiss of atmospheric noise or the voice of an announcer from an AM station. There was something wrong with the Second IF Amplifier section of the radio at it was time to troubleshoot. After close inspection of the copper circuit side of the printed circuit board I discovered that one of the leads of an IF Transformer in the Second IF Amplifier section had a cold solder joint and wasn't making a good connection. This was easily cured with the solder iron and the addition of new solder to the joint. Success! I could immediately hear an AM station once the power was applied to the radio.
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Alignment
Each IF Amplifier is coupled with a transformers that has an adjustable Ferrite slug. The Ferrite slug allows you to adjust the transformer's frequency response and that of the tuned amplifier circuit. In addition, the oscillator circuit has a transformer with an adjustable Ferrite slug used to tune the circuit to exactly 455Khz above the frequency on the tuner dial. The last two adjustments are located on the back up the tuner capacitor, they are the Antenna Trimmer, used adjust the antenna to resonance in the middle of the standard AM broadcast band and the Local Oscillator Trimmer which is used to adjust the oscillator circuit for the high side of the AM broadcast band.
I use an old Eico Model 324 Vacuum Tube RF Generator for alignment purposes. I let it warm up for a least a half hour before using it at its frequency output tends to drift before being fully warmed up. The tuning dial on the Model 324 is not very accurate so I also use a Yaesu FRG-7700 Shortwave Radio Receiver (perched over my workbench) to accurately set the dial on the RF Generator. I set the FRG-7700 to the desired frequency then vary the frequency of the RF Generator until I can hear the 400Hz Amplitude Modulated signal from the RF generator on the FRG-7700. Dead silence is heard from the FRG-7700 if the RF Generator is sending an unmodulated carrier only. Below is the order of the alignment tasks: 1. IF Alignment - A 455Khz unmodulated carrier is injected into the input of the 1st IF Amplifier using a RF Signal Generator. An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Ferrite slugs of the IF Transformers are adjusted for maximum amplitude on the Oscilloscope. 2. Oscillator Alignment - The tuning capacitor is rotated fully counter-clockwise. A 540Khz amplitude modulated carrier is injected into the input of the 1st IF Amplifier using a RF Generator. An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Ferrite slug of the Oscillator Coil is adjusted for maximum amplitude on the Oscilloscope. The second part of this procedure involves rotating the tuning capacitor fully clockwise and then injecting a 1600Khz amplitude modulated carrierinto the input of the 1st IF Amplifier using a RF Generator. An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Local Oscillator Trimmer on the back of the Tuning Capacitor is adjusted for maximum amplitude on the Oscilloscope. 3. Antenna Alignment - This alignment procedure is done without any external test equipment. The Tuning Capacitor is rotated fully counter-clockwise then rotated clockwise until the first AM station is heard. The antenna coil is then moved along the Ferrite core for maximum station loudness. In the second part of this procedure, the Tuning Capacitor is rotated midway through its full rotation until an AM station is heard. Then the Antenna Trimmer on the back of the Tuning Capacitor is rotated for maximum station loudness.  Final Assembly
You can see all of the solder flux residue on the copper clad side of the printed circuit board from soldering the components in place. Very unsightly and unprofessional!
I use a spray can of Flux Remover and an old toothbrush to remove the solder flux. Pressurized air from my air compressor quickly dried the printed circuit board. Be sure to use the Flux Remover in a well ventilated area. I typically deflux outside during the warm Ohio months.
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Time to mount the tuning knob. It is held to the shaft of the Tuning Capacitor with a small machine screw.
It is important to properly align the tuning dial faceplate. The number 53 should be pointed towards the Ferrite loop antenna when the Tuning Capacitor is rotated fully counter-clock wise.
Now its time to mount the thumbwheel for the Volume control. The thumbwheel is held to the shaft of the Volume potentiometer with a small machine screw.
There is a groove on the edge of the thumbwheel that indicates the OFF position. This groove should be pointing away from the circuit board when the Volume potentiometer is rotated fully counter-clockwise. 
I install the self tapping screws into the standoffs in the front cabinet before mounting the printed circuit board and then remove them. This will make it a little easier to screw them in, and its reduces the chance of the screwdriver slipping and damaging components on the printed circuit board during final assembly.
When I test fit the printed circuit board in the front cabinet, I had a alignment problem with the Volume Control thumbwheel making contact with the opening. See area circled in the picture below.
I used an Xacto knife to trim the Volume Control opening in the front cabinet until the thumbwheel no longer made contact.
The face of the tuning knob was also making contact with the front cabinet when the printed circuit board was mounted inside. I used a file to remove some of the plastic from the opening until the tuning knob was no longer making contact.
The printed circuit board is held in place inside the front cabinet with four self-taping screws circled below. I had to substitute small wood screws as the original screws had been lost years ago.
The rear cabinet of the AM radio snap fits to the front. It was impossible to get the rear cabinet snapped in place until I used an Xacto knife to trim the tabs.
Here is a picture of my finished Graymark Model 536 AM Radio, only took me 30 years to complete the job ;-)
 My Graymark Model 536 AM Radio in Action! |
Jumat, 24 Mei 2019
My Graymark Model 536 AM Radio
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