Building Elenco AM/FM 108CK kit - question about signal generator setup - Page 1
Forum section: electronics / beginners

author: Hank30721
author_rank: Contributor
timestamp: February 01, 2023, 05:46:55 pm
content: Hey folks…I posted this in the test equipment thread and got an email to post it here - sorry for a double post. I'll add some more details.I’m taking an online course building the Elenco AM/FM 108CK kit. A friend let me borrow a Koolertron CDJS98 (rebadged Juntek PSG9080) for the kit construction. He has only used it for generating waveforms for audio equipment - never used it for much else.The instructions for the kit say to set up the generator for a 1 kHz wave, 455 kHz am modulated carrier, at 80% modulation, minimum voltage (Vpp) and connect it to a test point on the kit through a 0.001uF cap. Then increase the generator’s amplitude (Vpp) until you hear the 1kHz tone from the radio’s speaker. So to set up the Koolertron, on the modulation menu screen, I set the depth to 80%, the modulation frequency to 455kHz and use the default sine wave. I switch to the waveform function screen and set the frequency to 1kHz and 5mV amplitude to start. Activate the channel and nothing - just some hum from the speaker at 15mV. I switch back to the modulation screen and now I can barely hear a tone. I switch back to the waveform page and the tone stops, I increase the amplitude just a few mV then switch back to the modulation screen and the tone starts again and is a little louder.For the AM detector test (I copied the instructions below) - I connect an oscope to see the waveform and get an amplitude reading from another part of the kit. The instructions say to increase the function generator's output until the oscope reads a certain value. Having to switch screens is a PITA and I keep overshooting then not enough signal. I am using the AWG correctly?Is there another way to set this generator up? I have used an older Heath RF generator and you could vary the amplitude in more real time - you activated the modulated carrier, set the tone frequency and dialed the amplitude up or down. The generator manual sux. I contacted Koolertron and they sent me a video showing to simply adjust the modulation depth to vary the Vpp. Is that correct for the kit? That is changing the modulation for AM carrier - shouldn't that be fixed?Any guidance is greatly appreciated.Instructions: AM DETECTOR BANDWIDTH TESTConnect your test equipment as shown in Figure 25.Set the generator at 455kHz with 80% modulation ata modulation frequency of 1kHz. Set the oscilloscopeto read 0.1 volts per division. Turn the power ON andset the volume at the minimum. Increase theamplitude of the generator until the signal on theoscilloscope is 4 divisions peak to peak. Check thesignal to make sure that it is free of distortion. Leavethe frequency of the generator at 455kHz, butincrease the modulation frequency until the outputdrops to 0.28Vpp. Record the modulation frequencyon the generator here:

author: Hank30721
author_rank: Contributor
timestamp: February 01, 2023, 05:54:21 pm
content: Here's a pic from the Elenco manual in case I missed something:This may all be for naught. The generator has stopped working altogether. My friend has contacted Amazon to check for warranty repairs. I still want to understand if I set up the test correctly. Was I even using the AWG correctly? I would still appreciate any comments.Per several suggestions here and from watching youtube videos, I have ordered a Sigilent SDG1032x to complete my project.

author: BillyO
author_rank: Super Contributor
timestamp: February 01, 2023, 06:59:38 pm
content: From you description of what you did, it sounds like you set it up as 1KHz being modulated by 455KHz. It should be 455KHz being modulated by 1KHz.Instead of:Quotetry:Quote

author: Hank30721
author_rank: Contributor
timestamp: February 01, 2023, 09:23:42 pm
content: Thanks for the reply! I have written the change in my notes in my book.I hope I didn't fry my friend's AWG?!

author: Hank30721
author_rank: Contributor
timestamp: February 02, 2023, 05:18:20 pm
content: Quote from: BillyO on February 01, 2023, 06:59:38 pmThanks again BillyO!The Sigilent SDG1032x arrived late yesterday. I finished the AM portion alignment with ease despite my lack of experience and old beat-up Tektronix TDS 340 oscope. It may be time for a new oscope (time to sell some plasma).I've attached a youtube video linkof the radio picking up KMOX last night from St Louis, MO - I'm located in NW GA. I was impressed with my own work - what a great feeling!On to the FM section....

author: RKanarek
author_rank: Newbie
timestamp: October 10, 2023, 02:09:44 pm
content: Greetings,First and foremost, belated congratulations to you on your success! While succeeding in building a radio to receive what is currently transmitted, particularly a radio without electron tubes, seems like a hollow victory, I'm sure its much better that the substantial defeats I'm used to. ;-)My main reason for posting is to inquire as to what online course you are participating in that requires the construction of a radio. It sounds terribly interesting. Is the course open to anyone? Can you supply a URL? Do they offer other courses? Is it engineering, technology, or just hobby focused? See, I said I was terribly interested.Thanks in advance for any info.Kind regards,RKanarek(First post after being a Pateron for years!)