AM Transmitter Build Noise Help - Page 1
Forum section: electronics / projects

author: Spankyty
author_rank: Newbie
timestamp: February 16, 2025, 10:41:43 pm
content: Hello, I recently restored my grandfather's radio. It is a Philco 38-12. There are no stations that have music or era-specific music for the radio. I fell into a rabbit hole in finding the best AM transmitter. It seemed to be the SSTRAN brand. They don't make it anymore so I decided to borrow the schematic and create my own. The problem lies in my schematic or PCB layout. When I turn the gain, mod, and compression pot all the way clockwise I can hear things! But........I am getting a lot of static and noise. I could just barely hear the music. There is some range to it though. I can hear it at 30 feet away in my car. It's just the static and noise that won't go away! I tuned to peak output with the variable capacitor, the 8, & the 4 dipswitches for 1MHz.The difference between the sstran and my borrowed schematic is that they used RCA white and red jacks. I thought 3.5mm jacks were line level just like RCA? I want to connect my phone or any MP3 player to this.I tried different power bricks, and they all do the same thing. I didn't think it would make a difference since it gets regulated anyway.Attached is the Schematic & PCB I made. I don't think there are any errors on the schematic.. but let me know! I think my issue lies in the PCB layout.Here are a few datasheets and the Gerber not that anyone wants to make this broken thing.https://drive.google.com/drive/folders/1VOuO5fYKRck24-3XtJHuropZIGxAtofv?usp=sharingThis is my second attempt at the PCB layout. My first was through-hole. I like the look of this SMD one better.Thank you if anyone has recommendations or finds something wrong let me know! I want to learn!

author: Analog Kid
author_rank: Super Contributor
timestamp: February 16, 2025, 10:49:31 pm
content: Quote from: Spankyty on February 16, 2025, 10:41:43 pmSorry can't help you, but:1. That's not a schematic; it's a circuit board diagram. Not the same thing. A schematic is a representation of the circuit, not a picture of where the components go on a circuit board.2. If you do want to post an actual schematic, better to just attach it to the post directly so it can be just clicked on, rather than having to download a PDF.Someone who can help you will be along shortly ...

author: moffy
author_rank: Super Contributor
timestamp: February 16, 2025, 11:34:32 pm
content: You will need a scope to look at the signals and the bias levels. First thing would be to bypass the compressor chip and feed audio into the modulator stage and see what happens. That is the most crucial stage as it combines everything together and you need to make sure the transistor bias levels are ok.

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2025, 12:30:27 am
content: https://www.eevblog.com/forum/chat/please-rate-my-designschematic-ltrantgt/

author: Analog Kid
author_rank: Super Contributor
timestamp: February 17, 2025, 12:43:58 am
content: This ^^^Nothing personal, but Ihateschematics drawn like that. I'm not going to bother mentally chasing labeled connections all over the place to see how it all fits together.Please draw this in the conventional way, with everything (as much as possible) connected together. It's OK if one or two things are left hanging, but please don't make us scramble our little brains trying to understand your circuit.

author: shabaz
author_rank: Frequent Contributor
timestamp: February 17, 2025, 02:57:13 am
content: Hi,Here's a better approach than your current one, that should work:You've not mentioned your source schematic, but it looks like it may be the one in theAM Transmitter Kit PDF doc here.The diagram is laid out differently in the PDF doc, and it would be good to stick to a similar layout in your replicated design, otherwise you're making life difficult for yourself when it comes to comparing and searching for errors.Where your layout has a slight edge, is that you've titled different sections of circuitry. You can still do that while maintaining the layout the same as the circuit in the PDF. Just put text titles close to different areas of the layout. You could also assign names to some of the signals if that helps understanding; your software will allow assigning that as a net label that can be displayed against the wire.Another point: your circuit appears to be missing some things; for instance, there's a filter that's supposed to be on the output, and I can't see it in your schematic. There may be other differences but it's hard to tell with the current layout. My suggestion is to keep all parts in the design, you can always patch it or use wire links if you decide you don't need all parts, e.g. if you decide you don't want to attach an external DC voltmeter like the circuit has shown.If you are able to do so, it's possible to simulate some portions of the circuit. There's a bit of a learning curve though; software called LTspice can be used. It's understandable if you're unable to do this as a beginner for now, and prefer just testing portions of the circuit for real.One you have the schematic entered in your CAD package, you could ask for a review; same with the PCB layout.Regarding schematic layout and PCB design, I don't know what software you're using, but if you use a popular free one (KiCad) then others can also go in and edit things; personally I'd prefer GitHub over Google Drive, but it's your decision; everyone has their preferences. If you're new to KiCad, you can pick it up within a few days. Try this(50 minutes) it contains everything you'll need.You'll also need at least some tools when it comes to troubleshooting. A 'scope is highly recommended (just noticed this has been mentioned above too) for the level of complexity of this project (it's still well within beginner capability however). It doesn't need to be an expensive 'scope; you could search ebay for one or borrow one (maybe some hackspaces have ones you could use), or purchase one for a few hundred $ (and then maybe sell it if you find you're not doing as many projects as expected).

author: Analog Kid
author_rank: Super Contributor
timestamp: February 17, 2025, 03:49:36 am
content: Here's the schematic from that PDF in case anyone wants to see it:

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 17, 2025, 02:25:36 pm
content: If you have restored the old Philco, you are doing a really good job.Since you have done so well.....How about getting an old RCA (or other brand) RF generator. (I had a RCA SENIOR) Some of them have connections for audio modulation, so you could put any audio into it. They do not put out much power but if the units are close to one another and you have a few feet of "antenna" on both units, it will work. Any RF generator that has capability for AM modulation should work.These things are very cheap and still useful.

author: Spankyty
author_rank: Newbie
timestamp: February 17, 2025, 03:38:29 pm
content: Quote from: moffy on February 16, 2025, 11:34:32 pmMy voltage regulator is giving out 14.8V not 15V, do you think that would be an issue...?

author: Spankyty
author_rank: Newbie
timestamp: February 17, 2025, 03:49:06 pm
content: Quote from: Benta on February 17, 2025, 12:30:27 amQuote from: Analog Kid on February 17, 2025, 12:43:58 amThis is good information. I appreciate the honesty. It's something to improve on.Quote from: shabaz on February 17, 2025, 02:57:13 amI do have a scope, I'm scared to blow it up lol. Also, I am unaware of the filter you say is missing. L8? I see that this schematic format can be confusing now. More things to work on. I use EasyEDA only because I like that I can access it anywhere and I like the look of it. I really want to use KiCad, but I don't like the looks and it feels messy. That's just me. I'll have to get used to KiCad.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 17, 2025, 08:54:04 pm
content: You could download LTSpice and use it for free. If you don't want to take the time to learn how to use it as a simulator (very useful but takes some effort), you could still use it just to draw schematics. It's intuitive enough that you could be producing a drawing in a very short time.I know nothing about EasyEDA, which you say is your preference. Any way to make it draw a conventional schematic, where things aren't put into little boxes but made into a single drawing?

author: moffy
author_rank: Super Contributor
timestamp: February 17, 2025, 09:29:25 pm
content: Quote from: Spankyty on February 17, 2025, 03:38:29 pmNo, that is well within +/-5%.

author: Spankyty
author_rank: Newbie
timestamp: February 20, 2025, 02:36:48 am
content: OK, I redrew these schematics without so many nets. Only going to different pages there is a net. Is it looking better? I know this schematic works from sstran.https://ia800409.us.archive.org/21/items/Transmitters-Antennas/TX%20TRANSMITTERS/AM%20TRANSMITTER%20-%20SSTRAN%20AMT3000.pdfThis guy built one from scratch too.https://aa7ee.wordpress.com/2024/12/23/micropower-am-broadcasting-a-scratch-built-sstran-amt3000/The next step is to make the PCB layout. V3 lol. Which is where I think the noise originates. Each side should have a ground plane? These two planes are connected with a bunch of vias as well. I want to keep the power tracks bigger than the rest. Thank you guys!

author: aeg
author_rank: Regular Contributor
timestamp: March 11, 2025, 12:20:00 am
content: As moffy pointed out, oscilloscope measurements would be helpful. What is the audio signal amplitude at J1? At U1 pin 13? At Q2 collector? What is the amplitude and depth of modulation at SW3 pin 1? Do the waveforms look right?Also... what are you using for an antenna?

author: jwet
author_rank: Frequent Contributor
timestamp: March 12, 2025, 03:11:03 pm
content: You've done a pretty nice job of this- I think you've captured all the pieces that matter and it should work pretty well. The problem might be as simple as the noise on the power supply- even though you're regulated, any high frequency noise on the power input will come right through and onto the air- this is one of the problems with AM TX is they need pristine rails. You mention a "brick" which is probably a noisy switcher? Can you power it from a quiet linear supply or a battery as a test. I suspect this the problem. Once you know this, you can make a linear power supply.In the original circuit, they had all those L's and C's up front for HF noise.