MP3 Player for Volvo AM Radio by SW-EM Guest Contributor David Smith [Comments added, Ron]

AM radio being the vast wasteland that it is, I built this MP3 player so I could play my vintage music collection through the original Volvo Bendix AM radio in my 1967 1800S. 

This system is based on a clever concept of operating an inexpensive 1.000MHz reference oscillator (normally intended for a steady input power voltage of 5V, and normally putting out an unvarying reference frequency for use in electronic systems like PCs) under-voltaged at an audio-modulated 3.3V.  It turns out that when operated in this manner, (definitely outside its' normal operating mode) the audio signal modulating the module power effectively modulates the amplitude of the base frequency, which coincidentally, happens to be the way Amplitude Modulation works(!) ...and VOILA, you have a miniature, low power  transmitter, operating in the middle of the AM broadcast band! 

A few details must be worked out to assure voltage and signal levels allow the whole thing to function as intended when built into a car, but this "hack" is a workable solution within the technological and financial grasp of any hands-on electronics projecter-type who has ever built an electronics kit, and who knows which end of the soldering iron to hold.  Ron

Here is the wiring diagram I shamelessly copied from the YouTube video "AM transmitter for antique radios and other project updates" that got me started.

Link:  https://www.youtube.com/watch?v=YNGs-IX_B2s&

I headed to eBay and got the following components for less than $20 in 2017 (with the exception of the oscillator which came from Digikey – but I’m sure it can be found on eBay as well for about the same price):

The following is a picture of David's info with inactive Links.  Active Links can be found below. 

...here are the components and current links (as of 6/2020):

 

When the MP3 player arrived, naturally it looked a bit different than the one in the video: 

 

So I had to figure out what capacitor(s) I could connect to. After carefully tracing the connections I determined that these were the candidate capacitors (boxed in yellow) and the ends to which I could solder (black circles):

Closer view: 

 

 

Now I could get started laying out the components so they would fit into a project box with the MP3 player as a front face. I placed the items on a piece of perf board as shown and started soldering:

 

The Motorola cable is cut in half and the outer insulation jacket stripped off. The woven copper shielding is the ground and the center core is the signal.

Connect the ground wires together and solder to common ground on the board and connect the signal wires together with the capacitor that will go to Pin 8 on the oscillator.

Cut the project box to accept the MP3 player and mount it with two countersunk screws.

 

Drill and cut holes/slots for the power and antenna cables. I mounted mine to the bottom of the radio with adhesive backed Velcro tape. Unplug the antenna jack from the radio and plug it into the MP3 player and the other cable into the antenna jack. I made a splitter and attached it to the back of the fuel gauge and used this to power radio and the MP3 player. I attached the ground wire to the radio case with a self-tapping screw.

Insert a SIM card with your pre-loaded music files. Turn on the radio and tune it to 1000 kHz and ensure you’ve got a good signal. Power on the MP3 player and (if you’ve got everything soldered up right) it will override the station and you should hear it start playing your tunes. The player comes with a remote so you can skip, pause, mute, or switch to FM band.

YouTube References:

AM transmitter for antique radios and other project updates

https://www.youtube.com/watch?v=YNGs-IX_B2s&fbclid=IwAR322BuRwO0yR8IQlzihX3C1CPiFOC_-g33CBQ119zlYd7-clmZQmVXDamI

AM & FM antenna repair 2

https://www.youtube.com/watch?v=w2BtIA8LAIs

Commercially available equivalent for price and design comparison (the YouTube transmitter video above also shows how to make this version if you want to be able to scroll through your device to select specific music files, but I wouldn’t recommend that – it takes your eyes off the road):

https://redirad.com/

Thanks to David Smith for compiling and contributing this information.  Questions, complaints and praise should be directed to him at:  1800stig AT gmail DOT com   If you find it helpful, please e-mail back with results and comments.

 

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