Red Sox Radio

Thread Starter

BobTPH

Joined Jun 5, 2013
2,056
Red Sox Radio

Okay, it's the off season, so I had some time. I finally finished up putting a project I had made a couple of years ago into a custom 3D printed enclosure.

It's an AM radio that is permanently tuned to WVEI Worcester MA, 1440 KHz, which broadcasts the Boston Red Sox games.

Here is the very retro looking finished project.

CroppedFullSmall.jpg

Here it is with the custom enclosure opened up.

OpenedSmall.jpg

Here is the schematic.

Schematic.PNG

I used the MK484 tuned RF AM radio chip, which I found to be very nice. Using just a single tuned circuit of the antenna with a capacitor, plus a variable trimmer, it is actually able to pick out 1440 from a Boston station on 1430. Though, sometimes I have to rotate it get the help of the directionality of the loopstick antenna.

The MK484 has a 3 stage RF amplifier covering the AM broadcast band, a detector and an AGC all built into a single TO92 3 pin package! I followed that with a single gain stage and a class AB output stage, all using 2N3904 transistors.

The antenna is 50 turns of enameled wire on a 10 cm x 1 cm ferrite stick.

The speaker is about 3 inches, salvaged from a boom box. It won't set any records for output power, but it can be heard reasonably in a quiet room.

Power is from a 9V rectangular battery, with 1.8V for the MK484 coming from the drop across 3 1N914 diodes. The MK484 is designed for 1.5V operation.

I printed the parts for the enclosure on my Prusa I3 clone printer in two colors of PLA. Used Autodesk Fusion 360 (with free hobbyist license) for the 3D design. I digitized the Red Sox logo by printing it, cutting it out, and pasting on my screen in Fusion. I then clicked around it with the mouse to create splines for the curves. Took me a while to figure that one out!

The schematic and board were developed using BobCAD, a schematic capture / PCB layout program I wrote myself because I hated all the options I had tried. The board was DIY using a pre-sensitized board and UV exposure from fluorescent light.

A lot of fun. Can't wait until spring season to start!

Bob
 
Last edited:

DickCappels

Joined Aug 21, 2008
5,950
What is the purpose of R3?

The MK484 is truly a wonderful chip, the rebirth of the ZN414 it is also useful as an IF amplifier for audio receivers.
 

Thread Starter

BobTPH

Joined Jun 5, 2013
2,056
What is the purpose of R3?

The MK484 is truly a wonderful chip, the rebirth of the ZN414 it is also useful as an IF amplifier for audio receivers.
R3 is the AGC feedback. The DC level of the output is related to signal strength and goes back to the input tank circuit. Don't know why this works as AGC, but it does. Quite amazing to get this functionality on 3 pins!

Could you rephrase or explain?
I used 3 forward biased diodes to give me a semi-regulated 1.8V to power the MK484. It uses very little current. It is basically the same as a Zener regulator. 1.8V was within the operating range and it actually works a little better than on the 1.5V that is usually used for a single cell earphone output radio.

Thank you both for your interest.

Bob
 
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