Introduction: Adding Auxiliary Input (for an Mp3 Player) to an Old AM Truck Radio

Picture of Adding Auxiliary Input (for an Mp3 Player) to an Old AM Truck Radio
This is a very simple hack that will allow your older radio to play from an auxiliary input.

In my case, I am using the radio from my '66 Ford truck and using the input to listen to music from an mp3 player. The radio is about as simple as it can get: AM only, mono output but this can be done for stereo just as easily (more easily in fact!).

Here is a quick video of it in action:



The parts count is fairly low:

any RCA jack with 2 NC shunts: (sourced locally at All-Electronics but available online here for $0.50)
Hookup wire: (I had this on-hand; about 2-3 feet is plenty)
Optional 80-220 Ohm resistor (this is only necessary if your stereo is mono)
Optional enclosure to hold the jack

Tools:
Soldering iron and solder


Step 1: Fix (Optional)

Picture of Fix (Optional)
My radio required some simple coaxing to get it working properly before I could begin hacking it. It powered on, but had a VERY bad hum that threatened to destroy any speaker connected to it.

Doing my diligent google research, I was unable to find any schematics online and only the tiniest bit of background information on my radio at all. Luckily, you do not really need schematics to do this hack (but it could make things simpler). This site is a decent resource for information on caps and radios for those that are interested.

In general, the simplest first attempt is to replace all electrolytic and paper capacitors. These capacitors almost always go bad in old radios because, over time, they dry out and their values stray. Replacing them is easy and cheap enough so this can be done very quickly.

In my radio, there are no paper capacitors and only two electrolytics. I used google docs to make an extremely crappy picture with their positions (electrolytics are polarized don't forget!) and their values. I replaced the caps, did a few other minor things, and it worked! The power cap in the picture was not replaced.

Here is a little video of it functioning:




Step 2: Background Information

Picture of Background Information

The first point of order is to understand your radio. If there are schematics available for your radio get them! It will take a lot of the guesswork out of this.

I was unable to find schematics for my radio but it is simple enough to generalize. Older radios are extremely simple and mine is no exception; it is an AM mono radio.

The key component, as far as auxiliary input is concerned, is the pot. The pot is typically broken up into three distinct functions:
1) On/Off
2) Volume Control
3) Tone Adjustment

  The middle of the two which governs the volume (see the closeup, picture #3 in this step). There are three prongs coming out of the pot. The middle connection, called the wiper, is wired directly to the amplification stage. This wire will be left untouched. The remaining two prongs are connected to the source (antenna in this case) and ground.

To be able to tell which prong is connected to which, position the radio so that the knob is pointed towards you and the prongs are pointed downwards towards the floor. In this situation, turning the knob clockwise increases the volume which means that the source is connected to the right prong (B in the picture).

Last is to map out the pins on the RCA jack. Using a multimeter, I mapped out all pins on the jack in the two situations where an external source is plugged in and when it is not (see last picture in the set).

Step 3: Cut, Solder, and Done!

Picture of Cut, Solder, and Done!

The modification that is required is really simple and can be quickly done. All that is being done is to add the RCA jack in between  the source and the potentiometer. Using the pin numbering scheme on the previous step:

Disconnect the wire that is currently the source on your potentiometer. This will now connect directly to the RCA jack.

Pin 1 will go to the ground leg of the potentiometer

Pin 2 will go through a resistor to the source leg of the potentiometer. The resistor value can be anywhere between 80 to 220 Ohms and will allow you to play a stereo source on a mono radio without loss of sound.

Pin 3 will go to the tuner (it will be the wire that was previously connected to the tuner)

Pin 4 will be unconnected if you have a mono radio like I do.

Pin 5 will go directly to the source leg of the potentiometer.

Slap it all in some sort of enclosure and you are done!

Notice, I cheated a bit because my pot is mounted directly onto a greenboard. I wanted to attempt this without making too many changes to my existing setup in case it did not work so I ran the RCA jack as a source in parallel with the tuner. The end result is that it functions but could end up having some problems with running two sources like this.

Now that I know it works, however, I plan on removing the pot from the greenboard and adding more wires to hook this all up properly. I'll update once that is completed!

Much thanks goes out to AMCForums where this was originally posted by member nali. Even more thanks goes out to forum member gryzynx for the extremely thorough explanation and helpful replies. The original forum thread can be found in its entirety here.

Comments

3366carlos (author)2017-01-17

now you can install it in your car, nice.

crispy429 (author)2016-05-24

Hey, thanks for the tutorial! Quick comment on a hickup I had, I (incorrectly) assumed that my potentiometer would have the ground leg in the center and it was acting funny. With the audio jack grounded, no sound (both when the audio cable was plugged in and unplugged), but when I removed the audio jack from the radio case, it was faint. When I touched either pin 2 or 5 to ground, it would work perfect. Just a heads up for anyone else attempting and having this issue! Thanks again!

IkeA2 (author)2016-04-22

Hello. Old post, but I'm hoping to get a response.
I have a 1979 dodge truck with a stereo AM radio with 8track. Would it be possible to disconnect the 8track and use that input for the auxiliary?
Any help would be appreciated.
Great tutorial, all the same!

FrankB73 (author)2016-03-01

I just made this modification to the AM radio in my 1963 Plymouth. Works great! Many thanks for the tutorial.

yabbadabba du (author)2015-07-24

very informative article. however i'm trying to add an audio OUTPUT in an Alpine audio out of a car. Is that possible?

JeremyK3 (author)2015-02-01

Just finished this modification! I did it on the am/fm on my '79 f100, and it works quite well! I just need to find a spot to mount the aux plug, and I'll be good to go! Thanks for making this easy to understand!

baskinator4 (author)2013-02-23

Thanks for the great write up! I just attempted this on my AMC Eagle's am/fm stereo and am having a couple issues. For reference, I used the same panel mount jack as you.
-My radio does not cut out when the jack is plugged in, I thought this would be the case
-The aux only plays music out the right speakers, but the radio still plays everything correctly

Since mine has stereo, I assumed I could just wire pins 2 and 5 without a resistor. Don't know if this is correct or not, but both pins go directly to the source leg of the volume pot. Pins 3 and 4 are both connected to the radio input wire, and Pin 1 is connected to the ground leg of the pot. Any suggestions?

Phil B (author)2011-03-05

Image #2 in step 2 does not load. There is an error message that says, "Image not found." The notebook drawing of an electrical plug in image #6 of step 2 is labelled "RCA plug," but shows a 1/8 inch stereo plug. The attached photo shows a male and a female RCA plug. I am a little unclear if the pin numbers are printed on your radio's green circuit board, or if you assigned those numbers. This was a very popular type of project at Instructables a couple of years ago. It is less frequent now, perhaps because many cars in the last couple of years come with auxiliary jacks built into the vehicle. Thank you for sharing.

nonoodlez (author)Phil B2011-03-05

I went over the images in step 2 and they all seem to load fine. I wonder, is there is anybody else that can confirm whether the images are loading properly or not?

You are correct on the RCA comment. I meant mini-RCA. I'll correct it and repost in a bit.

Concerning the numbering, I did not have a datasheet on hand so those are numbers I just came up with to clarify things for myself.

Just for you to know, this was for my 66 Ford truck and this hack would be particular to this era of radios. This is really a brute force approach. If I am not mistaken, later radios (i.e. 80's and 90's) are a bit more difficult to patch into thus requiring a different approach.

Thanks for the comment!

NGinuity (author)nonoodlez2012-11-10

Not to nitpick (I suppose I am though) but just a minor clarification. That's a "TRS" connector, not an RCA connector, full or mini. TRS stands for tip-ring-sleeve and can have a common slang name of "stereo jack".

nonoodlez (author)Phil B2011-03-05

Wow! I just noticed that you are quite the contributor!

Phil B (author)nonoodlez2011-03-05

When I was 27 years old there would have been no way for me to submit many Instructables. But, I turn 65 in less than two weeks. I have had a lot of time to learn about a lot of things, usually the hard way. Some of them are worth passing on. I hope somebody benefits from them. Often people who look at my stuff find ways to improve upon what I did.

OldHarley (author)2011-08-08

In order to get my AM radio working again, I replaced the electrolytic capacitors just like in nonoodlez diagram above; however, my local electronics store informed me that both the 50uf and 500uf capacitors are no longer made.

Instead, I used a 47uf and a 470uf, both rated at 35 volts and it works like a charm.

Incidentally, these capacitors have a (+) and a (-) side. Just be sure to orient the new ones like the originals. Both sides will be marked on the old and new, so it is easy to tell.

Thanks to the author for this article!

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