Adding aux-in to an old car stereo

It's a bit pointless trying to have a decent audio in a convertible. If the exhaust system isn't loud (mine is stock, but not quiet), the wind noise will overwhelm basically anything that isn't ridiculously loud. A radio is a bit more usable when the top is up, even with the rear window down, and it's really nice to have in a modern drive-in when you have to tune in a local station to hear the movie.

Anyway, my car came to me with this sweet vintage tape deck:

It's a Kraco, model KID-585. These were pretty low-end universal replacement units from the i dunno, early 80s? It has a working cassette deck and AM/FM radio, connected to a pair of 4" Sparkomatic speakers mounted along the outer kick panels. 

Like most people nowadays, i'm spoiled by using my phone for GPS directions when traveling someplace new, and wanted to have this option built in to the car instead of having to use earphones every time. 

There are a limited number of modern options to replace a 2-post car stereo with a retro-styled unit, even with Bluetooth connectivity, but none of those are cheap, and i'd rather not spend much on something i'll barely use. There are easy options to add an aux-in port to head units with CD changer ports, but this radio was made before CDs existed. Happily, i found this link during my research:

So i opened up the radio.

kraco tape deck inside

From the article and an image of a schematic for my radio in an ebay auction, i was able to parse out where the signal inputs go in to the volume (lower post in the above image).

In this closeup, there are 4 rows of connectors for the volume potentiometer. The top row is the on/off switch for the main power. The 2nd 2 rows are the audio input for each channel. I don't know what the lower one is.

stereo volume pot

The very fine wires seemed to come from the radio selector, so the far left terminal in each row seems to be the input, and the 2nd terminal is maybe the output. I decided to start with them.

The key to making the aux-in work is a 3.5mm input jack with 2 switches. I got this one from DigiKey (and a couple of spares in case i wrecked one).

aux input jack

The way these work is that each channel has 2 connectors that go to a normally closed switch. By redirecting the input signal through these switches before it gets to the volume pot, the radio will be on (since the switches are closed) until an audio jack is plugged in, which opens the switches and takes in the signal from the plug. 

The image shows the pinouts for this particular switch, it's best to use a meter to figure out the connections for the one you're using. You want the tip and middle segment of the stereo cord to connect through when plugged in, and the radio input switched off. 

My first test was successful, but the thin wires are from the cassette, so i'd need to have a tape in the deck to enable the aux audio input. I'd rather have the aux input switch from the radio, so i changed to using the larger light and dark blue wires instead, and gave it a test.

aux in testing

Everything seemed to check out, so i knotted the wire bundle as a strain relief, and happily there was already a hole in the case where the wires could come out. The ground wire is brought up from the jack to make a common ground on the back of the radio case.

aux in wires installed

I installed the jack on the passenger kidney panel, right next to the USB charging port i'd added earlier. I even sprang for a cheap of new co-axial 4" speakers for marginally better sound.

aux in port installed

The new setup works great, i have normal radio when the unit is turned on, phone audio input when the 3.5mm jack is plugged in, and it still switches to cassette when a tape is put in. 

This was a fiddly project, but cost all of $2.50 for the input jack, and $25 for new speakers. I modified the kidney panels i had made earlier and used the spare wire from the speaker set for the input jack wiring. Between this and the USB, i feel better equipped for longer trips and navigating when i'm driving solo.