Add AUX IN to an Old Car Cassette Player

I have an old car which cannot ply on road due to its end of life. It had a Kenwood KRC-443ii cassette player audio system installed with two front speakers. I thought of extracting it from the car and make a usable audio amplifier which can perhaps fit under my home work desk for music and audio. I unscrewed the amplifier and the two speakers from the car and started using it as audio amplifier in my work from home setup.

Initially I used car cassette tape AUX adapter for connecting phone or computer as audio source, but later on the mechanical part of tape started creating problems, like not always switching to tape when cassette adapter is inserted and audio also got poor with time. So I thought of adding a 3.5mm AUX directly on PCB bypassing the tape altogether.

I started searching for how to add an AUX to it and learnt that a car audio system should have points on the PCB which can be used to add AUX, but could not find any resource which was specific for this car audio system.

After scrambling through various videos and literature on Internet and doing my own hit and trial method I finally could add a 3.5mm AUX input to the car audio system, directly on the PCB for a crystal clear sound.

In this project, cassette tape playing part was removed and a 3.5mm audio input jack was added. This project requires you to do a bit of soldering on the audio system PCB.

This project would need a soldering iron, a 3.5mm audio jack wire and a few tools to open the car audio box.

After you get the cassette audio player extracted from the car, open up the top cover and you should see a mechanical cassette tape player part - a big portion of the overall system containing springs and levers. You need to carefully take out the tape player part out because it would no longer be required. It has micro switch which when depressed, when a tape is in, the switch state changes to tape from radio; this is when the audio output from a cassette used to play in speakers. There is a small electric tape motor also which spins the cassette during play.

There will be two connectors attached to the tape player part which needs to be released to pull the tape mechanism detached from the system. One of the two connectors is for input from tape head, and another is for switching source between tape and radio. We need to identify two pins from the switching connector which when short will turn the source to tape. For this car audio system these pins are marked in the picture.

The AUX inputs are generally found around the pre-amplifier chip. Look up the chip number on google to find out the pins for stereo output (Yes, pre-amplifier output pins are AUX input we will use for left-right channel). We actually do not have to solder the AUX input jack (left-right channel) onto the pins of the chip itself but on the connecting capacitors C23 and C24 for this audio system. The ground connection can be taken from the tape head connector as shown in the figure.

Once the wiring and connections are done as shown in above step, we are ready to reassemble it and power it with 12v 10amp power supply. Once powered you can use the front panel push button to TAPE mode and connect the AUX 3.5mm jack to your audio source like mobile phone or computer to listen to the crystal clear sound.

One thing to note is, on this PCB we found two separate set of left-right channel inputs. One near the pre-amplifier and another near the main amplifier chip. The one near the pre-amplifier will give you clearer sound than the other one but it has low top volume - but good enough to fill your room. The inputs from near main amplifier chip will give you louder sound but there may be distracting ground humming - so I prefer the one with clear sound outputs. Later on I shall add Bluetooth as well.

You can use this project as a reference to add AUX to a similar car audio system. All the best!