Hack an Aux In to an old sound system

[YPOC]

Hello,

I'm trying to hack an old sound system to receive an aux in signal. In hope I could easily find a line signal going to the amplifier where to solder some wires to, I cracked the case open but soon was lost.

I've made you some pictures and tried to align both sides as best I can so you can easily flip between the images.
The output is on the right side, the two cinch connectors in the center, also there is a headphone out just above. There are 4 inputs: CD in (where the cable is connected to), right next to it is tape in, AM antenna just below the cinch connectors, and FM antenna on top next to the fuse. Power comes from the two blue cables.

Where can I find the line signal?

I hope you can help me with this,
Yannick

[Seekonk]

The first place to start is the volume pot.  If they have gone digital, you are likely screwed.

[Benta]

Don't understand your question. Are you trying to send a line signal to the PCB in your photo, or are you trying to get a line signal out of the PCB?

[YPOC]

The volume control is digital. There's another circuit board wrapping around on the left side, but I think it only controls the buttons and display. I could snap some more pictures if you think you could use them.
I am trying to get a line signal into the PCB for it to be amplified and output via the cinch connectors. Basically I've built a little web radio receiver from an old wifi router and want to connect it through the sound system's amplifier.

[james_s]

Can you not just use the CD or tape in? You could add a switch to select between CD and the device you want to connect.

[Benta]

Tape in = line in. Very simple.

[Zero999]

Tape in = line in. Very simple.

Yes.


http://www.ebay.com/itm/New-Philips-Audio-Car-Cassette-Tape-Adapter-3-5-MM-For-iPhone-Ipod-MP3-AUX-/271034048754

[james_s]

I remember those things.

If he has a tape input on the stereo though, that input can be used directly as line in.

[Benta]

Tape in = line in. Very simple.

Yes.

Somewhat beside the point, don't you think? The OP states that he has a "tape in" input. Well, that's a line input.

[RoGeorge]

Instead of the cassete-as-jack-input, an iPod headseat can do a similar job:
https://hackaday.io/project/1477-the-worlds-simplest-hack-ipod-party-amplifier
 

[peter.mitchell]

signal goes ta2078p>pt2387>pt2256>amplifier ic. find the traces that go to the ta2078ps inputs, cut them or take out a series component and solder in a switch there, connect your line in to the switch, and the original signal to the other side of the switch.

[raspberrypi]

I did that on an Mercedes factory stereo. I put the line in right where the tape head reads then press tape to use the aux in.

[Audioguru]

I did that on an Mercedes factory stereo. I put the line in right where the tape head reads then press tape to use the aux in.

A line level is much higher than a tape head and will overload the preamp. The tape playback preamp has a lot of "tape equalization" that will seriously mess up the flat frequency response of line level sounds.

[raspberrypi]

I did that on an Mercedes factory stereo. I put the line in right where the tape head reads then press tape to use the aux in.

A line level is much higher than a tape head and will overload the preamp. The tape playback preamp has a lot of "tape equalization" that will seriously mess up the flat frequency response of line level sounds.

This was just a stock stereo system, it actually sounded better then stock in this case. If hes hooking it up into mediocre equipment he might not even notice.

[Brumby]

I'd find the point between the cassette deck pre-amp and the main amplifier and use that.

I found an old half-size jukebox by the side of the road (literally) and took it home.  Looks like someone got ticked off with it and kicked the crap out of it - but most of the cabinet was salvageable and the electronics worked.  It had a cassette deck in the side of the cabinet that was pretty crappy, so I ditched it and used that section of the selection switch to wire in my own sources - a tablet, a BT receiver and an external input through my own 3 way selector switch.  If I wanted to keep the cassette deck, I could have simply made it a 4 way switch.

It comes out at get-togethers where some background music is wanted.  The BT receiver tends to be the most used - as there is usually one or two who come that have music on their phone that suits the group.

[boffin]

See that nice jack in the middle of the board marked "CD IN", there's your audio in.....

[Brumby]

... and there's another one marked "TAPE" something (possibly "TAPE I_O") right next to it.

[ludzinc]

*than

[YPOC]

I've had partial success. I didn't even think that the signal the board receives from the tape deck and the cd player could be pre amplified already, I thought they'd send some sort of raw signal.
So I went poking around for a bit and found suitable pins on the tape input with which I got some quite good sound quality. I soldered some wires to these pins, hooked them up with an headphone jack, but suddenly the speakers were dead. I checked if I accidentally bridged some pins, but they're all clear. Can't even get some buzzing or crackling, from neither sound source.
Somebody know some first aid? Note that I'm posting in the beginner's section for a reason.

[Brumby]

If your flying lead lash-up gave you good results, then your problem is likely to be a result of your rework for the permanent solution.  This could be directly from the changes you deliberately made - or by something incidental, such as the solder bridges you mentioned or accidentally yanking on a wire too hard to damaging something on the PCB.

First thing I would check is whether other sources produce output - just to make sure the amplifier is actually working.  If not, then you may have dislodged a wire somewhere (hopefully) or damaged something which would need to be found (hopefully not).

If the amplifier is working, I would next check the "headphone" jack... Is it stereo or mono?  Is it switched or unswitched?  Getting the pins wired up wrong could be a simple fix.  Is it insulated or uninsulated - and if uninsulated, is the body in contact with electrically connected metalwork?  If so, something could be shorting.

If you can't work it out, try a few tests to get an idea of what works and what doesn't and, if you can, take a photo or two of what you've done, showing the connections and solder joints.  (Please make sure they are well lit and focussed - otherwise it makes it hard to see the detail.)  Bring this information back here and we will see if we can spot something.  No promises, but we will try.

[Brumby]

I've had partial success. I didn't even think that the signal the board receives from the tape deck and the cd player could be pre amplified already, I thought they'd send some sort of raw signal.

CD players have a somewhat digital flavour with their raw signals, so they have to have digital to analogue converters to send sound out to an analogue world.  In truth, the output of CD players can be a little high (~1V - 2V) for "line" level signals (500mV), but in practice you can often get away with it.  When an amplifier has an input labelled "CD", it will be expecting the higher level.

Tape decks are another thing.  The low level output of the head will need a preamplifier - but you will usually find a preamplifier in the tape deck to bring it up to line level.  For integrated systems, there might be such a board near the transport mechanism or there might be a pre-amp section on the main board.  Either way, you don't tend to take low level tape head signals on long journeys if you can help it.

[tooki]

I had that problem with a Sony MiniDisc deck (an MDS-JE500 or 510), with its line out that was too hot for the line-in (labeled DAT) on the Sony stereo. I had to build a level attenuator.

[YPOC]

Alright, I've put two images together, both with front and back side, one side is mirrored. Boy, on camera it looks far worse. Please prepare for some crusty images.

The first image is where I've soldered the wires to. I've just seen that one pin is bent but that shouldn't be the culprit, I'll fix that later when I got the sound working again.
The second image is where I've "stolen" the headphone socket from. I had to apply quite some mechanical force, but I removed it before I attached the wires, and testing after removal didn't show any errors. Don't know what these six small pins on the headphone socket are for, maybe they are responsible for muting the speakers when headphones are plugged in?

I have tested hooking up the CD player and the radio antenna, but neither produce any kind of sound. The display and buttons do work, controlling the CD and finding a radio station frequency work as well. So I guess something happened with the amplifier. Am I right in thinking that the amplifier is in the top right corner, with the big heatsink? (see my first post) I can't spot anything obviously faulty there.

[tooki]

Most likely the extra pins are switches. In older and cheaper equipment, the audio was often fed right through them (double-throw switches that directly switched the audio output from speakers to headphone when inserted), in a lot of modern equipment, the switches just tell the microcontroller to switch the output.

Especially if it is the first case, then it's possible that by removing the jack, you've completely disconnected the output signal.

[Brumby]

Most likely the extra pins are switches. In older and cheaper equipment, the audio was often fed right through them (double-throw switches that directly switched the audio output from speakers to headphone when inserted), in a lot of modern equipment, the switches just tell the microcontroller to switch the output.

Especially if it is the first case, then it's possible that by removing the jack, you've completely disconnected the output signal.

Yep.  That's what you've done.

The simplest thing to do is find which pins of those 6 at the back are shorted together with nothing plugged into the jack and then bridge those points on the PCB.

[YPOC]

YES, awesome guys, that fixed it! I was so sure that I tested the board after removing the headphone socket and before soldering the wires to the pins, but whatever. Now it works! Thank you so much!