I need to hack into a car stereo board to inject an audio signal

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
146
Got an old car stereo lying around and noticed it might be good for making a home sound system for my computer. For using inside home it has a decent amount of watts per speaker, 35 watts if I'm correct. The thing is, it is too old and doesn't have an aux in. I'm looking at the schematics trying to figure out where's the left and right audio channels input either in microcontroller or in the amplifier, but I can't understand much of what's written. Can anyone help me figure out where it is?

Here are the pages I'm looking into pages 76 and 77 (pdf attached).

It's a Pioneer DEH-436.

Here's the full service manual:
https://www.audioservicemanuals.com...eh-436/2464988-pioneer-deh-436-service-manual
 

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sghioto

Joined Dec 31, 2017
5,369
Have you tried injecting a signal at the front or rear pre-out jacks?
From the schematic they appear to be directly tied to the output of the electronic volume IC which connects to the power amp.
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
146
@Audioguru again Thank you my good man. Perhaps I can get around 25 watts without clipping and distortion. Maybe I don't need more than 15 watts after all, since it's just for home use.

@sghioto you mean wiring to the audio outs? They're outputting more than 6 volts each.
 

bassbindevil

Joined Jan 23, 2014
818
There are spots on the schematic where it looks like a signal could be injected into the electronic volume chip or the power amp chip. But, for under $10 there are very good class-D amplifier boards on eBay with the same watts/channel, or in the $20 range you can get one in a case with volume and tone and maybe Bluetooth.
 

sghioto

Joined Dec 31, 2017
5,369
@sghioto you mean wiring to the audio outs? They're outputting more than 6 volts each.
No. The block diagram shows two jacks labeled Front Pre Out and Rear Pre Out. Connect the computer to one of these jacks and turn the volume down to zero and see if the computer audio is getting through.
 
Last edited:

sghioto

Joined Dec 31, 2017
5,369
From the schematic (not the block diagram) there are 4 outputs from the electronic volume control chip. Two of those outputs connect to the RCA jacks on the back panel labeled Rear Output. You should be able to back feed an audio signal from the computer into the power amp from these jacks. The volume on the radio will need to be set at zero. The volume level for the computer audio from the computer is adjusted by the computer.
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
146
I managed to inject the audio by first removing the tuner L and R, then I used a switch to toggle between aux and the AM/FM transmitter. I don't really need the radio part, but I'll leave it there just in case. The only problem is that now when I connect the right speaker, the entire stereo is getting in protection mode, turning off. Maybe there is a short circuit somewhere, but I'm not being able to find anything.
 

MisterBill2

Joined Jan 23, 2018
17,887
Are you connecting the speaker with one side connected to common (ground), because if the output is typical neitherside is common. Or one side of the connection is grounded someplace. Or your connection might be too low an impedance.
Be glad for protection, it is preventing a big smoke release event.
 

sghioto

Joined Dec 31, 2017
5,369
I believe I found the part where I can inject the signal. I'm using a switch to toggle between the tunner and the aux, this way I don't need to reduce all the sound.
That will work too. I was using a way as not to have to modify the radio. Are you using the Pre Out jacks as the aux input?
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
146
Outcome: Managed to get the thing working, used it for a couple hours, but then it was not working when I tried to use it again. I believe the amplifier burnt. I was connecting a PC audio out to it, not much voltage in the audio output, but I believe it might have burnt the amplifier anyway.
 

MisterBill2

Joined Jan 23, 2018
17,887
Common mode voltages can be rather sneaky and lead to some interesting failure modes. And unfortunately that amplifier module may be a serious pain to replace. And also rather expensive to get. So you will need to do some detailed evaluation to see which part actually has failed. Of course, there are also some security functions built into some automotive audio systems that demand periodic contact with the vehicle main computer module.
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
146
"So you will need to do some detailed evaluation to see which part actually has failed. "
It is randomly working. If I don't manage to find an easy fix, It was very didactic.

1644952797168.png
 

MisterBill2

Joined Jan 23, 2018
17,887
On several occasions I have repaired equipment that was "Hopelessly fried" by re-connecting a broken wire. Of course if you were blasting rap at max volume the radio did the only smart move, and died.
Not saying that you were, but some do.
 
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