Hello experts!

I'm an electronics engineer student trying to hack a HiFi and I have a simple problem but I'm not sure which is the (best) solution.
The thing is that I found a Sharp Micro HiFi (similar to this one) having a decent loudspeakers. The CD is broken but both the radio and cassette work perfectly, and I thought I could use the whole HiFi as an external amplifier+speakers for my laptop/mp3/etc and so my idea was to find a way to add a stereo audio input to the amplifier board so that I only had to plug a regular headphone output to it.

The HiFi has a dedicated board for amplifying the audio signals, drive the speakers and handle the radio system. I found the two traces in the PCB which carry the radio audio signals prior to be amplified and I disconnected them and made a simple board with a DPDT relay to switch between the existing radio signal or my external headphone signal. The problem is that when I switch to the external signal (coming directly from an MP3 stereo jack) the volume is really low and barely audible.
The PCB trace comes from a DC-blocking capacitor which is connected to the radio IC. I have no clue of its output impedance or the input impedance of the adjacent amplifier...

At first I thought the problem was the voltage level, but it's not. If I plug in my Android mobile, which happens to have a higher level, I can certainly hear distortion at higher volumes. Besides, the voltage levels when the radio is connected are pretty much the same. So I guess the issue must be related to unmatched impedances then? What do you think?

Thanks in advance!!

 

Besides, the voltage levels when the radio is connected are pretty much the same.

1. Which "radio chip" is it?
2. How did you measure the voltage levels? Oscilloscope / multimeter? Did you couple your external audio signal via a capacitor too? If you did, what's its capacitance?

I did this once in a hotel room radio system (without cutting traces or adding relays of course) and it was working quite well. Are you sure you got the right signal traces?

If you want it the easy way buy this or this.

 

1. Which "radio chip" is it?

They erased the part number of either the radio chip and the amplifier chip... Or they are custom ASIC's which I doubt...

2. How did you measure the voltage levels? Oscilloscope / multimeter? Did you couple your external audio signal via a capacitor too? If you did, what's its capacitance?

I measured them using an oscilloscope and yes, I tried coupling the external signals with a 1uF cap.

I'm pretty sure they are the actual audio traces because when I switch back to them I can perfectly hear the radio again, and when I just reconnect the traces to the external signals the radio disappears at all.
Don't you think it's an impedance issue then? Can my MP3 supply enough power? If it couldn't, this would be the effect? I though current wouldn't be a problem because the original signals already came from an IC...

I also tried adding a voltage follower between the MP3 and PCB and it hasn't made any difference at all...

 

This is entirely do-able but there must be a mistake somewhere. Perhaps a missing ground connection? Can you post a drawing of how you connected things?
Did you know that audio relays have gold plated contacts? That might be part of the problem, but not likely if you used a new relay.

Edit: You have revealed that you are not so much an amateur, which makes my post look a bit lame. I still wish for a drawing of some sort.

 

Did you measure the voltage while the MP3 was connected (having the amplifier input connected)?
I doubt that an MP3 cannot provide enough power to drive that amp.

Even though you found the signal traces for the radio, how do you know at what point in the signal chain you are?

As 12 said, it is doable, just have to find the right amp inputs.

What's the exact model number of the Hifi system?

 

You have revealed that you are not so much an amateur, which makes my post look a bit lame. I still wish for a drawing of some sort.

Don't worry for that, I do consider myself a poor amateur... Here you have a schematic. I don't think there's a mistake there, since it's really really simple, but it'll help to explain the connections... Nor I think it's a soldering error.

In a few minutes I'll look for the exact HiFi model number and post it here. For me, it all looks like an impedance stuff or... GND problem. I recall I used the GND connection used by the IC, but maybe there's something like an analog ground for the OC outputs (which, of course, I don't know if they are or not since the part number is erased...).

If I can get a proper image of the board, I'll try to upload it too. And if not, I'll try to reverse-engineer part of that board and get a quick schematic out of it, but it'll take me more time.

Thanks for your help!!!

 

Did you measure the voltage while the MP3 was connected (having the amplifier input connected)?
I doubt that an MP3 cannot provide enough power to drive that amp.

I'll double check that. I think I always measured it having the MP3 connected, though.

 

Problem solved. I feel really stupid... All that time I had a bad connection between the board and the MP3 and I didn't notice... What I was hearing was some sort of coupling through the wires when the input level was really high, but the connection wasn't done at all. Silly me, I know...

Now I wonder if it is worth adding a simple amplifier/attenuator for those inputs not capable of delivering enough current. I tried to plug in an MP3 and a cell phone and both sound loud enough, but when I connected an old fashoned portable FM radio I didn't achieve as much volume at all. Do you have any advice?
By the way, it is a "Sharp XL-40 Microcomponent System". In the attatchements you'll find two pictures of the radio IC sitting in its board and the traces I used.

Thanks a lot!!