Audio help needed.

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fingaz

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I'm trying to repair a stereo system it's a Roberts mp-sound 43.
It basically sounds very distorted at all volume levels but worse the louder it gets. Sounds very bass. It's based on JRC NJM2706 and ET2314. It worked fine, went into storage for about 6 months, now sounds awful.
Strange thing is that through headphones it sounds fine with no distortion.
Datasheets for the 2 chips attached.

Any suggestions or help would be greatly appreciated.
 

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  • datasheet.pdf
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For a start I'd try a different pair of speakers on it - headphones are very often fed from the speaker outputs anyway (via attenuators).

But unfortunately Roberts these days is just cheap Chinese imports with a big price tag, and little in the way of spares or service information. I used to work for a Roberts dealer.

If you go to the Middle East you can often buy identical looking products with different names on the them, at about 1/4 of the price.
 
Can't find a service manual on it, but this type of problem is almost always related to the amplifier output sections - either a failed output transistor or a bad speaker relay (if it has one). The output stage is able to supply enough current for headphones, but not enough for speakers.
 
These are the only audio related chips o the board. It's only 10w output into 8 ohm speakers.
 

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Ylli said:
Can't find a service manual on it, but this type of problem is almost always related to the amplifier output sections
Click to expand...

Unfortunately Roberts have long since stopped supplying manuals and spares.

They were never highly thought of anyway, always just the poor man's Hacker
 
Nigel Goodwin said:
Neither of the two chips you listed are power amplifiers.
Click to expand...

Here is the whole board. The chip at the bottom right is tda2822, to the left of that ET2314, to the left of that NJM2706. As I said, its low power.

The TDA2822 is not faulty, already tried replacing it.
I should say that with the left speaker disconnected, there is not sound distortion. It appears to be only on the left channel.
 

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Last edited:

The TDA2822 is a low power audio amplifier, and presumably feeds the headphones?.

From that picture I would suggest the power amplifiers are at the top right of the PCB, possibly on the other side of the board?.
 
Can you determine where the wires from the speakers connect to the board? Closer view of that part of the board?
Nigel Goodwin said:
The TDA2822 is a low power audio amplifier, and presumably feeds the headphones?.
Click to expand...
Could be, but a separate amp for the 'phones is not a hallmark of cheap amplifiers.
 
Ylli said:
Can you determine where the wires from the speakers connect to the board? Closer view of that part of the board?
Could be, but a separate amp for the 'phones is not a hallmark of cheap amplifiers.
Click to expand...

There's probably a good reason it's done that way - it's quite common in TV's, when you find a TV with a headphone socket.
 
Nigel Goodwin said:
The TDA2822 is a low power audio amplifier, and presumably feeds the headphones?.

From that picture I would suggest the power amplifiers are at the top right of the PCB, possibly on the other side of the board?.
Click to expand...
Nothing on the underside of the board.
 
Ylli said:
Can you determine where the wires from the speakers connect to the board? Closer view of that part of the board?
Could be, but a separate amp for the 'phones is not a hallmark of cheap amplifiers.
Click to expand...
Yellow/black/red to the left is for left speaker.
White/black/red to the right is for right speaker.
 

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fingaz said:
Yellow/black/red to the left is for left speaker.
White/black/red to the right is for right speaker.
Click to expand...

Like I said, top right of the previous picture, some of the components round there are the low-pass filters for the class-D amplifiers.

The amp could even be the small SM transistors nearby.
 
Nigel Goodwin said:
Like I said, top right of the previous picture, some of the components round there are the low-pass filters for the class-D amplifiers.

The amp could even be the small SM transistors nearby.
Click to expand...
Yeah, I think I'll leave it at that. I'm not messing around trying to identify SMD transistors. Not to mention there's a lot of glue on the board there covering a lot of the SMD components.
I wanted to try to get it working for the challenge, but it doesn't seem worth the effort now.
Thanks for the input
 
Nigel Goodwin said:
Like I said, top right of the previous picture, some of the components round there are the low-pass filters for the class-D amplifiers.

The amp could even be the small SM transistors nearby.
Click to expand...
I agree it's the top right - but I suspect there is an IC on top of the board, with that tubular looking object on the top of it; right in the centre of the glue area.

Note the custer of parallel tracks heading under the glue from the left - to IC pins nearby??
 
I may have another look around that if I can get all the glue off.
 
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