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doubled 10th April 2010 06:39 PM

Left channel dead
 
Hi All. I purchased some no-name ("Dual") integrated amplifier and the left channel is dead.

Listening with head phones, no problem.


I cracked it open, and there was an un-hooked jumper between the left channel and the headphone-jack circuitboard. I reconnected them, and the channel is still dead. A wire plugged into the left channel does connect properly to the headphone-jack circuitboard properly.

I'm new at amplifier repair, but have the basics of circuitry. All connections look solid, and nothing looks leaky or burnt out. Looking inside there tons of little capacitors, two gigantic ones, and exactly one mosfet.

Any ideas/suggestions/help? The circuitboard says TT8-541A on it.

Thanks!

Mooly 10th April 2010 07:10 PM

You would at least have to post close up pictures etc to stand much chance.

If it's discrete (no power IC's for the outputs) then it's a case of checking the basics... DC operating conditions ie is the output pair midpoint at zero volts and is there audio there ?
It may all be physical... damaged cracked print etc.

It sounds an easy fix... the fact headphones are OK means that if it uses the main amp to supply those, then it really does seem like a bad connection/break etc

AndrewT 10th April 2010 07:12 PM

If I read you correctly you say there are two working channels at the headphone outlet, but only one working channel at the speaker terminals.

This could indicate that both power amplifier channels are working and you have a wiring fault.
or
The headphone outlets have their own amplifiers separate from the speaker outlets.

doubled 10th April 2010 08:15 PM

Yeah, I am thinking it should be an easy fix too, but I am stumped.

Here are some annotated pics.

I've discovered that on the headphone-jack board there is a five jumper cable, and the brown and red ones form a closed circuit with the R+ out, but not the L+ out. So that helps narrow it down, but from there it is messy. I am hoping to get some ideas on which components may be broken or how to trace out the circuit.

Also, no matter how loud I turn up the amplifier, I hear absolutely nothing from the left speaker. It is not just faint, it has absolutely no signal.

doubled 10th April 2010 08:25 PM

I thought I just posted - apologies if this is a double...

Yes, I think it should be an easy fix! Here are some pics.

The whole thing, from above. The blue line follows the left channel jumper that was disconnected when I purchased the amplifier (it seems the previous owner had intentionally disconnected it!)

Close up at the speaker ouptuts.

Close up where the headphone-jack circuit board has a jumper to the main board. I noticed that the red and brown cables form a circuit with R+, but not with L+.

From below.

Also, I tired just cranking the volume, to see if it is merely a weakened output, but there is absolutely no signal going out on L+.

Thanks for your input!

doubled 10th April 2010 08:40 PM

Hi Andrew - pretty sure that the headphone jack does not have it's own amplifiers. You can see in the pictures that there is only a small circuitboard at the head phone jack (upon inspection, it looks like all it has are two resistors) and then from there, the cables head out to the speaker outputs. But there is something else going on, as the signal of the speaker output is coming entirely from the headphone jack board.

RJM1 10th April 2010 09:14 PM

1 Attachment(s)
The amplifier is obviously working. The only thing that could posibly be wrong (depending on what amplifier it is ) is output fuse, protection relay contacts, or speaker selector switch contacts. Just ohm it out from the output of the amplifier (with the power on) to the speaker jacks. Check the contacts on this relay to see if they are burnt.

doubled 11th April 2010 09:49 AM

Thanks for the advice RJM1. I don't have a proper toolkit, I'm just using a 9V battery with a light to see where connection work. When I test the relay, it made a loud clicking sound, so I assume it is working alright. The contact underneath look normal. Anything else I should be checking?

jaycee 11th April 2010 11:07 AM

The headphone socket will have switches in it to disconnect the speakers when you insert a headphone plug. More than likely, these have failed. Seems a bit silly to do it that way when there is a speaker protect relay!

doubled 11th April 2010 11:42 AM

Yes, jaycee that's it! I tested the headphone-jack board, and there is a connection involving the jumper to R+ that works when the jack is empty, but does not work when I plug in the headphones. The corresponding connection for L+ never works, regardless. I shorted that connection, and bingo, functional L+.

Should I buy a replacement jack? I'd rather not have to remove it and seat a new one. Do you think it is possible to fix the jack? For now I will leave it shorted so L+ is always on (I don't use headphones too often anyway).

Many thanks for your help!


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