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Troubleshooting a broken amplifier

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ArcaneDreams

New Member
Well first here is a picture of the Mobo:
http://i11.photobucket.com/albums/a154/ArcaneDreams/Random/Sarah050.jpg



Here are the specs:
http://www.electro-tech-online.com/custompdfs/2006/12/480824.pdf


I know I will probably never find out exactly what is wrong with it, but I was just curious what you guys had to say.

The thing powers up, and the protection light stays lit. I have checked all of the fuses inside and they are fine.

It was never overpowered or put under any stress at all. In fact, the gains were never set over 50% of the way.

It is also highly unlikley that it was a power surge, do to there being a total of 3 of these things plugged into a surge protector, and the other two work fine.


I know the best route would be to send it to the manufacturer, but this is more of a curious/bored project type thing.


So, where would you start if you had to troubleshoot this?
 

Sceadwian

Banned
Give the board an exaustive physical lookover with as much light as you can, the board looks generally clean so a burnt out component should stick out relativly well, look for the obvious stuff, lose connectors, broken wires, scratched PCB. If you can't find anything visably wrong with it start checking transistors. You should be able to do a simple diode test on the transistor leads using a multimeter to find out if there are any obvious shorts.
 

ArcaneDreams

New Member
I have been doing the physical lookover and wire checking, and I found nothing.

However, after checking transistors all of them register something on my ohmmeter, however I came across an A1837 transistor. I cannot get any reading at all on what I'm assuming to be the emitter.

I get a resistance reading from Base to collector. However I get an open circuit when I place the leads across the base and emitter, as well as the collector and emitter.

I have never tested transistors before, so if I'm doing it wrong please tell me.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
By far the most likely problem is failure of one or more output transistors, as it's a stereo amp it's probable that only one side has failed.

These types of faults aren't trivial to repair, and it's unlikely that replacing just the failed parts would cure it - it's usually best to replace all the transistors in the DC chain, or they often just fail again.

You should be able to identify the faulty channel by measuring the DC voltage on the output - but BEFORE the relay, they should be 0V, the faulty side will probably be either high or low.
 

fingaz

Member
The (2S)A1837 Transistor is PNP, data sheet is attatched below. you should get a resistance reading between both the base colletor pair and the base emmiter pair (though try reversing the polarity of the test leads, you should only get a resistance reading one way round) You should only get resistance reading on the BE and BC pairs, and open circuit on any other pin combinations.

Do listen to what nigel said, otherwise you may end up destroying the components you replaced.
 

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ArcaneDreams

New Member
Thanks for the help guys. And thanks for that spec sheet site, it is quite helpful.

Nigel your info is great, however I have no idea how to find the relay.

As of right now I am going to test all of the transistors I assume to be in the left and right output circuit.

Here is a picture of the transistors I am going to test:

http://i11.photobucket.com/albums/a154/ArcaneDreams/Random/transistors.jpg


I am going to record every reading my dmm gives, and report back.
 

ArcaneDreams

New Member
Ok, I'm totally in the dark here.

I had to stop after my first couple tests because nothing is making sense. I am going to guess it has something to do with the transistors still being connected to the motherboard.

---------------------------------------------------------------------
(I used the diode selection on my ohmmeter, which is also the 2000 ohm selection)


Transistor: C4793

BE: 293
BC: 531
EC: 1317

And the reverse polarity

EB: 293
CB: 1253
CE: 729
--------------------------------------------------



Now, I get these kinds of readings on everything. So unless EVERYTHING is bad, then I am going to guess I am doing something wrong, or I am going to have to de-solder each transistor to get a good reading.

:(
 

Nigel Goodwin

Super Moderator
Most Helpful Member
ArcaneDreams said:
Nigel your info is great, however I have no idea how to find the relay.
No disrespect, but if you can't recognise a relay, how do you hope to repair a high power amplifer?. Without a circuit I can't really advise you where to measure the voltages, but the relay is dead easy to spot! (there may be one for each channel, or one with two lots of contacts).
 

ArcaneDreams

New Member
Well if its dead easy to spot, then why don't you tell me what it is? Simply because I have not had experiance with relays does not mean that I cant learn about them.

This whole thing is a project for me to learn on.

It is quite frustrating to be spending thousands of dollars on college, and all I have to show so far is the memoriziation of a bunch of equations. There has not been much practical application (like troubleshooting) at all.

I mean, you mentioned it is most likley a transistor problem. So, I have been nose deep in all my books researching them, and google-ing like a mad man. I spent around 7 hours last night just reasearching for the sake of learning.

Its not that I am unable to learn, its simply that I have yet to be exposed to alot of the things you have been.

Throw me a bone here...


And, as I said, when I test the transistors I get very odd readings when compared to what I am supposed to get. I am making an assumption and guessing that I will need to remove the transistors to test them correctly. If this assumption is wrong please tell me.

Please remember that I came here to this forum to learn, and this is a project for me to learn upon.

If you think I should research say...relays...then tell me so, and perhaps point me in the right direction. If I should research something else..then tell me the same.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
ArcaneDreams said:
Well if its dead easy to spot, then why don't you tell me what it is? Simply because I have not had experiance with relays does not mean that I cant learn about them.
A relay is a VERY simple device, essentially an electromagnet that moves switch contacts - it's extremely common, you find them in cars and all over the place. If you don't have even the slightest electrical knowledge (and knowing what a relay is would be very slight!) you don't really want to be messing with a high powered amplifier - you're more likely to cause more damage.

This whole thing is a project for me to learn on.

It is quite frustrating to be spending thousands of dollars on college, and all I have to show so far is the memoriziation of a bunch of equations. There has not been much practical application (like troubleshooting) at all.

I mean, you mentioned it is most likley a transistor problem. So, I have been nose deep in all my books researching them, and google-ing like a mad man. I spent around 7 hours last night just reasearching for the sake of learning.

Its not that I am unable to learn, its simply that I have yet to be exposed to alot of the things you have been.
To repair things you need to understand how they work, to do that you need to understand individual components and how they interact - this isn't a 'read a web page and understand' thing, it's years of studying and hard work. As with anything you start small and work up, there are lots of things to understand before you get as far as transistors.

And, as I said, when I test the transistors I get very odd readings when compared to what I am supposed to get. I am making an assumption and guessing that I will need to remove the transistors to test them correctly. If this assumption is wrong please tell me.
You need to remove them!. But ohm testing isn't a very good way of repairing things! - voltage testing is far more informative.

Please remember that I came here to this forum to learn, and this is a project for me to learn upon.

If you think I should research say...relays...then tell me so, and perhaps point me in the right direction. If I should research something else..then tell me the same.
Look on Digikey (or any other retailer) and search for relays, you will find plenty of pictures of them.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Gaston said:
the relays are the two blue boxes in the upper right corner of the picture you posted.
I'd forgotten about the pictures! :eek:

Even better, the two coils just below the relays (brown and shiney) will connect to the relay contacts - these are the points you need to measure the voltage on. They should both be zero voltage, it's likely one will be positive or negative a LOT - possibly 50 or 60 volts - this would be the duff channel.
 

Shultzie

New Member
Are the white square components on the board next to the transistors made of a ceramic material? If so I have found precision resistors like this used in audio equipment and quite often fail when transistors fail. This could point to the bad area as well.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
ArcaneDreams said:
So I'm guessing its bad that one reads about 80V and the other reads about 50V
Yes, it looks like BOTH channels have blown - unless there's something common between the channels that might cause it (but unlikely, as the voltages would be the same in that case).
 

Hero999

Banned
noblevision said:
hello
can any body help me with a circuit diagram of a class D audio amplifier
thank you
Yes, but start a new thread rather than making off topic replies like this.

Reading this reply and looking at the photograph, has made me suspect that this might be a class D amplifier. I can't see and huge heatsinks, those inductors look pretty huge and 81% efficiency is too high for it to be class B or AB.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Well it's got large fan cooled heatsinks, so it might be a class AB amp?, and there doesn't seem much in the way of low-pass filtering on the outputs if it's class-D?. I'm not sure what the two low inductance toroids are doing at the bottom, but they aren't near the speaker outputs.
 

Hero999

Banned
I think I made a mistake calculating the efficiency, perhaps it isn't class D after all.

Here was the page I mis-calculated the efficiency from, as you can see, it is a bit confusing.
 

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