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Old 31st August 2004, 03:37 PM   #1
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Join Date: Aug 2004
Location: Bois-Colombes
Question Class A repair : where to start ?

I took the oportunity to grab a Luxman M-05 (dual 105W class A) but it needs some repair
I know the previous owner tried to change to outpu transistors, but had them burn again a few minutes after putting the power on

I have the service manual including the schematics, but would appreciate some advice on the methodology :

I'm planning to :
- open it and search for any burnt component,
- compare the IC and transistors with the part list

- then put some power and check various points in the signal

is there some better ways to diagnose what's wrong ?
any places to look at first ?

I also have a practical question is about the testing load :
I don't think I can test it without a load, and don't want to risk my current speakers
using some power resistors in parallel should be ok ?

or can I have a go without any load ?

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Old 3rd September 2004, 07:29 AM   #2
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btt as I'm a bit concerned to make mistakes when putting it on
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Old 4th September 2004, 01:31 AM   #3
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Location: Recife - Brasil Northeast
Default Repair stations, repair offices, service places..

They do not loose time.... as 90 percent of the problems are in the supply and semiconductors, they go first at the supply...being good, they replace the transistors....hehe...crazy isn't...because spent less time, pay for them all... the burned ones and the no burned ones.

Of course, here third world, and third world methods, i believe in your place things different.

Check if fuses are the correct ones...if not replace them.... switch amplifier on without speaker and see if fuses blow.

If not burned, the fuses, check now if you have DC voltage in the output ( do not put speakers...hehe...they can melt).... if you measure DC voltage in the output, burned transistors in the power amplifier....

Check for a burned resistor that is easy to see...measure the bigger ones, the emitter ones, if your Class A use them, because not too common in class A amplifiers.....

Now, if resistors good.... disconnect transistor, at least 2 wires...and measure them...better to remove them...go from the output to the input....from the last transistor, the bigger ones to the smaller ones.... this way the amplifier will work....if have a short somewhere, maybe in output, because of melted parallell wires....or shorted electrolictic condenser in the audio amplifier...but this is much less probable.

Use the same types you see in the schematics.... this is better to diagnosis...because if burn again, you will always think that the guilty is the transistor, this will make confusion in your mind...this way, put the correct ones.... just to avoid doubts..... despite that hundreds of types will work there.... some of them will work better than the ones you have...because of material evolution...noise..... termical transference, power ratio, frequency and gain.

Resistance in series, to limit Class A amplifiers can be used too...hard is avoid the resistor go on flames...because class A uses a lot of current.... you may use low values....0,33 ohms by example to limit current..... measuring over it you will see the current passing into the amplifier.

Good luck.

Sorry, Brazil only to be sold... top secret ultimate schematic, not to share...encapsulated VAS:
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Old 4th September 2004, 01:38 AM   #4
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This is my first post on the forums.. but this is what i like to see, and why i enjoy this forum.

Members helping members.. what a good and detailed answer

I think that I will really enjoy my stay here
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Old 4th September 2004, 06:07 PM   #5
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Location: Recife - Brasil Northeast
Default This place is very special, i ensure you.

There are people that came here to have fun, so good in their enginneering habilities, that in real life we will not be able to meet them, so busy they are, so important they are.

But they came here, alike one more, and try always to teach us some...unfortunattelly i cannot understand all them say, because some base of knowledge is needed to follow them, also they use some "abreviatiated" forms, short forms that is a "hell" to me...CCS....constant current supply, Long tail (you find and cannot see that long tail.... is short tail).... and a lot of codes i do not know.

And they have a lot of patience with us... and some of them share with pleasure their knowledge with us.

Alike every place, alike every society, you will find non cooperative people too.... also the ones that feel pleasure to not agree, to disturb, to make some noise....others are funny, you have 1 or two bad guys too.....but we are the selection.... all of us crazy enougth to try to improve sound.... when alive sound is 100 times better, we will never reach that!....but we will go trying.

And this is passion!....non sense too, but wonderfull!

Welcome, each day will be alike a party to you...have fun!

Sorry, Brazil only to be sold... top secret ultimate schematic, not to share...encapsulated VAS:
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Old 6th September 2004, 10:41 AM   #6
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Location: Bois-Colombes
Thumbs up international forum

destroyer X
thanks for your anwser, I'll have a go from there and look for what's burned or damaged first

I'll be very carefull, as 100W in class A means high currents

just a slight note about this project and the support I received :

Destroyer X is in Brazil, I'm in France, and I found the technical manuals from a guy in Sweden : international team work in progress

I'll keep you informed on the progress
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