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Find Fault in my built amplifier

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Please fill in the blanks on Kris' image if you have any further problems.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Serves me right for reading the thread on my phone...

No excuse. It shows up fine.

I'll blame it on the rain.
 

Andrew Borg

Aug 10, 2014
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Hi Kris and Steve if it would be possible I post the readings again No problem.
The thing is now I notice that the transformer I'm using is +11v and -11v input that it's not good for this circuit.
I read that this circuit needs 18v least to work.
So I change the trafo and send you results back.
Thanks.
 

BobK

Jan 5, 2010
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I simulated it, but without the correct transistors, there were no darlington models in LTSPICE, so I made them out of a power transistor and a 2N2222 / 2N2907. The simulation actually works well at 11V.

Bob
 

Andrew Borg

Aug 10, 2014
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Hi Bob thanks for running the simulater to check.

I'm posting some vids on youtube who built this amp:

 

(*steve*)

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Yeah, I'd try to get it going on ±11V first. Higher voltages will only make things go wrong faster, more spectacularly, and with more collateral damage.

So, replace that transistor, power it up again without the speaker and give us a clear image with the voltages.

If you can't mark up the image, I'll post another version of the image with the boxes marked with letters and you can simply post the voltages in the text of your message beside the letter.

150wamp ready for voltages markup2.png

Do something like:

A 11V
B -11V
C 4V
.
.
.
 
Last edited:

Andrew Borg

Aug 10, 2014
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Today I changed that transistor and the amp was working. After sometime it stopped :(.
Don't know what happened.
I will post those markings.
Thanks
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If you can, post the values just after the transistor was replaced and again after it stopped working.

The first will indicate the conditions that were (possibly) present to cause the failure, the latter will confirm the failure of the transistor. It is possible that the failure of the transistor is masking the real problem.
 

Andrew Borg

Aug 10, 2014
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Hi Steve the thing is I did not take values when it was working.
Here are the present markings now:
A= 11.18v
B= -8.8v
C= -8.8v
D= -8.1v
E= -9.5v
F= -10.16v
G= 1.3v
H= -5.8v
I= -0.12
J= -5.2

Thanks
 

(*steve*)

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OK, here is the circuit marked up with your voltages:

150wamp marked up.png

There are a couple of standout problems:

150wamp marked up2.png

Notice the voltage in orange is (or should be) your negative voltage rail. This should be the most negative voltage in the circuit. However the voltages in red are more negative.

I suspect that you have either misread the voltage, or you have something wired up incorrectly.

The difference between the voltages circled in red are such that I could almost imagine you have read the wrong terminals of the transistor.

Perhaps you should ensure that the negative supply rail is directly connected to the point you measured B -- A voltage here of -11.8 would be (perhaps) reasonable.

Q5 is showing signs incompatible with life. Check very carefully you have it wired up correctly.
 

Andrew Borg

Aug 10, 2014
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Hi Steve thanks for response. I checked again certain marks.
A=10.98
B=-10.9
Other markings are mostly the same.
I= - 0.01

Is this maybe cause I used a 50v 10uf capacitor instead of 16v as mentioned in circuit?

Thanks
 

Andrew Borg

Aug 10, 2014
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Hi it's me again. Today I found the negative connection was not soldered as it should be.
When I hooked the speaker with transformer earth a little gain was coming but very distorted; the thing is when I connect the other earth of the input to it no sound comes.
Do you know why is this? Any help please?
Thanks
 

(*steve*)

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Sorry, I meant to get back to you earlier.

Using 50V caps instead of 16V caps will not cause a problem.

The new voltages you've given make things appear much better. I was going to update the diagram with the voltages on it so I (and others) can think more closely about it.

The major problem (which would prevent me connecting a speaker to this is that the output voltage is resting at -8.8V. This means that there's going to be significant DC flowing through your speaker. Also it is quite likely that Q2 will get warm or hot even when there's no output.

I would recommend getting an 8 ohm 50W resistor to use in place of the speaker until this problem is resolved.
 

(*steve*)

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150wamp marked up b.png

Q5 is definitely dead.

As shown in this circuit there is no reason for this to happen. I suggest you check very carefully for a wiring fault around Q5
 

Andrew Borg

Aug 10, 2014
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Thanks Steve for having patience to help me.
Today I checked around Q5 and changed even C3 cause I thought it's taking voltage from there but still invain.
Hope to solve this.
Thanks
 

Andrew Borg

Aug 10, 2014
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Hi Steve I noticed that the tip 142 was not touching the heatsink. It got out from the screw that I did. So I pulled it out to see if it was ok and found it burned.
I will change this and see if the amp works now.
Will report back.
Thanks
 

Andrew Borg

Aug 10, 2014
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Changed this transistor but same problem. A guy on youtube told me that this works on 25v 6amp transformer least voltage.
Maybe he' right.
 
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