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Old 10th October 2003, 08:46 PM   #11
darkm4n is offline darkm4n  Romania
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Shure i have .. but from where it comes ??? A damage capacitor ?
A damage transistor ? or .. ?
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Old 11th October 2003, 09:42 AM   #12
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Have you meassured with an oscilloscope??
Low frequency hum/distortion can be an effect caused by high frequency oscillation. And how do you know your problem is in the 60-120Hz region???? Thats normally supply hum (100Hz)..
Have you meassured the ripple on the supply???
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Old 11th October 2003, 10:40 AM   #13
darkm4n is offline darkm4n  Romania
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Hi ,

I measure by the ear with a tone generator ( on the computer )
)

I hope that the power supply to be the problem ... but it isn't ...

Bias ?? Maybe bias current ??
I tested all the componenet's on the board .. they are all good ...
power supply checked .. only bias ..

AND THE BOARD WITH THE PROBLEM IS GETTING VERY HOT !!!
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Old 11th October 2003, 11:09 AM   #14
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If it's getting hot, when there sure is a problem
However oscillation will make your output hot until it blows up....
You have two boards; One is ok and the other is not...
Place the two boards side by side, and carefully compare component colour code/type numbers etc......
The turn the boards upside down and carefully compare the PCB's and all the solderings. I normally uses a magnifier glass for this, and it have saved me many troubles
If you haven't found any fault, then power up both prints and carefully check DC voltage ratings on each components, making notes on a schematic (both for the good and the bad working PCB). If you find any interesting deviations, let us know
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Old 11th October 2003, 12:32 PM   #15
darkm4n is offline darkm4n  Romania
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Default Nope ..

There is no diference between the component's ... all it's exactly copied everywhere ...

I changed all the capacitors , the transistors ( not output ones ) ... and still is the distorsion ...

Thanks, Mihai
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Old 11th October 2003, 01:25 PM   #16
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If you made your own boards there is a chance that a microscopic trace is left between two seperate traces. The other possiblity is if you overheated the trace during the soldering of a part you can get 'seapage' inside the layers of the board that you cant see from the surface. Measure between the E, B, and C of all transistors with the amp off to see if there is a short. The diode setting on your meter is good for this.
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Old 11th October 2003, 03:08 PM   #17
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It will help to change your view that the two boards are exactly the same. They are not. One is getting hot, the other not. So, there IS a difference. Now find it. Jan's advice above is a good starting point. Remember: you are SURE there is a difference. Where? I noted you didn't change the output transistors because they didn't get hot. Yet, one of them could cause the extra current drain from the driver. These are important pointers.

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Old 12th October 2003, 11:52 AM   #18
darkm4n is offline darkm4n  Romania
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Default Again ...

I clean the soldring , check evey little component , changing all the capacitors again ... verifing all the wireing .

I don't know what is telling me that the bias current it's the sucker ...

That board was burned because i set the bias to low resistance , my output blow up , that was in the past . I repair the board , changed all that was damaged .. only thing i didn't change was the resistors ( they all are good ) and the bias current pot .. maybe that pot is damaged .. When i mesure it with the multimeter it's working fine .. but i'am thinking when it's in use .. that pot may create problems ?

And it's normaly that you board to heat if the bias is set to maximum !?

And my distorsion .. may be a crossover one .. that's way i thinked at BIAS .

I think it should be normaly the board to heat at a high bias current .. not a low one ...

Thank's very much . Mihai .
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Old 12th October 2003, 02:20 PM   #19
Diode is offline Diode  United States
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Mihai,

Jan is telling you some very good information and you should do as he says. I had to do the same thing on some amps that I built, but in my particular case, it didn't help with troubleshooting but I sure learned my amp design inside and out. THEN, I put a scope across the output and there it was.... It was singing like a bird. I tried putting a capacitor in different places but the bias was my problem too. In the bias circuit was a capacitor for filtering and anti-oscillation and all I did was increase its value by about 3X from 100pf to 320pf and it stopped the problem. Changing parts won't necessarily fix this problem. This is where the "black magic" aspect comes in, sometimes it just happens and you filter it and go on. If you are using PC boards, that's about all you can do but if it is your own layout, it will be in your parts placement, which can be changed without parts changes from the original design.

Jan, great ideas and posts!

Good luck,

Let us know what you do to fix the problem!

Chris
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Old 12th October 2003, 03:53 PM   #20
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Default M250 pcb

Can someone please point me where to find the pcb drawing for the m250? I hope it's in pdf or jpeg only.

Thanks
JojoD
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