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Old 21st July 2007, 04:56 PM   #11
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Hello,

@Carlos:
I don't want to name the brand here, but I send you the details via mail.

Any help for my problems?
Quote:
1. Can anyone guess what the VCC/VSS voltage should probably be.
2. Can anyone give me a hint what is the best way to make an analytic failure search? Are there some stages which can be run and tested stand-alone. I only have a multimeter, an oscilloscope and limited know-how.
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Old 21st July 2007, 05:26 PM   #12
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Default Remove transistors, one by one and test them outside the board.


And measure every resistance that seems burned, replace the ones have errors, obtained during measurements, bigger than 20 percent.

Lift one side of diodes and test them into the board.

This is the most fast method to fix an amplifier...the time people will lose trying to track errors by voltages is bigger than the time you spent to remove, to test and to put them back again into the board.

I am telling that:

two guys starting the same time, same equipment, same damage:

The one that will remove transistors, and test them out of the circuit, will be 3 times faster than the other.

Also, someone not experienced, cannot "detect" defects easier watching voltages...but someone not skilled can discover burned transistors measuring them.

I have made tests about, during my work time in Motorolla.... the beginer, novice guy into repairs, were faster then skilled folks.... one analising circuitry without dessolder and other removing parts.. because of that were developed VHF radios using plug in sub circuits to allow fast corrective maintenance.

But the one that remove parts will also never learn how to debug amplifiers...a matter of option, and what is more important to each life moment.

Dx voltages, the standard Dx amplifier voltage chart, will not help you too much, as you are using different supply voltage...also the voltage measurements is the analitic method... depends on you if skilled or not.

If you did not accept my suggestion about the transistors removal, parts removal, and testings.... tell me your supply voltage and i will prepare a chart of voltages to you...or a diagram with some voltages on it.

Repairs, fast service, corrective maintenance, is more adequated to "changeneers" (replacement parts engineer...non engineer... clever folks trained to be fast)... circuit analisis is to engineers... repairs is a waste of time to creative engineers... unless very well paid to do that.

If my life will depend of how fast, and good can be someone, to fix the Cardiac electro mechanical pump (heart aid machine)... i will prefere my life in the hands of the changeeneer.

To create those machines.... of course i would prefer the Engineer.

"Changeeneer is a replacement technical man...skilled to replacement of parts and skilled to measurements and adjustment...the name to do not exist in Dictionaire... creation of Brazilian Broadcasting Personnel"

They were called also "Operational Engineers" for some years... a 3 years University...a "half" EE degree they had...now not accepted as full EE anymore.

regards,

Carlos
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Old 21st July 2007, 07:48 PM   #13
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Hello,

@Carlos
I am happy beeing "changeeneer"
I already removed all active parts (diodes and transitors) checked them and replaced the blown ones plus the resitors with "hot spots".

So I will also remove and check all resitors and capacitors

Do you think it is possible that my amp is equipped with a wrong transformer? I still wonder that someone uses only 50V caps if the idle rail is 49 volt.

Best wishes
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Old 21st July 2007, 08:26 PM   #14
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Default It is very common to use those condensers


Because those ones specified as 50 Volts....they do not explode if you exceed that voltage...sometimes they work, continuously into 75 volts!

They are just giving us the guarantee of 50 Volts when you buy the unit....not limited, in the reality they can hold more.

This voltage turns the output in risk... even using 8 ohms the maximum dissipation may be exceded during normal audition levels.

Output may be under risks... i am not sure, as i had not time, and care, to read all your thread in detail and with double readings...as i am not good to read in english and to understand first time.

regards,

Carlos
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Old 23rd July 2007, 07:09 PM   #15
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Hello,

after changing Q10/Q11 and R69/R73 and replacing R64 with a trimpot it's working gain
Thanks to all that helped for their support!!!

The only minor problem that's left is that my bias is still too high even if I turn the R63 trimpot to 0. With the 10 Ohm test resistors a suggested on the Dx page I still have 1200/1300 mV.

Does a higher rail voltage also mean a higher bias current or should I increase R62?

Best wishes
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Old 23rd July 2007, 07:43 PM   #16
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Default Do not increase R2...... keep it the same value


Increase the trimpot value you have installed.

Adjust your amplifier to low current.... 50 miliamps each rail, to avoid the protection be near to start to work.

The protection will colect voltage produced into the emitter resistances...this voltage depends from the current is flowing into the emitters....too much current means too much voltage there....because of that the protective transistors may enter in operation too much soon.

regards,

Carlos
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Old 23rd July 2007, 08:11 PM   #17
jaycee is offline jaycee  United Kingdom
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Don't increase R62! that will do the opposite, and increase the bias.

Try increasing R63. 330 or 470 ohms might work well here with a 1K trimpot being used.

I would measure bias by measuring the voltage across one of the emitter resistors, R82 for example. I'd go for about 10mV across this resistor, that would be a bias current of 45mA approx.
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