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Old 18th June 2012, 04:19 PM   #1
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Default adjusting VBe

Hi
I just replaced burned output transitors in my car amp using 2SA5200 output stage, there are bias adjustment pots , i am not sure how much VBE should be there ?? any idea ????
rail voltage is +/- 30 V and is rated at 75W into 4 ohm.

any suggestion please its a focal 4.75 car amp, i have written for them for scematics but of no use .
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Old 18th June 2012, 04:24 PM   #2
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Adjust 22mV across each of the big resistors in the source traces.
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Old 18th June 2012, 04:37 PM   #3
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Normally, you donīt care Vbe adjustment, in place you need to adjust the output quiscient voltage. With 5-10mA is normally sufficient, but if you have a audio sine generator and īscope, then adjust bias till no crossover distortion near 0V axis, then increase a bit more to ensure that when cold, it donīt distort. Or use the current suggested by the manufacturer.
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Old 18th June 2012, 06:01 PM   #4
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the big resistors are in the emitters. with pins on the other side of the board. There are bias check point but it doesnot change with adjusting pots? but Vbe varies from 50mv to 800mv, i have set it for 400 mv at the moment by the bias post.

result. almost at room temprature on no signl and load, on a 5 ohm resistor at 1 Khz sine wave and 18V output volrage , it getts prettyt , just can touch the 5200 for a minute moment.
its a non inductive resistor load on a heat sink , with 18v sine wave the load is 18x18/5=65 and 65x0.707=45 watts only whereas the 5200 is usually rated for a 100w amplifier ????? any help here?
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Old 18th June 2012, 07:04 PM   #5
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oK the resistors are in collector path and are 50mOhm wirtten on it, i assume its 50 mill-ohm, however i assume 0.05 Ohm is quite low, any way as suggested by richie boy 22mv would mean a collector current of around 0.5 Ampere ???? By the way i have measure the VCe to be 29Volts, so Pd with 0.5A would be around 15Watts?????
please advice
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