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Old 5th February 2014, 01:33 AM   #11
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Join Date: May 2010
Location: Washington State
The bias transistor is the MPS6521 in line with the output FET's on the MESHA board but not soldered to it.

The A06 and A56 are the drivers for the outputs.
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Old 5th February 2014, 01:37 AM   #12
tiomilo is offline tiomilo  Costa Rica
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Thanks again! Gonna replace all before next test.

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Old 5th February 2014, 06:16 PM   #13
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The bias drift might be improved if you find the bias tracking transistors near the MESA/Heatsink and bend them over to make contact with the hot heatsink. Use thermal grease if you have it. It will be a NPN transistor, probably MPSA06 or MPS6521.

The heatsink will go up to 90 degrees celcius and thermal protect when run at full power so that does not sound so bad. The efficiency could be better. Make sure you measure with about 1% THD clipping, 1kHz, and measure the B+ voltage right at the amp input terminals. The method of current measurement must be accurate - not just reading the dials on an ASTRON. Even so, if it doesn't blow up at full power and makes it to thermal I think you fixed it ok and would go ahead and run it.
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Old 7th February 2014, 05:14 PM   #14
tiomilo is offline tiomilo  Costa Rica
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thanks a lot ! already replace MPSA06, MPSA56 and biasing MPS6521 on both channels now is running like charm, a little warm @ full power. consumption decreases to 0.035 at idle cold and 0.112 idle warm, very clean sound an plenty power !! thank you people !
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