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need service manual for yamaha DSP-A780

 
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ranco
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PostPosted: Wed Jan 16, 2008 2:19 pm    Post subject: need service manual for yamaha DSP-A780 Reply with quote

Hello j need service manual for yamaha dsp-a780 that no power on.
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ges
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PostPosted: Sat Aug 22, 2009 2:24 pm    Post subject: Re: need service manual for yamaha DSP-A780 Reply with quote

ranco wrote:
Hello j need service manual for yamaha dsp-a780 that no power on.




[b]please someone can help me,j need schematic diagram for amplifier yamaha modl DSP-A780
Turn on 1-3 sec and shut down
Thanks in advance.
[/b]
Rolling Eyes
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DSP-A780
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PostPosted: Thu Oct 28, 2010 6:29 pm    Post subject: protection mode Reply with quote

MODEL DSP-A780
TYPE SYMPTOM/REPAIR
SUBJECT PROTECTION

SYMPTOM: UNIT GOES INTO PROTECTION AND TURNS OFF APPROXIMATELY 4 SECONDS AFTER TURN-ON. ALL AMPLIFIERS CHECK OK WITH NO DC OFFSET OR OVER-CURRENT.

CAUSE: ONE OR MORE AUDIO MUTING TRANSISTORS (Q408,409,411, 413,415)HAVE BECOME LEAKY. DURING THE FIRST 3 SECONDS, THEY GROUND THE AUDIO PATH. SIMULTANEOUS WITH THE SPEAKER RELAY CLOSING, THE MUTING TRANSISTORS ARE TURNED OFF BY APPLYING APPROXIMATELY -14 VOLTS TO THE BASES. IF THE TRANSISTORS ARE LEAKY B-E, A HIGH NEGATIVE DC VOLTAGE APPEARS ON THE AUDIO PATH, SENDING A -DC TRANSIENT THROUGH THE AMPLIFIERS. THE PROTECTION CIRCUIT SEES THE DC AND TURNS OFF THE UNIT. THIS TRANSIENT CAN BE OBSERVED WITH A SCOPE AT THE OUTPUT OF EACH DEFECTIVE CHANNEL.

SOLUTION: REPLACE DEFECTIVE TRANSISTORS ONLY WITH THE ORIGINAL TYPE 2S1915 (VK432900).
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