DSP-A780 |
Posted: Thu Oct 28, 2010 6:29 pm Post subject: protection mode |
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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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