PIONEER VSA-AX10AI-G/NA
Service Manual

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7.1.2 FAILURE DIAGNOSIS OF THE POWER AMPLIFIER
If the power is turned off after "AMP ERR" flashes on the FL, the power amplifier is damaged, and DC voltage is output. In this case, either or both of the current amplifier section or/and the voltage amplifier section is/are damaged. Diagnose the sections in failure, as indicated below. The main cause for power amplifier damage is damage to the MOS-FET caused by current overload as a result of shortcircuiting of the speaker terminals, use of speakers with impedance lower than guaranteed, etc. Therefore, first diagnose the current amplifier section, then the voltage amplifier section.
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1. Damage to the current amplifier section
Many cases of MOS-FET damage caused by current overload are results of short-circuiting between the drain and source. The drain resistor opens because of excessive drain current. In this case, the channel in failure can be identified by measuring the resistance of a MOS-FET; if the resistance between the drain and source of a channel is 0 ohm, and the drain resistance is � ohms, that channel is in failure. You can also visually confirm the channel in failure from a damage scar on the surface of the part generated as a result of opening of the drain resistor. If one of the paired MOS-FETs in a push-pull circuit is damaged, another MOSFET may be damaged even if the resistance measured by a tester shows no sign of short-circuiting of that MOS-FET. Replace both parts when either needs to be replaced.
Drain resistor � Open Nch MOS-FET � Short-circuiting between the drain and source Power-amplifier output Pch MOS-FET Drain resistor
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2. Damage to the voltage amplifier section
The voltage amplifier section may be damaged by oscillation caused by unusual load connection, high-frequency output outside the audible frequency range with high amplitude, or the effect of damage to the power amplifier section. One possible part to be damaged is the IC, and two power resistors and four output resistors may be open. As a MOS-FET and IC are simultaneously damaged in most cases, both the MOS-FET and IC must be replaced when the MOS-FET is damaged. Therefore, if either the N-ch MOS-FET, P-ch MOS-FET, or IC needs to be replaced, replace all of them. In a rare case when only the IC was damaged, the channel in failure can be identified by searching for a channel that outputs DC voltage when the power is turned on. If is recommended that failure diagnosis of the power amplifier of this unit be performed with the power off, because the power amplifier section, which has a small number of components, can be easily repaired by replacement of semiconductors or according to measurement of resistance values. If failure diagnosis of the voltage amplifier section must be performed while the power is on, be sure to disconnect the connection between the voltage amplifier section and the MOS-FET. If a measuring instrument touches the gate of the MOS-FET being powered, the MOS-FET may be damaged by its oscillation.
Power resistor Output resistor

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To MOS-FET

Power resistor Output resistor
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3. Damage to other parts
If the power amplifier is damaged, the bias protection diodes may be degraded or open. After repairing the power amplifier, check that the four bias protection diodes of the channel in failure are not degraded or open by measuring their Vf (forward voltage drop) with a tester. If these diodes are degraded, distortion may appear or the frequency characteristics may be affected. If they are open, when any abnormality occurs in the power amplifier next time, the damage to the MOS-FETs may be expanded.
Bias protection diodes

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VSA-AX10Ai-S
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