Onkyo A-9555 Amplifier Protection

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XgN

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Joined 2014
Hi guys!

This time got Onkyo A-9555 amplifier for repair.

Amplifier is in Protection mode - Standby LED is blinking. And Reset doesn't help as in many cases desbribed here on the internet.
So long story short:
  1. Of course, started by measuring all the voltages and checking components for visual defects.
    Found Zener short to GND. Replaced both of them. Please refer to the schematic below.
    Onkyo1 — imgbb.com
    These two signals belong to SPRDET and SPLDET circuitry which responsible for protection and/or speaker/voltage detection at the output.
    Now the voltages are good at the nodes. Previously one was pulled-down to GND by the short Zener.
  2. Found that R645 is burned [for which Zener was blown]. Placed a jumper instead.
    Onkyo2 — imgbb.com
  3. Amplifier is still in Protection mode. Tried to disconnect power plugs P503 and P504. Vuola, without -57V amplifier starts and works properly (of course without PA part).
    Onkyo3 — imgbb.com
  4. Checked what's wrong with this -57V rail. So whenever amplifier is starting up and checking the voltage at the ouput this -57V comes into play and is present at the both outputs. So the protection works and the amp goes into Protection mode.
    Onkyo4 — imgbb.com
  5. Checked mostly of active and passive components in PA with continuity [diode] function and they are all seem to be good - their values in proper range for diodes, transistors, etc. Power mosfets in half bridge seem to be good too. Haven't checked with component tester though as it is hard to pull them out of the PCB.
Maybe some new ideas to try and check.
Thanks in advance.

P.s. Full schematics in .pdf here:
WeTransfer
 
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