Hello all,
I am having fun repairing an Onkyo TX-SR577 AV receiver. It was shorted so it was powering down immediately once the ON/OFF button was pressed and my current limiter (light bulbs) was confirming a short (bulbs were flashing bright).
I found two shorted transistors and a few more bad components on the AMP board.
I've replaced them all and the short is gone (no more lightbulb flashes) BUT the Onkyo still refuses to start.
Let me explain.
The Onkyo powers a small Stand-By transformer when mains is applied. That powers up the CPU and enables the ON/OFF button. When ON/OFF button is pressed, a CPU line goes to ground, the main 240V relay energises and voltage is applied to the main transformer.
What happens is that the main 240V relay engages but disengages after about one second. But my "current limiter" (light bulbs!) does not show a short anymore.
So I attempted jumping the main transformer to 240V directly - so it gets powered regardless (yes, I do have a current limiter connected!).
Weirdly enough, the AVR works! It powers up ok (even though it tries to power down by opening the mains relay, but mains is jumped to the main transformer) and all channels work, including the one I have repaired.
Now, the service manual has all the schematics but does not say what is the "logic" behind the scenes.
In my case, the APOWER line from the CPU is controlling a transistor which in turn grounds POWERD which drives the mains relay.
There must be something the CPU is sensing which triggers a full shutdown.
Does anybody have any ideas? Is there any documentation on how that works?
Thank you!
I am having fun repairing an Onkyo TX-SR577 AV receiver. It was shorted so it was powering down immediately once the ON/OFF button was pressed and my current limiter (light bulbs) was confirming a short (bulbs were flashing bright).
I found two shorted transistors and a few more bad components on the AMP board.
I've replaced them all and the short is gone (no more lightbulb flashes) BUT the Onkyo still refuses to start.
Let me explain.
The Onkyo powers a small Stand-By transformer when mains is applied. That powers up the CPU and enables the ON/OFF button. When ON/OFF button is pressed, a CPU line goes to ground, the main 240V relay energises and voltage is applied to the main transformer.
What happens is that the main 240V relay engages but disengages after about one second. But my "current limiter" (light bulbs!) does not show a short anymore.
So I attempted jumping the main transformer to 240V directly - so it gets powered regardless (yes, I do have a current limiter connected!).
Weirdly enough, the AVR works! It powers up ok (even though it tries to power down by opening the mains relay, but mains is jumped to the main transformer) and all channels work, including the one I have repaired.
Now, the service manual has all the schematics but does not say what is the "logic" behind the scenes.
In my case, the APOWER line from the CPU is controlling a transistor which in turn grounds POWERD which drives the mains relay.
There must be something the CPU is sensing which triggers a full shutdown.
Does anybody have any ideas? Is there any documentation on how that works?
Thank you!
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