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date = "2017-09-11"
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title = "Denon AVR-4520 Repair"
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tags = ["repair", "audiorepair"]
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categories = [ "Audio Repair" ]
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description = "Sold from 2012-2015 as the top-of-the line AV receiver from Denon, I got my hands on a unit with power protection issues. Here's the repair log the Denon AVR-4520."
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![Denon AVR-4520 - Front view](/img/denon-avr4520-frontview.jpg)
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## Background
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Recently I came across a **Denon AVR-4520** on _Marktplaats_. I was immediately interested because it's
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a Denon, an undamaged high-end model, and sold with a defect. The seller was dumping his entire AV
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stack, including Blu-ray player and Jamo floorstanding speakers. Reason: the AVR would _sometimes_
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go into power protection mode.
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I tend to stay away from AVRs that have no audio or video. 9 out of 10 times there's an issue with
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HDMI that can only be fixed with an expensive replacement of the entire HDMI board. Power protection
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faults can be caused by many different issues, but I'm mostly can be fixed by replacing a few damanged
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components. After some haggling I bought the unit and received it a few days later.
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![Denon AVR-4520 - Back view](/img/denon-avr4520-backview.jpg)
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## Diagnosis
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With the **Denon AVR-4520** on my bench I was ready to start diagnostics.
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Before hooking up any new device to mains power and powering it up I like to lift the cover and do
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a quick visual inspection. Although this unit is _packed_, I was able to check the amp boards and
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see no obvious damage.
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The first time powering up a unit is exciting. Will it blow up? Will it release its magic smoke?
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Not this Denon. It turned on just fine. _Wait what?_ I then performed a factory reset as described
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in the service manual - just to erase any problematic settings.
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The previouw owner claimed that the unit would _sometimes_ go into power protection mode. Ok,
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let's see if I can reproduce that. I turned the amp off an on about 30 times. No problems.
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Time to hook up a source and a pair of headphones and see what happens. I prefer starting out
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with analog stereo input, as has fewer parts that could have failed already. The amp played
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perfectly fine. Next I hooked up a pair of speakers and enjoyed my tunes for a bit.
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![Testing the Denon AVR-4520](/img/denon-avr4520-testing-01.jpg)
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I let the amp play for about an hour without issue. No power protection, just music. I hooked
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up HDMI to my monitor and attached an UTP cable for network connectivity. HDMI Out seemed to
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be working fine as well as Internet radio. Why was this amp going into protection mode? And _when_?
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Then I turned it off and on again: power protection mode!
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Aha! My assumption was that the amp does not fail when getting hot, but does fail when hot and
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powering on. First test: let the amp cool down and see if it turns on. After resetting the power
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protection it started up fine.
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Next I checked the error messages to see what _kind_ of power protection was triggered. The service
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manual tells me to power on the amp with `ZONE/REC SELECT` and `STATUS` buttons pressed, then
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select `2. PROTECTION`. This restarts the amp in diagnostics mode. Pressing the `STATUS` button allows
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you to browse view the last type of power protection triggered.
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![Power Protection: DC](/img/denon-avr4520-dc-protection.jpg)
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So, **DC Power Protection**. The service manual provides the following bit of schematic. The `DCDET`
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(for DC Detection) net goes to the CPU and should be _active low_, meaning that measuring ~0V triggers
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protection mode.
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![DC Power protection circuit](/img/denon-avr4520-dc-protect-circuit.png)
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This circuit monitors the amp outputs (e.g. output to the speakers) for a DC voltage. Normally, these
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outputs are _very_ close to 0VDC and have only an AC voltage. Having any significant DC output can and
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will damage your speakers, so it's a good thing the amp shuts itself down when there's a problem.
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To figure out which channel(s) are outputting DC I first measured each channel with my multimeter. To
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do this, turn on the amp, select an input that has no signal coming in and turn the volume all the way
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down. You cannot measure the DC directly on the speaker terminals on the back, because they may not be
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enabled (e.g. if running in `STEREO` mode, only FL+FR are enabled).
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![Measuring DC on speaker outputs](/img/denon-avr4520-dc-on-output.jpg)
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All channels were withing 3mV of `GND`, so no issues there _when running the amp_.
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Then I remembered that the problem with DC protection only occurs when powering up the amp _and_ when
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the amp is hot. To do this, I let the amp run for about 30 minutes, cycled the power and it went into
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protection mode.
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Each channel has its own little amp board with an easily accessible connector for the output. This output
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connects to the DC protection circuit shown above and the speaker temrinals. So I started unplugging
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connectors until the amp would start up without issue. This way I quickly figured out that the `SBR` or
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_Surround Back Right_ channel was the one causing the DC Protection mode. With just that board disconnected,
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the problem does not occur.
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Knowing the issue lies in one specific channel helps a lot (especially since this is an 11.2 channel amp).
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Next I wanted to see how the DC voltage on the output behaves on a good channel vs. the bad `SBR` channel.
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The working theory being that the `SBR` channel must be doing things the DC protection circuit does not like
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_during start up_. I already confirmed the DC on the `SBR` channel was well within parameters while running
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the amp and measuring the voltage.
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This is where a 2+ channel oscilloscope comes in _very_ handy. I hooked up my scope to the `FR` and `SBR` channels
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to see what the DC voltage on each channel does during start up.
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Blue is the `FR` channel, which works correctly, yellow is the bad `SBR` channel.
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{{< youtube QnaiWPzQGj0 >}}
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It's clear that the blue `FR` DC output stabilizes much quicker than the `SBR` channel. Here's a close-up video
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of the scope output.
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{{< youtube rtnBzLxHoA0 >}}
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This means one (or two) of two things:
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1. There's a bad capacitor on the `SBR` amp board
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2. The output transistor(s) are faulty when warm
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Next step: take out the board and check the capacitors.
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![](/img/denon-avr4520-sbr-amp-board.jpg)
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I always mark the capacitors with a permanent marker by ticking one side (e.g. )
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## Repair
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[hifiengine]: https://www.hifiengine.com/manual_library/denon/avr-4520.shtml
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