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