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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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date = "2017-11-21"
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title = "Repair: Denon AVR-4520"
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tags = ["repair", "audiorepair", "denon"]
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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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description = "This unit sold from 2012-2015 as the top-of-the line AV Network 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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@ -11,49 +11,51 @@ description = "Sold from 2012-2015 as the top-of-the line AV receiver from Denon
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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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a Denon, a high-end model, undamaged, 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 because the AVR would _sometimes_
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go into power protection mode, altough it was described as _it doens't work, only a red light is blinking_.
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He also mentioned that the problem was not consistent. Sometimes the unit would start up and work fine
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for hours, other times the unit would start up to a blinking light.
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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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A red blinking light means the Denon unit is going into protection mode - most likely because of a
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power supply failure or other fault condition that might damage the amp or connected speakers.
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Power protection faults can be caused by many different things, but can often be fixed by replacing a
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few damanged components. After some haggling I bought the unit and got it in the mail a few days later.
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## Diagnostic
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With the **Denon AVR-4520** on my bench I was ready to start diagnostics.
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With the **Denon AVR-4520** on my bench I was ready to start diagnosing the problem.
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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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Before hooking the devise up to mains power (I should _really_ get an isolation transfomer) I opened
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it up and checked for any obvious failures: burned parts, blown fuses, etc. All seemed fine,
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which was consistent with the what seller told me.
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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 first time powering up a unit is always exciting. Will it be able to contain the magic smoke?
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The unit turned on fine. I then performed a factory reset to get rid of 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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Time to hook up a source and a pair of headphones and see what happens. 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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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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up HDMI to my monitor and attached an UTP cable for network connectivity. Did you know this thing
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has a built-in network switch? HDMI worked fine as well as Internet radio. Why was this amp going
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into protection mode? And _when_?
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Then I turned it off and on again: power protection mode!
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After listening to some music from about two hours, I turned the amp off and on again: _power protection mode_!
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Finally!
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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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That's kind of weird. The amp can run hot without issue, but starting the amp back up again whe hot
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is a no-go. But since we now got into protection mode, I could check what the amp had to say for itself.
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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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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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@ -62,39 +64,41 @@ you to browse view the last type of power protection triggered.
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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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protection mode and any voltage (probably 3.3V or higher) means that everything is dandy.
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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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outputs are _very_ close to 0VDC and have only an AC voltage. Having any significant DC output
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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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enabled. E.g. if your running in `STEREO` mode, only FL+FR are enabled with relays. The rest is simply
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disconnected.
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All channels were withing 3mV of `GND`, so no issues there _when running the amp_.
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All channels were withing 3mV of `GND`, so no issues there _when the amp is running_.
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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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Remembered that the problem with DC protection only occurs when powering up the amp _and_ when
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the amp is hot. The amp did not go into protection when running and getting hot. That means
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that the fault condition that triggers the DC protection _only_ occurs when booting up the
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amplifier.
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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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connects to the DC protection circuit shown above and the speaker terminals. I started unplugging
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connectors one by one 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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the problem did 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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Knowing the issue lies in one specific channel helps a lot, especially since this is an device has 9
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discrete channels.
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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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I wanted to see how the DC voltage on the output behaves on a good channel vs. the bad `SBR` channel. Presumably
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the `SBR` channel does not stabilize quickly enough. The amp takes maybe 5-10 seconds to let the amplifier
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circuit stabilize. The `SBR` channel, when hot, seems not to be able to stabilize fast enough.
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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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@ -103,18 +107,21 @@ 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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It's clear that the blue `FR` DC output stabilizes much quicker than the `SBR` channel. Also, the `SBR` channel does
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some weird stuff after it seems to have stabilized as well. Here's a close-up video of the scope output.
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{{< youtube rtnBzLxHoA0 >}}
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This is good. I now have a provable and reproducable fault condition. The output of the `SBR` channel will get
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a load of DC voltage during start up and seems to have trouble stabilizing when hot. Cooling down resolves the issue.
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The `SBR` channel is not in use, so any heat it takes is from the heatsink.
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This is good. I now have a provable and reproducable fault condition. The `SBR` channel will output
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a DC voltage when the amp expects all channels to have stabilized. This happens only during start-up of the amp when
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hot. Letting the amp cool down resolves the issue.
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Now as to what is causing this and how to fix it!
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Eventhough the `SBR` channel is not in use (I was running in stereo mode), the problem occurs. The heatsink
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will warm up the attached darlington transistors either way.
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Now to find out which components are causing this mayhem.
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Above is the `SBR` amp board taken out of the amp. I have three things I'd like to check:
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@ -123,11 +130,11 @@ Above is the `SBR` amp board taken out of the amp. I have three things I'd like
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3. Power darlington transistors: any obvious shorts? How do they react to heat?
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I always measure resistors in circuit first. This works fine most of the time and when I get a weird
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read out I can desolder them when I'm working on the capacitors. All of them seem fine.
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read out I can desolder them when I'm working on the capacitors. All of them seem fine and within spec.
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Next I desolder one (or two) legs of each electrolytic capacitor. I always mark their orientation in
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Next I desolder one (or if need be two) legs of each electrolytic capacitor. I always mark their orientation in
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relation to the board before taking them off. Some PCBs have polarity indicators, some do not. All
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electrolytics measure within their specifications for capacitance and ESR. So no issues there.
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electrolytics measure within their specifications for capacitance and expected ESR. So no issues there.
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That leaves the darlington transistor pair. My gues is that they start to misbehave when hot. Keep in
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mind that the `SBR` channel is not used in my stereo configuration, so they do not produce any heat
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@ -139,9 +146,9 @@ darlingtons is simple:
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3. Freeze spray each of the darlingtons to see if it resolves the problem
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Freezing the `DHCT-A3` has no effect on power protection. Freezing the `DHCT-C3` however makes
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the problem go away.
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the problem go away immedately.
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With that, I've narrowed the problem down to `Q808`, a `DHCT-C3` darlington transistor.
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@ -149,36 +156,39 @@ With that, I've narrowed the problem down to `Q808`, a `DHCT-C3` darlington tran
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As with almost every project, diagnosing the issue is much more time consuming than the
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actual repair. I ordered a _new old stock_ pair of transistors, this is a set of complimentary
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`-A3` and `-C3` transitors including two insulating washers.
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`-A3` and `-C3` transitors including two insulating washers. These are
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genuine Denon parts.
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Next it's a simple job to remove the old darlington transistors...
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and place the new ones.
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Finally re-insert the amp board into the amp with some fresh heatsink compound.
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**Do not
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forget to add the insulating washers to prevent them from shorting out via the heatsink!**
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## Conclusion
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Replacing the DHCT-A3/C3 darlington transistor pair has resolved the power protection mode
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issues. I've been running the amp in my living room for a few days now and not seen any
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issue at all.
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issues. I've been running the amp in my living room for a few ~days~ weeks now and hae not
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seen any problems at all.
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So, for the cost of a used entry-level AV receiver and about €15 in parts I've got
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an almost brand new Denon AVR-4520 top-of-the-line AV Receiver. :-)
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an almost brand new Denon AVR-4520 top-of-the-line AV Receiver. :-) _I'm not reselling this
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one any time soon_.
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_For those interested, I'm using the `SBR/SBL` channels to bi-amp my speakers._
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_For those interested, I'm currently using the `SBR/SBL` channels to bi-amp my speakers. No, I
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don't hear any difference, but I've got 6 unused channels in my setup, so why not use them?_
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[hifiengine]: https://www.hifiengine.com/manual_library/denon/avr-4520.shtml
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