+++
date = "2017-11-21"
title = "Repair: Denon AVR-4520"
tags = ["repair", "audiorepair", "denon"]
categories = [ "Audio Repair" ]
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."
+++

![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, a high-end model, undamaged, and sold with a defect. The seller was dumping his entire AV 
stack, including Blu-ray player and Jamo floorstanding speakers because the AVR would _sometimes_ 
go into power protection mode, altough it was described as _it doens't work, only a red light is blinking_.
He also mentioned that the problem was not consistent. Sometimes the unit would start up and work fine
for hours, other times the unit would start up to a blinking light.

A red blinking light means the Denon unit is going into protection mode - most likely because of a
power supply failure or other fault condition that might damage the amp or connected speakers.

Power protection faults can be caused by many different things, but can often be fixed by replacing a 
few damanged components. After some haggling I bought the unit and got it in the mail a few days later.

![Denon AVR-4520 - Back view](/img/denon-avr4520-backview.jpg)

## Diagnostic

With the **Denon AVR-4520** on my bench I was ready to start diagnosing the problem.

Before hooking the devise up to mains power (I should _really_ get an isolation transfomer) I opened
it up and checked for any obvious failures: burned parts, blown fuses, etc. All seemed fine,
which was consistent with the what seller told me.

The first time powering up a unit is always exciting. Will it be able to contain the magic smoke? 
The unit turned on fine. I then performed a factory reset to get rid of 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. 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. Did you know this thing
has a built-in network switch? HDMI worked fine as well as Internet radio. Why was this amp going 
into protection mode? And _when_?

After listening to some music from about two hours, I turned the amp off and on again: _power protection mode_!
Finally! 

That's kind of weird. The amp can run hot without issue, but starting the amp back up again whe hot
is a no-go. But since we now got into protection mode, I could check what the amp had to say for itself.

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 and any voltage (probably 3.3V or higher) means that everything is dandy. 

![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 
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 your running in `STEREO` mode, only FL+FR are enabled with relays. The rest is simply 
disconnected. 

![Measuring DC on speaker outputs](/img/denon-avr4520-dc-on-output.jpg)

All channels were withing 3mV of `GND`, so no issues there _when the amp is running_. 

Remembered that the problem with DC protection only occurs when powering up the amp _and_ when
the amp is hot. The amp did not go into protection when running and getting hot. That means 
that the fault condition that triggers the DC protection _only_ occurs when booting up the
amplifier. 

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 terminals. I started unplugging 
connectors one by one 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 did not occur.

Knowing the issue lies in one specific channel helps a lot, especially since this is an device has 9 
discrete channels. 

I wanted to see how the DC voltage on the output behaves on a good channel vs. the bad `SBR` channel. Presumably
the `SBR` channel does not stabilize quickly enough. The amp takes maybe 5-10 seconds to let the amplifier
circuit stabilize. The `SBR` channel, when hot, seems not to be able to stabilize fast enough. 

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. Also, the `SBR` channel does
some weird stuff after it seems to have stabilized as well. Here's a close-up video of the scope output.

{{< youtube rtnBzLxHoA0 >}}

This is good. I now have a provable and reproducable fault condition. The `SBR` channel will output
a DC voltage when the amp expects all channels to have stabilized. This happens only during start-up of the amp when 
hot. Letting the amp cool down resolves the issue. 

Eventhough the `SBR` channel is not in use (I was running in stereo mode), the problem occurs. The heatsink 
will warm up the attached darlington transistors either way. 

Now to find out which components are causing this mayhem.

![The Surround Back Right amplifier board](/img/denon-avr4520-sbr-amp-board.jpg)

Above is the `SBR` amp board taken out of the amp. I have three things I'd like to check:

 1. The 1/4W and more resistors (basicall all non SMD resistors on top). I'd like to check for any shorts, open loops, or a deviation in resistance from the spec?
 2. All electrolytic capacitors: how does their capacitance and ESR compare to the spec? 
 3. Power darlington transistors: any obvious shorts? How do they react to heat?

I always measure resistors in circuit first. This works fine most of the time and when I get a weird
read out I can desolder them when I'm working on the capacitors. All of them seem fine and within spec. 

Next I desolder one (or if need be two) legs of each electrolytic capacitor. I always mark their orientation in 
relation to the board before taking them off. Some PCBs have polarity indicators, some do not. All 
electrolytics measure within their specifications for capacitance and expected ESR. So no issues there.

That leaves the darlington transistor pair. My gues is that they start to misbehave when hot. Keep in 
mind that the `SBR` channel is not used in my stereo configuration, so they do not produce any heat 
themselves. They get heat from `FR` that's mounted on the same heatsink. The plan to test these
darlingtons is simple:

 1. Run them amp until it gets hot
 2. Try to get the amp hot enough to get into the DC Protection mode.
 3. Freeze spray each of the darlingtons to see if it resolves the problem

Freezing the `DHCT-A3` has no effect on power protection. Freezing the `DHCT-C3` however makes
the problem go away immedately. 

![Your friendly neighbourhood Darlington Transistor Pair](/img/denon-avr4520-darlingtons.jpg)

With that, I've narrowed the problem down to `Q808`, a `DHCT-C3` darlington transistor. 

## Repair

As with almost every project, diagnosing the issue is much more time consuming than the
actual repair. I ordered a _new old stock_ pair of transistors, this is a set of complimentary
`-A3` and `-C3` transitors including two insulating washers. These are
genuine Denon parts.

![New darlington transistors](/img/denon-avr2520-new-transistors.jpg)

Next it's a simple job to remove the old darlington transistors...

![Transistor Transplant](/img/denon-avr4520-new-and-old.jpg)

and place the new ones.

![Shiny new Darlington transistors](/img/denon-avr4520-new-transistors-soldered.jpg)

Finally re-insert the amp board into the amp with some fresh heatsink compound. 

**Do not
forget to add the insulating washers to prevent them from shorting out via the heatsink!**

![SBR Mounted back onto the heatsink, ready for action](/img/denon-avr4520-final-solution.jpg)

## Conclusion

Replacing the DHCT-A3/C3 darlington transistor pair has resolved the power protection mode
issues. I've been running the amp in my living room for a few ~days~ weeks now and hae not 
seen any problems at all. 

![Current Denon AVR-4520 Setup with my Technics SL-3310 turn table](/img/denon-avr4520-final-setup.jpg)

So, for the cost of a used entry-level AV receiver and about €15 in parts I've got 
an almost brand new Denon AVR-4520 top-of-the-line AV Receiver. :-) _I'm not reselling this
one any time soon_. 

_For those interested, I'm currently using the `SBR/SBL` channels to bi-amp my speakers. No, I 
don't hear any difference, but I've got 6 unused channels in my setup, so why not use them?_

[hifiengine]: https://www.hifiengine.com/manual_library/denon/avr-4520.shtml