devroom.io/content/electronics/repair-denon-avr-4520.md

+++
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)

## Diagnostic

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 is good. I now have a provable and reproducable fault condition. The output of the `SBR` channel will get
a load of DC voltage during start up and seems to have trouble stabilizing when hot. Cooling down resolves the issue.
The `SBR` channel is not in use, so any heat it takes is from the heatsink.

Now as to what is causing this and how to fix it!

![](/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. 

Next I desolder one (or 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 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. 

![](/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.

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

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

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

and place the new ones.

![](/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!**

![](/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 now and not seen any 
issue at all.

![](/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. :-)

_For those interested, I'm  using the `SBR/SBL` channels to bi-amp my speakers._

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