198 lines
10 KiB
Markdown
198 lines
10 KiB
Markdown
+++
|
|
date = "2017-11-21"
|
|
title = "Repair: Denon AVR-4520"
|
|
tags = ["repair", "audiorepair", "denon"]
|
|
categories = [ "Audio Repair" ]
|
|
summary = "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."
|
|
aliases = [
|
|
"/2017/11/21/repair-denon-avr-4520/"
|
|
]
|
|
+++
|
|
|
|
![Featured](featured.jpg)
|
|
|
|
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?_
|
|
|
|
_Update: for those looking for the service manual, you can find it at [HifiEngine]._
|
|
|
|
[hifiengine]: https://www.hifiengine.com/manual_library/denon/avr-4520.shtml
|