How to fix sporadic dropouts on a FRITZ!Box

Introduction

My FritzBox 7362 SL had more and more dropouts, i.e. the internet network access was not usable for some minutes. This did not happen during the daily reconnection with the provider (around 6:00h in the morning for me), but it happened at totally random times.

At first the problem occurred only every few weeks, eventually several times a week.

When I went to http://fritz.box and looked under System/Events, there was always only the log starting with the respective dropout. The Fritzbox had "forgotten" all previous events. As if someone had briefly pulled the power plug to force a restart of the FritzBox.

Finally, the dropouts became so frequent that I was able to watch one of them live: the FritzBox briefly flashed red and then restarted - as if someone had pulled the plug for a moment.

Step 1 Reading the log

How to fix sporadic dropouts on a FRITZ!Box, Reading the log: step 1, image 1 of 2

How to fix sporadic dropouts on a FRITZ!Box, Reading the log: step 1, image 2 of 2

Note the date and time of the last Internet outage.
Open a web browser and log into the user interface of your FritzBox by entering http://fritz.box.
Select "System" from the left menu, then "Events" below that.
Scroll all the way down in the events and look at the date and time.
If the lowest entry, furthest back in time, coincides with the last Internet outage, there is a good chance that it is the error described here.

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Step 2 Self test of the FritzBox

How to fix sporadic dropouts on a FRITZ!Box, Self test of the FritzBox: step 2, image 1 of 2

How to fix sporadic dropouts on a FRITZ!Box, Self test of the FritzBox: step 2, image 2 of 2

Start a self test of the FritzBox to make sure that the error does not lie with the FritzBox.
To do this, click on Diagnostics and then Function and start the self-test by clicking on Start.
During the self-test, the FritzBox functions continue to work and there is no interruption of the Internet connection.
If the self-test found errors, the power supply is not the cause.

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Step 3 Determine the type of power supply

How to fix sporadic dropouts on a FRITZ!Box, Determine the type of power supply: step 3, image 1 of 2

How to fix sporadic dropouts on a FRITZ!Box, Determine the type of power supply: step 3, image 2 of 2

Look at the power supply of your FritzBox. If it is the power supply shown here, there is a good chance that it is the described problem.
On the nameplate it says: FW7577/EU/12/D

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Step 4 Select new power supply

How to fix sporadic dropouts on a FRITZ!Box, Select new power supply: step 4, image 1 of 1

If the previous points apply, a replacement of the power supply will probably help.
By the way: I had also opened the power supply, cleaned contact points, re-soldered solder joints and checked electrolytic capacitors by capacitance measurement. Unfortunately, I could not eliminate the error in the power supply. With sporadically occurring errors it is also not so easy.
The new power supply does not have to be an original FritzBox power supply. Important is the following:
Voltage 12V direct current (DC); current at least 1.4A; the hollow plug, which is plugged into the FritzBox, must have the following dimensions: Diameter 5.5mm outside, 2.5mm inside (pin). The positive pole must be inside (on the pin).
A higher-quality power supply is recommended because it has a longer life expectancy or higher reliability and consumes less power via better efficiency.

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Step 5 Replacing the power supply

How to fix sporadic dropouts on a FRITZ!Box, Replacing the power supply: step 5, image 1 of 1

Unplug the old power supply from the power outlet.
Trace the cable coming from the old power supply to the FritzBox.
Unplug the old power supply from the FritzBox and replace it with the new power supply.
Connect the new power supply to the power outlet.
If someone has managed to repair the old power supply: Please post instructions here, so that others can benefit from it :-)