Replacing faulty electrolytic capacitors in AVM Fritz!Box 7270 DSL router and installing active cooling in it

I have an AVM Fritz!Box 7270 DSL router, which started to show failure symptoms after about three years in use. First, little by little, DSL connection started to become more and more unstable and with external DSL-modem, it’s routing speed slowed down to as low as 1Mbit/s. Rebooting the device helped for short periods, but that was also problematic. When I let the device to cool down, after that it required 5-10 minutes of warming up, before it started properly. When I first plugged the power cord in, it lighted all LEDs on red. Then after about 10 minutes, if I pulled the power cord off and put it immediately back, it started properly.

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Device ran quite warm and I first thought it is the heat that makes the device malfunctioning. So I opened the casing and put a computer fan to blow towards it. As a result, box crashed after few minutes. I repeated this test few times and it crashed every time I cooled it down with fan. So all symptoms referred that device’s electrolytic capacitors are failing. I also measured voltages on capacitors and they were more or less of their tolerances.

So I checked all the capacitors and ordered same or similar ones from Capking. Replacement caps doesn’t have to be exactly same valued. 100V caps can be replaced with 63V caps, I measured that highest cap voltage was 51V (48V normal) You can also unify capacitance values a bit by rounding them up.

1000 µF 16V 3 pcs *same*
220 µF 10V 1 pcs *same*
100 µF 10V 3 pcs *replaced with 100uF/16V*
68 µF 16V 1 pcs *replaced with 100uF/16V*
47 µF 16V 1 pcs *replaced with 100uF/16V*
33 µF 16V 1 pcs *replaced with 100uF/16V*
10 µF 35V 1 pcs *replaced with 22uF/100V*
10 µF 100V 2 pcs *replaced with 22uF/100V*
4,7 µF 100V 2 pcs *replaced with 10uF/63V*

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These pictures are taken still with old C(r)apxon capacitors. I just don’t understand why to ruin feat of German engineering with low quality capacitors. I replaced them with Rubycon and Nichicon.Those three chips ran hottest and were uncomfortable to touch so I took heat sinks left over from Arctic Cooling’s Accelero graphic card cooler. They have glue sticker in the bottom so they are easy just to press on place.

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Nichicon HZ 1000uF/16V caps are bit taller than original Capxons so the top cover won’t completely fit anymore. I cut the original fasteners from the bottom frame of and fastened top cover with tie wraps.

Like I said, box runs quite warm, so in addition replacing the faulty capacitors (which fail faster if ambient temperature is high), I also decided to install active cooling in it. I made a hole to a top cover and installed a fan in it. Fan is 60mm and powered with home made splitter from box’s power supply. Voltage is lowered to about 8 volts with potentiometer+transistor circuit. Fan is inaudible after about two meter distance even in silence at night. Fan grill is home made from some suitable steel grid and painted black.

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Want to adjust fan speed but can’t find potentiometer with sufficient power handling capacity? Simple and cheap option is to use normal low power potentiometer and bipolar transistor. Potentiometer can be around 2-5 kilo-ohms and transistor can be basically anything as long as it has TO-220 or some other “big” casing which can dissipate enough heat. Connect the center of the potentiometer to transistor base, one end to the collector and the other end to the emitter.

Now only transistor type (PNP or NPN) defines how this staggering two (2) component circuit is connected to series with fan. If it is PNP-transistor, it is connected on a positive side of the fan, collector to power supply positive and emitter to fan positive. If it’s NPN, you put it on negative side of the fan, collector to fan negative and emitter to power supply negative/ground. Transistor heats a bit, but it should not require additional cooling. If when fan running slowly you can touch transistor, then it is OK as is, but if it feels too hot, you can put small heat sink or metal plate (2x1cm or something, it doesn’t have to be big) on the back of the transistor. Note that the back is connected to transistor collector so don’t ground it to anything.

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So now I have fully working router again and it hopefully lasts “till the end”, since it now has quality capacitors and active cooling.