As mentioned in my last blog post the GuruPlug Server Plus needs some major refactoring before it can be used. Not doing so will make you end up with a fried brick. There are various stories in the forums about cooling it properly, so you might want to have a look first instead of following mine blindly. And of course - whatever you do - it is your fault when you end up with a brick, not mine!.

Replacing the power supply

As I would never ever trust the PSU (see my last blog post for the gory details) and some additional free space in the case is necessary, it is time to rip it out. As replacement I've ordered an external 5V/20W PSU. To connect it to the plug I've drilled a hole into the case and added a proper connector after soldering the original cable from the old PSU to it. Make sure to add some retainer to stop the cable from touching the CPU/memory heat sinks later.

GuruPlug Server Plus PSU replacement GuruPlug Server Plus PSU cabling

Proper heatsinks

One of the biggest problems in the original GuruPlug design is the heat spreading and collecting piece of alloy, which probably makes things even worse than better. After some discussion in #debian-devel the idea came up, that the board should be able to run fine without a heatsink - at least when idling. That is the case, indeed. So let's get rid of that piece of alloy, together with the holders and springs.

Unfortunately heat things are necessary to handle some work load. To mount them make sure you get some good thermal glue as you don't want to insulate the chips from their heat sinks. I've used Arctic Silver II thermal glue - not cheap, but working well. Make sure to clean the heat sinks and chips properly with alcohol (2-Propanol is my favourite for that), so there is no fat and dust left. When you use the glue be careful to read the documentation properly and make sure you apply only as much as necessary - a thin coating is enough. After applying the glue and mounting the heat sink you need a clamp to put some pressure on it while the glue dries.

Heat sinks for the memory

As there is a lot of space around the memory chips, I've got some 14x14x6mm heat sinks. Luckily there are only small parts on the other side of the board, so it was not hard to attach clamps to ensure the glue is able to dry properly. In case largish amounts of glue are squeezing out between heat sink and chip you probably want to start from scratch and clean everything (quickly!) with alcohol again.

GuruPlug Server Plus memory heat sink clamping

Gigabit Ethernet PHY and CPU heatsinks

PHY and CPU need proper heatsinks as they're becoming pretty hot under load. Due to space limitations I had to stick with a 17x17x20mm heat sink for the PHY. I've tried to get a largish heat sink with BGA mount for the CPU, but I was not able to find one in short time and without ordering 1000 of them. So I've got another one of the 17x17x20mm model for the CPU. Later I found two other heat sinks which might fit on the GuruPlug's CPU: Malico 19x19x25mm or a Advanced Thermal Solutions Maxigrip. But for now the smaller one will have to do the job, also I'm scared to removed it without destroying the CPU.

To mount the heat sink on the PHY a bit of preparation is necessary to ensure that the glue doesn't result in short cuts on the IC's pins. The arctic silver documentation recommends to use silicone on the pins to stop the glue. As I neither had the proper silicone nor did I want to make such a mess, I've decided to get a sticky insulating tape and some tooth picks to mount it properly on the IC pins. Make sure to do a proper job here! With good clamps you can work on the PHY while the memory heat sink glue is still drying.

GuruPlug Server Plus - preparing the PHY

The other problem on this side of the board is that some higher parts are mounted on the lower side of the board, so make sure to get some foam to protect them from your clamps. Except of the size of the heat sinks there is no real difference to the small ones: add glue, mount the heat sink and fix it with a clamp.

GuruPlug Server Plus - mounting the PHY heatsink

Before the glue dries completely make sure you remove the duct tape strips again. Be careful not to wipe any excess glue onto the IC pins. If you manage to do so you might want to remove and clean everything properly. So just be careful and everything should be fine.

GuruPlug Server Plus - mounting the PHY heatsink 3

Mounting the heat sink on the CPU is easier as the CPU is a nice BGA package. No need to protect pins an there are none.

GuruPlug Server Plus - mounting the CPU heatsink


As you're able to see on the photo I did not the very perfect job, but fine enough to work properly.

GuruPlug Server Plus - heatsinks

First tests with the new heatsinks

Some tests with stress and some heavy network traffic showed quickly that the heat sinks are working well! But not well enough to be able to mount them into the original closed case (how the hell was this supposed to work before!?). Without any air to breath the temperature on the CPU heat sink went up to 85 deg C quickly. So on the CPU's die are > 100 deg C, which is at least close to the max allowed temperature.

GuruPlug Server Plus - still too hot

Adding air holes

Obviously some additional air holes are necessary. After meeting my drill and cutter, the original case looks a bit like a piece of cheese now.

GuruPlug Server Plus - case with air holes 1 GuruPlug Server Plus - case with air holes 2

Some more tests show that the temperature sticks around 76 deg C under heavy load now. That is still pretty hot, especially when the temperature in the room becomes hotter in summer. As I'm not going to run a lot of traffic trough it (my cable connection is limited to 20 MBit/s anyway), I guess it will work. Below is a graph I've recorded with my multimeter and qtdmm. It shows that the temperature rises to ~76 deg C and stays there.

GuruPlug Server Plus - temperature curve

Not sure if I could introuce some more load, the CPU had a load of 6 running stress. Network traffic was around 2x100MBit/s again. I could imagine with 2xGBit the CPU and PHY would become much hotter (no idea if it would be able to handle it at all, though). The PHY was usually a bit cooler than the CPU, so I didn't spend time to measure the temperature properly.


My opinion is still the the original design of the plug is an insane QA and design failure. There is no way that it could ever work as shipped originally. If I'd have to design a new case for it, it should have a single insulated compartment for a PSU to ensure that no contact with high voltage is possible. The case around all other parts should be made of alloy with a lot of air holes. So the case could be used as heat sink (properly done, not with these insulating duct tape strips) and at the same time allow air circulation and a nice view on the LEDs on the board. Of course the case would be much more expensive than, but I'm sure it would work properly and would be worth the money. And if done peoperly, it would look much better than this blinking plastic thing. I could imagine it would sell well!

So at the end I'm wondering what GlobalScale Technologies will do. I doubt it will be possible to create the 'professional upgrade kit' as they had announced before. Replacing all units is the only proper way. We'll see what happens. At least my GuruPlug is working well now and it will be used as replacement for my OpenWRT router soon. WiFi is not working, but that will be handled by my media center anyway.


I've done some more modifications - keep on reading here.

GuruPlug Server Plus - running plug