The bosch tassimo drinks machine model TAS 6515GB has a circuit board inside with a large resistor mounted close to a heat sink.

SOURCE: Tassimo machine leaking water underneath

I'm having the same problem with my machine. I rang the Tassimo careline and they told me to descale the machine but, so far, this has only made matters worse. I'm now going to take it back to where I got it from because it's still within its warranty.

SOURCE: Bosch Tassimo machine leaking from inside! How do I dismatle it?

Before you attempt to dismantle the Bosch Tassimo, you need to determine how/where it is leaking. (1) Before making a drink with the unit remove the cup tray and ensure the bottom where it fits is dry. Replace the drink tray (2) Make a drink or use the cleaning disk, put a cup under as normal and start the cycle. (3) After the drink is made or a clean cycle remove the bottom cup tray and see if the bottom where it fits is wet.
If it's wet you have two (2) problems
(1) The pressure valve on the top of the heater is staying open, usually due to using water that has lots of calcium in it. (2) The pipe that leads from this pipe to the bottom tray is leaking (it's not a good fit in any of these machines, and causes water to drip from underneath.
Don't take it apart yet! Buy some de-scaler from AP-Wagner.com and run this a few times, it should clear it. If it doesn't you have no choice but to open the thing.
If you have the model with a filter use it and change it when it tells you. If you don't have a built in filter, don't use water directly from the tap, run water through a Brita filter or similar
If the bottom tray doesn't get wet, take it apart to see where it's leaking. 99.99999999999999% of the time it the valve.

So how to take the thing apart.
(1) The sides (silver or black) slide to the rear. However, you have to put something in the back between the side and the rear cover to pry them apart while pushing to the rear. It's very difficult. I use a 6" steel ruler. You have to bend the sides to overcome the catches that hold them in place. You'll see a small rectangular slot at the back of each side. They will come off eventually.
(2) Once off you'll have to remove the lid where you put the disc in. Remove the 4 screws under the lid deck. (3) Remove the water pipe screws (2) and the bar code scanner screws (2). Be careful of the barcode scanner the cable is very, very delicate (I always unplug it from the circuit board and remove it completely, lift the black tabs at either side of the cable). Lift of the lid assembly. (3a) Remove the screw from the underneath the unit at the back, this allows the front to move out. (4) At each corner of the top carefully pry the locking mechanisms until the top is free. Remove the pink (thick) pipe and the pink (thin) pipe and turn the top around out of the way of the heater unit. A coiled steel spring will drop out, it's easy to replace when the top is back on. (5) Where the thin pink pipe goes to is a back pressure release valve carefully pry the clips apart and remove. Take out the rubber O-ring and the valve. Carefully remove any calcified deposits from under the rubber (blue) valve. (6) Reassemble valve (7) Reassemble top, and lid taking care with the bar code reader. (8) Test before replacing the sides
The thick pink pipe that descends from the top, that you removed earlier goes to the bottom tray, you will notice it is very loose (they all are) and it leaks. Put a tie wrap around it to seal it.
Good luck, it's not an easy fix, they were never meant to be taken apart to be repaired - not very ECO friendly. Let your MP/Congressman know, these should end up in a landfill because of 50 cent valve, and they don't come apart.

SOURCE: I have a tassimo tas

You should be able to get descalers from your local supermarket.

SOURCE: how much bicarb of soda do i use to descale my bosch tassimo

Sodium bicarbonate is not a descaling product. It makes a nice minimally abrasive cleaning agent when mixed as a paste. Sodium ions will exchange with calcium in a water softener when the calcium is dissolved in water. Once the calcium salt (calcium carbonate) forms, it doesn't go back into solution in a neutral or basic environment. A bicarbonate solution will be basic.

Descaling requires an acid. The Tassimo products say "Do not use vinegar" (acetic acid). I suspect they use citric acid in their descaling product. You can try the lemon-flavored Kool-aid (the version packaged in the envelopes that needs a sweetener added when it's a drink). Mix 1 tablespoon of the powder in 1 pint of water. However, you'll need to make more than a pint of the solution since you need 500 ml of liquid per cleaning cycle. (1 pint is ~473 ml.)

I hope this helps.

Cindy Wells