HRM-dual - how to clean the chest-strap? It stinks after a few uses and cant get the smell out of it

Take the module off and machine wash the strap weekly as recommended on the care instructions. 

I use dish soap. Video tutorial: https://youtu.be/cZ-DHex_Iag?feature=shared

I put an asterisk * after the name of an activity to know when I cleaned it, and count 7 activities before cleaning again. Also rinse after every use and hang to dry.

Leave it on while you shower, wash it then.  Rinse after every use if you don't shower.

I dont really dare to put it into a machine wash just for the fact that one already got damaged. 

Ok but looks like the secret is to never forget to wash it ever.

But what if you dont have a chance to wash it on every occasion like me, is there any chance to remove the smell? I mean I had other kinds of stuff that was smelly but was easy to clean, but the strap is like holding on to the smell no matter what you do. 

I would try hydrogen peroxide. Kills most germs. Leave it on for a minute or two.

did someone try that already? I fear that this will dissolve the essential layer of the contact patches and render the strap useless. 

If you are concerned about damaging the sensor, put it on the elastic only. That's where the stink is coming from anyway.

Remember, it reacts mostly with organic matter. It shouldn't damage the strap.

I take the sensor off after every run and rinse it under tap water (I don’t use any detergent because it might affect the rubber inside). Then I wash the strap with body soap, let it air dry (avoid wringing as it may damage the electrodes). 

Lots of sensible tips already given here. 

I have mostly good experiences with Polar brand belts to clip my HRM-Run on. They smell a little bit when.new, but it gets better over time. I had just one that stank right out of the box, which makes me think that it maybe is temperature related damage that occurred during storage or shipping, or degradation of materials or the bactericidal fabric/coating during long term storage.

Immediately after use I rinse the strap in clear water,  then let it soak for a while, then rinse again, then hang to dry. I alternate two, so they get a chance to properly dry before the next use. Using laundry detergent may damage the bactericidal coating/fabric, so I don't use that. Baby shampoo is probably fine though.

Whatever you do, avoid the spin dry cycle if you put it in the washing machine, and always rinse directly after use. Which is not practical on race day. Once a strap stinks, either before first use or after a long time, it will remain smelly. Keeping a stinky one in a tin for use on race days only may be an idea.

the elastic part is not the part that is smelly, only the fabric where the electrodes are attached too. This seems to be a very special compound.