Perhaps the BL3500 is in L.OPT 0 where it doesn't respond to IR codes. To put it in L.OPT 2:

• If using Be0link 1000 remote, press LINK 2 STORE.
• If using Beo4, press the POWER (red) button and LIST together.  Then press LIST until OPTION is shown, press GO (centre button), and then press LIST to show L.OPT.  Then press 2.

Now try MENU MENU 04 GO again.  

Hi Jg1976. You mention you can't activate your BeoLab 3500 MK1 to listen to the MCL socket. What does your display show after pressing 'Menu > Menu > 0 > 4 > Go' ? (This sequence should switch the BeoLab 3500 to the MCL input) You could also check your BeoLab 3500 in 'Test mode'. To do this press 'Menu > 0 > 2 > Go' And then check what you see in the display:

• ML OK = Master Link Connection is OK
• NO ML = Master Link is not connected
• Error 1 = Address configuration not possible
• Error 2 = Master Link data pulled low
• Error 3 = Master Link data pulled high
• Error 4 = Data collision on Master Link

If you see 'ML OK', the BeoLab functions properly, and your cable will probably be wrong. Connections and pins used might be the problem. If you are handy yourself, you can make such cable yourself. Lots of info on this forum. You can also ask @Steve of SoundsHeavenly to help you with the right cable.  

Thank you for all help! The Beosound 3500 is still stone dead. Nothing happen.. It is now in L-opt 2 (thank you Guy) I have also tried all other L.opt. KolfMAKER, nothing happens when I press MENU 0 4 GO. Nothing happens when I press MENU 0 2 GO.. Maybe it is broken?

Ok, so I connected it to my normal Masterlink. It works. But only when press ON on the Beosound 3500 panel and not via BEO4 so it seems to be a remote control issue.. It won't respond to any command so it definately look to be an L-opt 0-issue.. I can not either change L-option due to the remote does not respond.. Are there any way to reset the unit without the remote?    

Concerning your remote, so it is a BEO4? Does the display of the BEO4 show responses when you press buttons? Maybe stupid, but did you check the batteries?

Yes, I have 9 of them or more so I tried several that also work in other rooms. So it looks to be an IR-issue on the Beosound 3500.. How to solve I do not know..

I find this article online: "Beolab 3500 nightmare!! Recently purchased a beolab 3500 with the intention to use it as a stand alone sepaker.It responds fine to the mute and timer buttons however doesn't seem to respond to a controller I know is working.Have tried putting it into option 6 with no luck.somone please help! Answer: It seems like this is a common issue my 3500 died this way too. Tried every reset command but the only way to fix this error is to change the entire IR board. But I didn‘t find any boards online and I think repair from B&O would be very expensive. I ended up buying a used 3500 for 100€ and be hold the old one for spare parts"

Very unfortunate to hear that, it is such a beautiful product. I just ha d a look in the service manual, and noticed that the power transformers are near the PCB's that are in the middle. Of course that are cooling elements. But I can imagine that high temperatures in such small casing, after years of use, do have impact in the electronics. What I am saying, is that the soldering of the IR-PCB might suffer from loose connections or cracks in the soldering. If you have nothing to loose, you could try to check that, and do some reflowing of the soldering with an soldering iron, or the 'oven trick'.  

Thank you KolfMAKER.. Maybe I will give that a try once I have the time.. Appreciate all the help..