First of there is both a MKI and an MKII version of the Beolab 3500 Both versions have Masterlink. Beolab MKI (serial numbers are lower than 19343452) has an 8 pin connector on the back marked MCL and can be used with the Master Control Link system Beolab MKII  (serial number 19343452 and up) has an 8 pin connector often refered to as powerlink on the back, but according to the servicemanaul for BL3500 MKII only intended for use with Beolink Wireless 1 (W1). I have done several tests in another thread that also suggest that this connction can only be used for this purpose, and not function as a normal powerlink speaker. With your Beocenter 8500 I see two options:

1. Get a Beolink converter type 1611 which connects to the AUX port on the Beocenter 8500 and converts it to masterlink. This way you can connect both MKI and MKII Beolab 3500, and you have also the option to add other zones with Beolink Active, Passive or the Beolab 2000.
2. Use the Speaker 2 connection on the Beocenter 8500 to make a MCL network to a Beolab 3500 MKI, it will not work for a MKII. To do this you have to use the thick 7 wire MCL cable, speaker connectors with 3 legs, and have an 8 PIN DIN connector mounted in the BL3500 end. You might be able to get some MCL cable with connectors mounted secondhand, but else it will be a DIY job. Since the MCL connection is from a speaker output it is depending on the volume from the Beocenter 8500. The BL3500 has some compensation for this, but it is not optimal.

I would suggest you go with option 1 as this is the more modern way of doing things, and you have more options to add more zones in the future. As for the ML/MCL converters. Those will not work for this setup. They are for connecting a Beomaster/Beocenter/Beosound with Masterlink to a MCL installation, so a way to add a more moderne main room system without replacing existing link rooms. Hope this helps, and fell free to ask more if in doubt

Very helpful, thank you.  As it happens I think I do already have the cable needed for option 2 - I used to have a Beomaster 3000 with the MCL30 multiroom box/transceiver, and that cable has 3 pin speaker cables on one end, bare cables on the other end but I could easily connect an 8 pin DIN.  I have sold the Beomaster 3000 now, but I still have the MCL30 - I was selling it on eBay. When I got the Beocenter 8500 (recently, as an upgrade from my Beocenter 8000) I did plug the MCL30 into it in case it worked, but it didn't.  The speakers work through it, and the physical mute button on the transceiver works, but the IR transceiver won't recognise commands from my Beolink 1000 or Beo4 remotes - it was built for the much earlier Terminal 3000 remote, so this isn't surprising I suppose. But you've got me thinking - I've took the MCL30 off sale on eBay (no bids anyway!) in case I decide to repurpose that cable by adding an 8 pin DIN to the other end, and go with your option 2. I'd prefer your option 1 but I cannot find a Beolink converter type 1611 on sale, but at least I've got options now, thanks. PS will anything other than a 1611 converter work, such as the BeoLink Converter NL/ML?  Thwre are so many of these converters available from B&O over the years, it can be a bit confusing to know which does what.

Hi again The NL/ML converter is for converting between Masterlink and the newer netlink products, so won’t work in your case. A Beolink converter type 1614 might work, I can test that tomorrow and report back

I have now tested with my Beocenter 9300 and a Beolab 3500. With the Beolink Converter 1611 everything works as it should. With the Beolink Converter 1614 nothing works. The 1614 converter does not supply power to the Masterlink network. Therefore a 1611 converter is needed if it is the Masterlink Route you take. If you choose to go the MCL way a cable pinout is available here (These pages has not been reestablished at Beoworld, so linking to the internet Archive) https://www.beoworld.org/beotech/wiring1.htm Not that the whirring is shown for a wall plate so you have to be sure to use the right PIN numbers on the Din plug. The pink wire is power, and is not used in the Beolab 3500, so you do not have to solder it to the plug, but keep it isolated so it does not short circuit.    

Thanks so much for your help on this - having thought about it overnight I think I'll wait until I see a Beolink Converter 1611 come up for sale - as you say that is the more modern (and more flexible) way of achieving things, and also ensures I can use a BL3500 Mark II rather than needing a Mark I. I don't actually want to put the 3500 in another room, I just want to put it on the other side of quite a large room, and if I can get a BL3500 in decent condition it would look pretty cool mounted on the wall.

Sound like a good idea. When you place the BL3500 in the same room you can choose to program it to option 4 where it will only react to Link commands. That way you can choose wheeter or not you want the BL3500 to play

You don't happen to know if the Beolink converter 1614 is a suitable alternative to the 1611?  It certainly looks exactly the same!

On CharlieWednesday said

You don’t happen to know if the Beolink converter 1614 is a suitable alternative to the 1611?  It certainly looks exactly the same!

The 1614 cant supply the voltage to drive the masterlink network, so for this usecase it wont work. The 1611 will do this.  

My goodness these converters are confusing. Thanks - I'll keep looking for a 1611 then.