I'm all new to the forum. I have tried to search here (and the "internet" as well) but without a hit. If this has been answered before I apologize.

I got an old, nice Beomaster 900K - all is working fine. Now, I want to add a Bluetooth audio receiver inside the Beomaster and connect it to the tape in connector. Yes, I know, I could just plug in a Chrome Audiocast or something like that to the tape in, but that would be all to easy and no fun, right. Now I have this tiny tiny Bluetooth receiver that needs 3,5-5,0 V DC. I have bought this similar tiny DC-DC stepdown that will take any (within reasonably range) DC and convert it to what I need. But I need to find a point in the Beomaster where I can find a steady DC supply. I did try right at the transformer (red and black wire) where I found approx. 16-17V. But it's not steady, it drops when I provoke a "hum" e.g. when touching the input with my fingers; and therefore I fear it would do the same when playing music, thus dropping to much on the DC-DC as well and switching of the Bluetooth board. 

Can you point me to where I can find a steady DC supply point anywhere on the inside of the Beomaster 900K?

I have the schematics (in this nice, brown envelope inside the box) but it's too many years since I've "played" with electronics so I can't quite figure it out. At least not find the spot on the actual boards.

Thanks for any help!

P.S. I have done a POC with the 16V, but not steady, supply of the whole signal chain to the tape in with great succes.