Where can I find a steady DC supply point on my Beomaster 900K

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drfresh Posted: Thu, Aug 12 2021 4:31 PM

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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.

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Dillen replied on Thu, Aug 12 2021 5:55 PM

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First thing you need to consider is, that Beomaster 900K has positive ground.
This means, that if your bluetooth device has common grounds for power and signal (as most have), you will short the Beomasters power supply.

All transformer leads are AC.
You can find DC on the filter capacitor - but it's still positive ground.

Martin

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drfresh replied on Thu, Aug 12 2021 6:59 PM

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Ahh - I did actually short the BM - but not in the first place, but after some tries. I did think it had some other reasons. But thanks for this insight. I'll have to study more on how to deal with that...

The filter capacitor - where is it? Sorry for a possible stupid question, but I'm new to the BM.

Thanks