Well, things are never quite as easy as I'd like them to be. After taking a closer look at the L&R sigmals, it was clear that I couldn't just tap into the L&R preamp signal without doing a little massaging. I threw the 5500 and 6500 schematic at Claude, and eventually came up with this:

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I'll take a second look at the electronics in the morning. For L&R, the 47 ohm, 100 ohm, and 1nF I have visually identified on the 6500 powerlink signal path, but I need to check the makeup of the circuitry around the transistor, and verify voltages. I also need to verify the suitability of using the TR2 collector for the speaker relay.

Very nice that you are able to compare against the 6500 and find what extra circuitry is needed. One thought however. Is the L&R preamp signal volume regulated like the powerlink L&R signal? If not you might need some extra circitry for the colume control

@Madskp In earlier times, I had mistakenly thought that Powerlink audio signal was line voltage. It isn't - it's pulled from the output of IC5, right after the inline capacitor.

I've traced everything back to where it is on the BM550 PCB3, and added the needed outputs. I've simplified the diagram (mostly due to an error on my part as to where the TR10 mute circuit comes into play), and located where on the board the jumpers go. My goal is to make a simple design to put all the needed outputs in the array of plugs outside, so all I need to do is fashion a powerlink cable to hook everything up and begin testing it. I've requisitioned the L&R Preamp RCA plugs as Powerlink-level audio in this version. Here is the pin matrix so far:

PL Pin 1: Sensor Pin 1

PL Pin 2, Casing: GND

PL Pin 3: Preamp L

PL Pin 4: Wire coming from PCB/TR2

PL Pin 5: Preamp R

PL Pin 6: Tape 2/Aux pin 6

PL Pin 7: Spk 2 Pin 4

It's a bit messy, and makes me wonder if I shouldn't just retrofit Speaker 2 or the 6-pin sensor with all the appropriate signals - that would certainly make the external hookup easier. I'll give it a think tonight and decide later.