Thought I'd post this in case it might help others with the same or similar setup. House came with a Music and Sound (M&S) 223 Music-Intercom System with front and back doorbells, front 2-way speaker, and 5 room wall speaker units (pictured). Having run low on coal to power it, we purchased an Arlo Essential Wired Video Doorbell AVD1001. While waiting for delivery, I removed the front door speaker unit and discovered the M&S used 14VDC to the door bell...not the expected (and now more common) 24VAC. Bummer. Two story house with door bell wiring coming from the attic, so rewiring would be complicated (and in a 130 degree space at this time of year in Houston). And, unlike converting AC to DC, DC to AC is more complicated and expensive (and no, to those who aren't aware, there's no such thing as a DC transformer).Anyway, investigating the intercom 'Master Unit' revealed its primary power supply to be at 21.5VAC. Ah ha. Not needing the old front door speaker any more, I hijacked its wiring and spliced it into the AC power. I also left the master unit end of the door bell button wiring intact. So now there was AC present at the door, and the existing chime wiring, which had been connected to the door speaker button, was also available. There isn't a separate chime box on typical vintage intercom units, and it's my understanding that they (and older NuTone's?) don't close a button circuit per se. The voltage drop caused by pressing the button is actually sensed and it 'rings' because of that trigger. Hence no reason for a 'standard' button voltage. I specified "3 screw" & "mechanical" when the Arlo setup asked what type of chime I had.Now for an actual circuit addition: a 24VAC SPST (single pole single throw) relay which I mounted at the door cavity rather than in the intercom unit box (pictured) but all of the following could be done at the intercom end. One side (either) of the door's ex-speaker/now-AC wire goes straight to the Arlo. The other goes to either coil connection on the relay. The Arlo's other voltage-in connection comes from the remaining relay coil contact. The 'power kit' is also wired across the coil with its switch in the "O" position. Again - no polarity involved. The door bell button's wiring is then connected to the relay's two NO (normally open) switch contacts, either lead to either contact. Makes no difference. The relay I happened to use was an American Zettler AZ2280-1A-24AF chosen for its small size, but any device with compatible coil voltage requirements would work. Its contacts are labeled COM & NO, and the coil isn't marked. Pinout diagram is available on Amazon listing.When the button on the Arlo is pressed, the relay is energized and contact is made between the original door bell button's wiring, ringing the intercom 'chime'. Oh, and I had to grind down the lower mounting screw head which prevented the doorbell from snapping into the frame. Pretty lame considering everything else seemed well thought out. No problems with setup except for the unnecessary, beyond annoying, insecure two-factor authentication band wagon Arlo appears to have jumped onto. No opt-out available, apparently.Hope this helps someone with their 'vintage' installation. It works fine so far.