Thank you everyone for the amazing help so far. I'm learning so much with this.
I feel like I should've perhaps mentioned that I'm pretty much a beginner when it comes to circuit boards. I've replaced fuses, capacitors and resistors before, after doing some research and finding the correct replacement parts, in order to bring back life to old/broken appliances/toys.
Also, something else to note is that the local (Cape Town, South Africa) place where I get all my electronic parts is Mantech (
https://www.mantech.co.za).
That said, with regards to the Fixed Positive Voltage Regulator (15+ V) in a TO-92 package (as mentioned by John & Peter), I found the following options:
https://www.mantech.co.za/ProductInfo.aspx?Item=14M8346
https://www.mantech.co.za/ProductInfo.aspx?Item=14M8345
https://www.mantech.co.za/ProductInfo.aspx?Item=14M4528
I've also followed the advice to remove the remaining epoxy from the chip and look at it under a magnifying glass (unfortunately I don't own a microscope).
I have uploaded some new images (viewed through the magnifying glass) to the same link (the new pics are at the bottom):
https://imgur.com/a/uBCXjwQ
There seems to be no part numbers on the metal layers. I'm also not informed enough to know whether it is "too complex to be a simple transistor and complex enough to be a regulator, or something in between like a Darlington".. To my untrained eye it seems pretty simple, so maybe a transistor then?
With regards to the TL431 (as mentioned by Piotr, Phil & whit3rd), I could find a few options at Mantech, but not sure which one could work:
https://www.mantech.co.za/Stock.aspx?Query=TL431and
With regards to the opto-coupler (mentioned by Phil), I've added a photo of that as well (
https://imgur.com/a/uBCXjwQ), the code on the opto-coupler is:
L1620
817BL
W
@Phil: If I understood you correct, you are saying the because of the blown part, the opto is likely blown as well? If so, which opto should I try to replace it with:
https://www.mantech.co.za/Stock.aspx?Query=817+optoand
Unfortunately I have no idea what happened as I got this from someone that was throwing it out, since they had already replaced it. I'm trying to see if I can fix it. It is from the handset/monitor side of the intercom system, so maybe it came loose of the wall mounting and took a heavy knock? Also, we have had load shedding happening in the past, which would some times have caused voltage spikes when the power came back on. Don't know if that could've cause something like this to happen? I know people have lost appliances to that, but not sure if it could be related.
@whit3rd: So you reckon, I just try to replace the blown part with a TL431 (any particular one from the list?
https://www.mantech.co.za/Stock.aspx?Query=TL431and) and then also replace the optoisolator (any particular one from the list?
https://www.mantech.co.za/Stock.aspx...=817+optoand)?
I'll do a check of the diodes. Would testing in circuit be sufficient or should I remove the diodes from the circuit for testing?