On Tuesday, August 27, 2019 at 2:39:00 AM UTC-7, wrote:

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:

That's an unlikely part, considering it's right next to the optoisolator.

https://www.mantech.co.za/ProductInfo.aspx?Item=14M7535 is the TL431 part

for the optoisolator,
https://www.mantech.co.za/ProductInfo.aspx?Item=14M7535

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?

If the diodes are short-circuited, they'll look bad in-circuit; if a multimeter that does diode check
is available, use that. Otherwise... play around with ranges on an ohmmeter scale, and
find out what a good example diode reads, both forward and reverse, and try to match that.
in a pinch (the diodes look like they're 1N4007) wire 'em in series with two batteries and
a flashlight bulb; should light forward-connected, not backward.