There's no point in 'following voltages' if you don't know what those voltages are supposed to be.
What is it that makes you think the fluctuating voltage is causing a shut down? How do you know it's not meant to fluctuate? And what does 'fluctuate' mean - in this context - anyway?
The...
Schottky diodes have a lower Vf than standard silicon devices (0.2V rather than 0.6V) so should show on a diode test meter assuming the diode is actually working.
With all due respect (prelude to me saying something you're not going to like to hear....lol)
"Probing around" seems to be your go-to fault-finding method which, as experience will eventually teach, hardly ever leads to anything useful least of all valuable knowledge on 'what you're doing and...
No you haven't - you've found a DESIGNATION of 'BC801' - the component itself is unmarked. As much as it looks like a resistor it could be a small inductor given the 'short' reading. Since that board has components on the other side we would need to see them (and the related position of BC801...
There's 'bugger all' to the basic design of the filter section that couldn't be actually, physically constructed (on protoboard or hardwired) with sockets for the relevant resistor/capacitor values and you could play to your hearts content until you found something that worked.
How about...
Pelican capacitors. Old manufacture. I recall using them in the 1970's......
1.5uF and 250V as above. Well known for losing their end wires. Polyester in construction and modern components are smaller (physically) plus have more robust wire terminations.
Replace 1 for 1...
So, basically, an income and/or ego-creating solution to you? If they offer an income then spend some of it on kennels for them.
TBH if you can't control your dogs then you shouldn't even own them.
Looks to me - according to your 'schematic' - that the diode is simply there for reverse polarity protection therefore is a 'simple diode' and not a zener.
Have you tested it and what made you remove it (symptom)?
Nope, me neither!
This is where another pic with the appropriate annotations on it would make a big difference. I also find it confusing to have to scroll back 'pages' to reference the 'small board' so please try to keep the posts relevant by including the information we need instead of us...