It's a BMS outstation, a computer type device intended to monitor &
controls heating & stuff.

On the pcb, it has an encapsulated transformer 0-24V pri/0-18V sec,
adjacent to the power input. The power is from a 24V ac mains
transformer. Also 4 electrolytic capacitors, but nothing I recognise as
an rectifier. There's a small battery on the pcb, but it's just to
maintain the RAM & programmed settings. The device fails if the 24 Vac
goes off. I'd like to monitor inputs and keep the alarms working. The
control valves are also 24 V ac, but I wouldn't intend maintaining the
power supply to these.

You could probably achieve what I want with PLCs but I'd have to learn
how to programme those, so that's relegated to plan B!