I have crates full of these old wall warts and similar PSUs. While I have many SMPS fail the old transformer-diode bridge - capacitor type seem to last forever.
The main objection is that they are unregulated and they are a pain to match to the load. If the load is not stable the voltage can be all over the place.
I often end up using a PSU with a lower nominal voltage because at the current I need the voltage it outputs is higher than the nominal voltage. Testing each time I need one is a pain.
Series regulation is not easy or convenient because there is really not enough headroom.
I have used in the past parallel regulation with a zener diode or zener plus transistor. It is a waste of energy but the amount is very small and the addition can be placed in the output cable or receptor without needing to open the enclosure which sometimes cannot be done without breaking it and there is no space anyway.
I am playing with a small transformer which I guess we could call "12 V , 200 mA". The following table gives the voltages at different currents.
mA V
0 19.4
10 18.9
15 18.6
20 18.4
30 17.9
50 17.0
75 16.0
100 15.0
125 14.1
150 13.3
175 12.6
200 11.9
225 11.3
250 10.8
So, let's say I want to limit the voltage to 14 V max. I would need to sink 125 mA @ 14 V which is a bit under 2 W. A zener and transistor can handle that easily.
This prevents up to 20 V building up in the capacitor and being dumped suddenly into whatever device is connected.
I'd like to hear thoughts and ideas on this topic.
BTW, I actually measured a number of points but then I put them into Excel and I obtained the following equation
V = 19.359 - 0.0492 I(mA) + 0.00006 I(mA)^2
So the table is an idealized table.