Sorry, I took so long in getting back to you. The SKR 240/04 are both unsuitable.
If you look at the first table of the datasheet, there are Rows of V and columns with diodes pointing in different directions. SKN means that the anode is connected to the stud.
So, lets, look at SKR 240/04 vs SKN 100/08 (Original)
The peak reverse voltage is 1/2 of the original, but the current is more than double.
If these are on the mains then 240 * 1.414 = 339 is < 400 with some safety. The PRV or Peak Reverse Voltage would have to be > 339, so 400-600 is OK, where 600 is preferred.
It also looks like the suffix UNF is used for english studs, otherwise the studs are metric.
The Diode direction is the #1 issue. The thread size #2, unless you can re-tap. #3 is the current. #4 is reverse voltage
You should check the stud size.
If these diodes rectify the AC mains then 400 is the minimum. If they rectify a much lower voltage, then it's possible that the PRV was higher than it needed to be in the first place.
A schematic could be useful as well as the "specifications" of the charger. Current can be higher and PRV could be higher, but they could have been oversized to begin with due to availability.
Thanks for the reply, I thought the thread was dead so I didn't check back.
Yes, I understand the difference between the SKR & SKN.
I didn't know about the PRV but I get it now.
So, the PRV is Peak voltage on the datasheet not RMS voltage, correct?
The diodes rectify the secondary voltage of 12vAC maybe a little more voltage at full power.
The SKN100/08 diodes were more than enough to handle the task, this power supply is built like a battleship.
A good friend of mine built the power supply & other fantastic control equipment to go with it for a special purpose so I have no Schematic or Specifications.
I could draw a schematic but i'm not sure my friend would appreciate it, one of the conditions of the build was not to share any of the details, so unfortunately I am on my own with it as I can't contact him, I just hope the rest of it works now.
I would like to keep my word to him at all cost!
So taking into account the PRV & that the SKN 100/08 was more than up to the task, the SKN 130/08 which I can get gives me the same PRV but a higher current rating even though not needed.
From what I can see, the SKN130/08 is a reasonable choice considering the availability, what do you think?.
I realise that I could use diodes with a lower current rating for what I need the control equipment for now.
Thanks for explaining a few things, it really helps!