What is the purpose of these caps in the ATX PSU?

[bte]

I have an old computer that I use from time to time. Recently the PSU in it (Thermaltake 400PP) died. It just doesn't start at all. After disassembling the PSU, I found out two electrolytic caps (2.2uF 450V) that only measure a few nanofarads each. They are marked C47 and C52 in the attached pictures. Probably they dried sitting next to those chunky resistors.

I would like to ask about the function of these capacitors. The primary reason is that I am trying to understand whether I could just use the brand name 6.8uF 400V caps I have at hand. I also really would like to understand what those caps in combination with the resistors do. I do have a general idea about how SMPS ATX PSU works but for that section, I can't wrap my head around the way they are used.

Thanks in advance for any insight.

Edit: Fixed a bug in the "solder side" picture.

[Psi]

Dunno what they are being used for maybe part of PFC or a RC filter.
But if you have more of those  6.8uF caps and you want to test it, you could put 3x 6.8uF 400V in series and that will equal 2.26uF at 1200V which is fine to replace 2.2uF 450V in most cases.  The ESR will be a bit different but i don't think those are being used for anything where ESR matters. Looks like 50hz mains stuff not high freq dcdc.

One other thing to check is the temp rating on the caps. 85C vs 105C you really want to replace them with the same type. Mainly due to the nearby heat in this case.


If it doesnt start at all then there might be something else wrong too,  its best to replace caps when the thing is working but unreliable (PC crashing).
If the PSU stops working entirely it can be because the caps got so bad it blew something else up. But you wont know until you test it. Sometimes you get lucky and the caps are so bad it just wont start but is fine otherwise.