replacement parts

Do you mind if I ask why replace them with the parts you're linking to? Just curious because apparently 2 caps in series act like resisters or something.

Cheers Michael

Since these series caps are essentially connected between mains supply and earth (via 12V zener which has negligible volt drop on negative half cycles and 12V on positive half cycles) you should not use any film capacitor rated simply at 250Vdc as Travis suggests. You could replace both caps with a single 470nF,250Vac X2 rated film cap such as Jaycar RG-5240. You need to make sure you have sufficient space on the board though.

On further inspection of the partial schematic you posted it appears that the right hand side forms the mains (250Vac) EMI/RFI filtering function of a switching PSU. I suspect that the PSU may not conform to the letter of the law as far as Australia is concerned, specifically with regard to the types of capacitors used for C801, 802, 820, 823 and C863, and possibly even C803 and C816. The first four mentioned are shown as 400Vdc rated when they should be 250Vac Y2 rated caps which are specified for connection between Line and Earth or Neutral and Earth. C863 is even worse since it is rated at only 250Vdc and it is also connected directly between Line and Neutral when RL801 relay contacts are operated.

C803 and 816 are shown simply as 250V MEFX rated caps and depending upon what this means they may also be incorrectly rated for use in Australia. Both C803 and 816 should be specified as 250Vac X2 rated caps since they are directly across Line and Neutral.

For further reading on the use of EMI/RFI capacitors read

Remember that Australia conforms essentially with the European IEC requirements as far as these components are concerned.

Sort of.

Thanks Ross, those caps seem to be doing the trick. I've got 2 monitors the same, working nice in both :)

Cheers Michael

Thanks for the info that this is a monitor we are talking about.

Most monitors are Asian manufacture and marked with a list of approval logos on the back, such as CE, UL etc. Unfortunately, I don't know of many which actually specify the complete UL (or other) approval number which thay are supposed to meet. On a 1996 monitor I have it gives it as UL1950 but gives no suffix. Most of us probably have monitors with a similar EMI/RFI design and we simply have to accept that they do meet the respective approval specs and use appropriately rated components which should comply with those approvals. It is usually only when a reasonable number of items are identified as having a risk of fire or other safety aspect that the relevant authorities raise the alarm and notify a recall. Other than that they are not usually test sampled for electrical approval when they are imported.

Provided that the manufacturer has actually tested the device for UL approval for 250Vac working the components such as the caps in EMI/RFI filter should equate roughly to the X2, Y2 rated devices. When capacitors which are connected between L, N & E of 240Vac supplies are not specifically marked for 250Vac working then they could be almost anything. For instance, the 400V caps in your monitor eg. C801, 802 etc., might be tested at 1KV and will work on 240Vac, they may not actually meet the stricter requirements for 250Vac working. Such caps should be flameproof and self healing and tested to much higher voltages than the general purpose 250Vdc rated types. By using caps marked X2, Y2 everyone knows that they are specified for 250Vac working.

That C863 connected across RL801/2,4 worries me. If it is not marked specifically for 250Vac working and is simply marked 250V then this will most likely mean 250Vdc working. I would be changing that to a

10nF 250Vac X2.

Which caps did you put in, the 1uF 250V back to back electros as original or the ones suggested by Travis? I certainly wouldn't use the back to back electros as they will most certainly fail again due to the high ripple current through them.

Its 250VAC rated from what I can tell. From all the monitors I've seen this cap seems to be fine... Normally I see the C843 and C844 mentioned with respect to this monitor, they have a habbit of blowing from what I can tell.

Before I asked what to replace them with, I stuck in 2x1uF 450V back to back electrolytics, as predicted they blew after about 24hrs. To fix it I put in those ones you noted from Jaycar, working the treat :)