Unusual resistor in power supply - help ?

[eccerr0r]

Re: Unusual resistor in power supply - help ?

looks like a typical 5% 4.7Ω resistor. Gold in band 3 divides the significant digits by 10. White is probably temp coefficient.

[Dannyx]

Good day folks. A colleague of mine asked me to take a look at this ATX supply which he said is completely dead. I didn't plug it in myself, but instead started probing around to see if I can find anything with the board "cold".

I started with the stand-by circuit which is driven by a TNY278PN IC which is not shorted. In that area however, I found THIS resistor which clearly had a hole in it and it reads completely open - no resistance at all. It goes between a leg of the small STBY transformer and the + leg of the main capacitors and for some reason it popped. The fuse is good, so it seems to me nothing exploded violently here, but the resistor still decided to give up....

No problem, I said to myself, I'll just replace the resistor and see what happens....that was until I looked up the color code and this is where I got stuck: there's no value for a gold band in the second or third positions ! So we've got yellow (4), purple (7), gold (?), gold (?), white (10% tolerance ?). Normally a gold band signifies a 0.1 multiplier, but not in the second or third position like that, so what is this resistor ? I thought maybe it's an inductor, but not only is the board marked R50, but it also doesn't quite make sense from an electronic point of view to have an inductor in series with the HV line feeding the standby transformer. It should be a fairly low value resistor in my opinion, something like 0.47 or 4.7 at most.....what do you guys think ? If we omit the last white band, that would indeed make sense as it would be 4.7ohm, since we have valid values for the gold bands in those positions. Let's start with this and I'll keep you posted on the repair as we go.