The Strange Failure of R98While repairing my Xantrex XKW 33-33 power supply, I came across a puzzling failure. I'm going to go into some detail on my thoughts and results because I've seen pictures of two of the Sorensen clones of this model (DCS33-33E) that suffered more serious damage than mine did. Hopefully, this info will be useful to owners of either the Xantrex XKW or Sorensen DCS power supplies
When I received my unit, R98 was burned so badly that it was open circuit and there was no way to determine its value. Luckily, MagicSmoker had a Glassman 80-13 power supply which is another clone from the same family as mine so I was able to get the value. Since I can't find a schematic, I traced out the circuitry around R98 (see attached ). The power supply shown as +AUX is the unregulated output of a full-wave bridge from a 16V transformer that is then filtered with a capacitor. It forms the source for the +5V and +12V supplies used elsewhere. C67 and C68 go to other parts of the circuitry. Q7 and Q10 are two of the six fets that drive the main switching transistors through T2.
Why put an 18V zener on a 19V, unregulated supply and why use a 5W device? Since all the other components shown test good, how could you blow R98? Looking at the board, R98, CR31, R104, and R106 are all spaced away from the board. I don't remember seeing 0.25W or 0.5W resistors mounted like that. Usually that's done with higher power devices to prevent the heat from degrading the board.
The only thing I can conclude is that the unit experienced a significant overvoltage condition on the incoming AC that drove +AUX higher for a significant time. It could have been a power line fault. It could also have been a user mistake where it was strapped for 110V and then plugged into 220V. If that happened, I'm surprised at how little damage there was, but there's no way to tell how much of that voltage made it through the transformer. Nothing else shows signs of heat or voltage stress.
After repairs were complete I did some temperature testing while the unit was outputing ~90W with some eye-opening results.
R98: 48C
CR31: 35C
R104 & R106: 80C
Now I know why R104 and R106 are spaced away from the board. But why not just make them larger so they can better handle the power? I wonder if R98, R104, and R106 are sacrificial components that are intended to blow under fault conditions while saving the board and the other components. Could R98 be a fusible resistor? When I look around the unit, R37 (1K, 0.5W) and R103A (15R, 0.5W) are also spaced away from the board. R37 is a normal-looking resistor, but R103A has a reddish-brown ceramic body which makes me think 'fusible / flameproof resistor'.
In the pictures of blown Sorensen supplies, it looks like larger components were mounted tight to the board. The boards appear to be badly damaged by the failure of R98 so it obviously wasn't a fusible resistor.
In summary, if you come across one of these power supplies with a blown R98, check the components in the partial schematic. If they all test good, the fault was probably caused by external forces. Replace R98 and see what happens!
Ed