I have an APC AP9320 Environmental Management System for a server rack that suddenly stopped working. After opening it up and applying power to the unit I can hear a faint high pitched sound coming from the area of the transformer (marked with a DANGER sticker) on the power supply board. After I removed the board to investigate further I discovered there were two resistors that appear to have overheated. Both resistors connect to the same points on the back of the board as can be seen in one of the pics. Both had some sort of black heat shrink material over them that crumbled apart when I removed the board. One resistor is so fried the color bands are no longer visible. The other resistor still has most of the color bands but I'm confused on what I am looking at. The bands look like they are black/grey, brown, orange/red, and gold. Looking up these possible combinations gives me either 81k ohm or 8.1k ohm with +/-5% tolerance. I've been unable to locate anything online for these exact specs so maybe I'm not reading it correctly? I'd like to try fixing this board vs. having to buy a whole new unit.





Thanks in advance!

 

What result do you get if you measure that resistor with an ohmmeter?
You may need to remove it from the board to get an accurate result.

 

That is my next step. Although I was concerned it wouldn't give me a correct reading if it was bad. I will have access to another unit next week so I'm hoping I can open it up and see the resistor a bit better, although it may be impossible if it has that heat shrink over it.

Is there a better name to describe it other than insulated resistor?

 

From other places, it seems that this is done when the resistor is being used like a fuse. Under a fault condition the resistor burns out to protect the circuit. If that is case for your circuit, then the burnt resistor is only a symptom and the real fault lies elsewhere.

 

Possibly the transformer that was making a high pitched sound as two pins come directly from the transformer on the same circuit to the burned out resistors which bridge to another circuit.

Wish I could find the schematic for this thing but I'm not very optimistic APC will provide one.

 

It could be the transformer but in my experience it is unlikely. Much more likely semiconductors feeding (transistor(s), MOSFET(s)) or fed by the transformer (rectifier diode(s)).

 

Ugh. This is rapidly progressing out of my limited skillset. Is there a repair service online that I could reach out to for a quote on fixing it? (I'm in the USA) I only have one local place I have found in the past and wasn't too confident in the shop as it smelled like an ash tray and the guy had a dog that basically lived in the shop so everything was covered in fur and dander.