HP6032A with Cooking 10kohm resistors

[CrazyTiger]

Hi All,

I got this HP6032A 0-60V/0-50A 1000W PSU which is faulty. The unit had a simple defect, blown fuse caused by a stuck blower fan. After replacing new fan and fuse. The unit is able to produce good output voltage, yet to fully load test, checked upto 200W, no issue.

However without load, I noticed the 10k 5W resistors of the power supply filter/doubler section is too hot to touch. My initial suspect was a bad filter cap. But taking it out and testing the caps, they measure good on my LCR Meter, the ESR is also reading 0.0Ohm on my DE-5000. The resistor itself is all measuring 10kOhm perfect. I also removed the the Mosfet board and turned on the unit, the resistor is still running hot. So nothing is shorting for sure.

 I am seeing the PCB board and trace also been cooking, on the other hand it could be a long time cooking scenario, I assume this PSU was used in a manufacturing factory. Maybe paranoia on my end? How hot is hot for a 5W resistor?






The circuit in question:

[wn1fju]

These power supplies develop 300 VDC across the filter caps.  So that's 150 VDC across each one of the 10K resistors, hence 2.25 watts.  That would certainly burn your fingers if you touched them. 

It looks like long-term heat marks on the board, something I've seen in a lot of power supply circuits.  You might try lifting the resistors off the board 1/4 inch or so if you are going to replace them.  And check for continuity of the traces near the solder points - some of them look a bit troubled.