RPSU Issue 1 Problem

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eljay
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RPSU Issue 1 Problem

Post by eljay » Wed Nov 28, 2018 1:36 pm

Just got round to building the RPSU Issue 1 for my HVM/SE330 which was previously being powered from the dizzy from my main Oakley PSU.

I've powered the RPSU from a PA-20 and whilst checking the output voltages prior to calibration noticed that R5 was starting to smoke (obviously overheating but why?). I removed the power, checked everything I could think of, no obvious fault or short that I could spot. I was able to determine when the power was on that I was getting approximately +15.4v and -15.4v at the appropriate pins on the PSU headers. The standby LED is working fine, all capacitors appear to be in the right way round. The voltage regulators are heatsink mounted to the 1U rack case using insulator pads and plastic mounting bushes. After further testing I established that R5 still overheats whether the PCB earth point is attached to the rack chassis or not.

Have I got a faulty power transistor? Where else do I look for a potential problem?

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Synthbuilder
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Post by Synthbuilder » Wed Nov 28, 2018 2:39 pm

Remove R5 completely and power up again. Measure the voltage across the two pads of R5. One end of R5 should be the connected to the metal case and the other to the RPSU's 0V. Really there should not be any voltage between them. If there is -20V or something like that then the insulation pad or bush between the LM337 and the case is not doing its job. You may want to measure the resistance between the middle pin of U2 and the case/heatsink with the power off. It should not be anything near zero ohms.

Tony

eljay
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Post by eljay » Wed Nov 28, 2018 6:20 pm

Many thanks for the pointers Tony. I've reseated both of the Voltage regulators on the insulating pads and bushes and although I didn't see anything obvious everything is now working Okay. I did them one at a time and I think it was the LM317T that was shorting to the chassis, perhaps I overtightened the fastening nut or a small piece of drilling swarf was present. This result obviated the need to remove R5 which I've measured in situ at 44R, should I replace it or let sleeping dogs lie?

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Synthbuilder
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Post by Synthbuilder » Thu Nov 29, 2018 2:27 am

eljay wrote:Should I replace it or let sleeping dogs lie?
If it smoked, or it's looking slightly frazzled, then you should probably replace it.

Tony

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