Switch mode power supplies

The last one of these that died on me, DrAudio helped me with. We finally chased it to an open resistor that was supposed to "tickle" the oscillator chip and get it conducting so it would switch. Applying a few volts from a battery would kick it off and then it would run but it would not self-start. The resistor looked fine but it was completely open. No idea why it happened, its the only non-original part in that power supply still. The caps were all fine.
 
Hi Scott,
The data sheet on the TOP246R is a wealth of info. Mouser has it. Check the resistance between pins 4 and 7. That’s the drain and source of the switching element. If shorted the IC has failed. I can’t read the nomenclature on the 8 pin part so not sure what it is. Good luck!
I measured B+ across those leads, so it is not shorted. Thanks!
 
The problem is the chicken and the egg, how can you test it if you can't get it to turn on? I am looking for someone with helpful advise.
It would help if we could see the schematic. If you don't have one, then start by understanding that the front end of switching supplies are referenced to the power line Neutral. BE CAREFUL! You should float your scope off Earth Ground temporarily while checking the hot side of the supply. Assuming it's only for 120v use, attach a DC voltmeter to the main filter cap. It should rectify the line voltage up to 168Vdc. If that's good, put your scope on the output of the FET driver IC. You should see it pulsing while powered. If not, the IC and some of it's associated components may have failed. Especially check any Zener diodes.
If they are OK, look for opened switching FETs. I check them in circuit by activating the drain-source diode. If shorted , the FET is shorted. If not, you may have to pull the FETs and activate them with a 9v battery and series resistor (10k) from Gate-Source, while an Ohmmeter is across the D-S. The D-S should show an open until the gate is activated. Then, shorting the G-S should turn it off again. If all that works, write back.
 
The primary side seems fine, the FETs are in the driver chip( see post 19) I still haven't gone to storage to retrieve my scope. I'm in the middle of several projects that take precedence over this one, I'll get back to it in a couple of weeks.
 
I have found that over time and with the heat generated, the capacitors and resistors drift, causing all sorts of issues. The equipment I work on (to support my audio habit :)) run several hundred smps 24/7. We have seen catastrophic fails (i.e. blown caps, ic's etc.) but the most common fail is turning it off. If the caps, or resistors drift far enough, they wont turn back on. Fortunately, we just replace them when they fail. Outside of work, I have had good luck with replacing the electrolytic's, maybe 4 out of 5 times it has resolved the issue, worst case was I had to replace the caps, IC, mosfet, and the optoisolator IC.
 
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