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Home Today’s Posts Search. Perhaps the large capxon cap started to go bad, and the PFC circuit worked harder to maintain the voltage, overheating the PFC coil and shorting dm131 the DM from high voltage spikes? Than it should be more than 70 V. The DM datasheet specifies 20 V at 3 mA max. So there should be some V drop with zener dropping it further to 20 V. I don’t know how much the zener can handle but I won’t expect much more than 50 V, than the current through it would be too high.

That is under normal operation though, if the DM draws more than 3 mA than it should, there can be higher drop on it than on the resistor. Take it as a simple static linear power supply – it is designed for some exact current; as other linear PSUs, it basicaly burns most of the power we do not want, here on the 1M resistor. Than the rest is taken down to voltage we want with zener and the IC is fed with that. That’s how I understand it anyway.

However, what usually goes bad because of excessive overheating due to using that GP diode and no cooling, and because of poor quality, datasheer the cap.

DM311 Datasheet PDF

Than the zener is supposed to do all the work and can go short…. That’s why I was surprised at first, I datashset I would be reading v here and that the resistor was bad. But digging further it looks alright.

But I don’t understand the purpose of the resistor, why have it there at all if the DM datashedt handle up to v on Vstr? Oh my bad, I’ve been talking about Vcc…the Vstr should consume uA. So I think huge resistor is there to ensure slow enough charging of the Vcc cap otherwise there would be huge spike through the IC, auxiliary transformer windig and to the cap. The Vstr resistor is a current limiter.


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It also drops the volts to something useful. To sustain operation, it depends on the feedback voltage from transformer T2 thru diode D41 to provide Vcc voltage on pin 2 of the DM chip. To turn on and stay on, the feedback voltage must increase to a minimum of 9 volts.

Typical operation on this power supply, it should reach and maintain 12 volts. If Vcc drops to 7 volts or below, the chip turns off. Also, if the Vcc goes above 20 volts it turns off. Check the voltage on pin 2. To operate it must be above 9 volts. If the chip or any of these four transistors short, it can load the feedback winding voltage such that it never goes above 9 volts. Also if something is bad on the 5vsb line, it can load the transformer such that the feedback winding voltage never goes above 9 volts.

Hopefully this information helps. The voltage on pin 2 of the DM must be above 9 volts, typically 12 volts. Measure it and let us know what you get. I did a comprehansive study on this power supply board a few years ago. It is the same board under the Sparkle brand. I am including some photos of that power supply and the drawings I made.

Hopefully they will help you to understand the circuits better.

And here are some drawings Attached: Anyway, thanks for your valuable input, I see these PSUs quite a lot, but never had time to dive so much much into these tanks of shit And shorted shottky diode on secondary side, I wonder if that was what killed the primary, or the other way around? Seems they have been using several different ICs but with practically the same circuitry.

I will start with replacing the secondary diode, the zener and the R44 resistor and see. There is chance the PWM is bad as well. Please keep us posted. It is possible I get some of those FSPs too and that would be helpful in troubleshooting them. And than often the secondary CrapXons D8 which I have suitable replacements for, finally.


So you need to replace them all, every time. Everells idea of replacing 1N with some fast diode together with quality caps should keep them alive for much longer.

Originally Posted by everell: Originally Posted by Behemot: It used to be problem though you CAN squeeze at least 16V D10 caps in there and one or two on other rails with some effort. Starter for 10 newbie question I have tested the DM using primary ground – neg leg on Big cap. I am now going to test CM well some pins Do I still have datashet use primary ground for this? Originally Posted by selldoor: Do I still have to use primary ground for this? I have not seen a PSU of this type where they were not so replacing them is a must.

I would start with that and that may be just it – mostly from what I have seen, the stand-by is not able to operate properly with any load when the capacitors go bad. It may start with no datashset making an impression it is actually OK. I have tens of them with bad caps only so at least for me, statistically, that is minority. Statistics are great for politicians.

But how come I end up with the really bad ones??? It had multiple problems. In the end, I took the basic 5vsb design using the DM from the Fortron GLN series, put the parts on a small piece of perf-board, and installed it on the Bestec.

Worked great and no more 5vsb headaches. Now I don’t fix two transistor 5vsb circuits on Bestecs.

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