TDK ZWS150BAF-12

[Keshenatech1]

Does anyone have a schematic or parts list for a ZWS150BAF-12 12V power supply? I have a dead one and I've determined the surface mount diode on the back side of the board that connects from the center leg of Q3 to the + side of the 420V 60uf capacitors has a 3.5 ohm short. I'm not having much luck trying to identify the diode as the part number is not very legible. In the attached picture, it's the black rectangular diode in the center. There are some numbers on it but I can't even tell if it's right side up or upside down. The picture is from a 2nd good working unit which is more legible than the original dead one, but not much. I've removed the diode from the original board and determined the 3.5 ohm short is the diode itself and not elsewhere. Would anyone be able to help me identify it so I can replace it with the correct type? Thanks

[прямо]

3.5 ohm is not a short. 0 ohm is.
And why aren't you using diode mode to measure a diode?
After you came to the conclusion that the diode is faulty, did you compare it with the 2nd working unit?

[Keshenatech1]

Not a dead short but 3.5 ohm is definitely not normal for a diode.

[Keshenatech1]

Q3 is a YG98156. Can barely see it between the other components. The diode goes between the center lead and the + of the high voltage caps. Also including a few more pictures of the unit.

[stj]

did you test it off the board?
wipe it with IPA, i think it has a film of flux residue on it

[Keshenatech1]

Yes measures 3.5 ohms removed from the board as I wrote above. And I also tried cleaning it and looked at a couple more boards, and they're all hard to read. The 24V power supply board made by the same company has diodes labeled "G6" in that same position so I'm thinking possibly a 60V 1A Schottky but I want to be certain before throwing something into the high voltage circuit. I even emailed the company and their reply was they can't disclose info about products currently in production for liability purposes among other reasons. This is used in the power supply for a Konami Concerto slot machine btw, this board and a similar 24V board both mounted in the same enclosure. I've posted on the slot tech forum too but no replies there either.

[прямо]

Never tested a diode using a resistance mode for my entire history of repairing electronics.
I always use the diode mode, and I replace that has a voltage drop reading out of spec.

You said 3.5 ohm is bad and thus assumed the diode is not normal, but did you even check if the other similar diodes on that faulty board and as well on the working board, have the same resistance?

If 60V 1A Schottky is somewhat not enough, go up to 3A, such as Vishay SS36. It should still fit within the component footprint.

[Keshenatech1]

Just to clarify, I was testing in the diode scale and ran across this one that made the meter beep and showed a 0V drop so then I went down to the ohms scale to see what the actual resistance is. And on a working board, it measures just like a normal diode even in circuit. Drops .458V across the junction in the diode scale and several meg in the resistance mode. Also I found a couple more good boards in machines we currently have in storage and found one that's legible. The markings on the diode are V6 on the top row and 58 below it. (new picture attached)

[momaka]

The pictures are too small for me to be able to trace out anything on the board... but going by your description ("The diode goes between the center lead and the + of the high voltage caps") and from what I can see from the general layout of the PSU, this sounds like the rectifier diode for the APFC boost circuit. In any case, since it's on the primary side, don't go with anything less than 600V for the rating. Being that it's a 12V 12 Amp PSU (~144 Watts), probably best to go with a current rating of at least 2 Amps.

Since this diode blew, it might be worthwhile checking all of the other components on the primary side. Check the APFC MOSFETs as well as the main switching MOSFETs. Also check the primary caps out of circuit with an ESR meter, if you have one. Not uncommon for APFC circuits to blow them open-circuit, which can then wreck havok on other parts.

If nothing else appears bad, just replace the diode and test the PSU without a load but with a series incandescent bulb. Go with a 100 Watt incandescent or halogen bulb or maybe a bit more, since APFC circuits tend to have issues when tested with such setups. If the bulb doesn't constantly stay on but only strobes, feel free to take it out and plug the PSU directly to the wall. Hopefully no bangs then.

Never tested a diode using a resistance mode for my entire history of repairing electronics.

It's actually not a bad test to do. I do it quite frequently myself.
The reason why is because in diode test mode, the multimeter test voltage/current is high enough to bias other diodes or semiconductor diode junctions in the circuit. This can sometimes give confusing results, depending on the circuit. Resistance mode, on the other hand, isn't supposed to bias diodes and diode junctions. So when used to test diodes... if you see a really low reading across a diode and there isn't anything else in parallel with it, it can be easier to find bad parts in some cases.

Again, that's not to say to not test diodes in diode test mode, but rather that depending on the situation, it may be more suitable to use resistance mode sometimes.
Also, diode test mode will not show a leaky diode when tested out of circuit. But if you test a diode out of circuit in reverse polarity and it gives a resistance back in the 10-100's of KOhms range (and you're not touching the probes with your fingers ), then you might have a leaky diode too.

Lastly, on some cheap multimeters (like my first 830 -based model), diode check and 2000 Ohm resistance scale give nearly identical numbers when testing diodes.