Sunkko 737G Spot Welder

[RustyCaliper]

Hello there.

I have a Sunkko 737G Spot Welder, cheap noname chinese device, that I just bought off Ebay. First time turning it on, it worked. Then I turned it off, inserted the foot pedal switch (not sure if that's related), turned it back on and the device popped: Huge bang, fuse of my home installation tripped. After untripping the fuse, the device is completely dead. There is a fuse 20x5mm inside the device as well, rated at 20A, which did not trigger.

Upon inspection of the device, there seems to have been a fault right at the very first component the AC goes into: The way this is built, AC goes to the PCB, which via a Triac (BTA41) activates a transformer (which creates the high amps for spot welding). What seems to have failed is an SMD component in series with line. The issue is the component has spectacularly failed and been obliterated and evaporated. No chance of identifying it (pic attached).

My two questions now are: What could that component have been? And, more importantly, is there a root cause that made it fail, i.e., what circuit behind it drew so much current? Interestingly, none of the components on the other PCB side appear damaged or having seen lots of current. The electrolytic cap looks a bit pregnant, but it's measuring no short. There seems to be an SMPS circuit and the small blue transformer measures very little resistance on both the output and the input (order of less than 1 Ohms). I would have expected at least something in the 100-1k range. Could that be the defective component? Optically it looks fine.

Any idea what the secondary voltage could be? Because then I could cut out the whole dodgy SMPS circuitry and just bypass it by a second SMPS, there's enough space.

Any advice welcome!
Cheers!

[RustyCaliper]

Quick update: I've desoldered the small SMPS transformer. For what it's worth, the SMPS IC is a BP9022A. While it's a CC in the primary, I doubt that when the primary winding has that low resistance it can still drive it, suspect the transformer to be defective. Not sure though, SMPS is not my area of expertise.

Injected +8V at the secondary because there was a 78L05 that I spotted (5V + Dropout); that turned the display right on and made it happy. It won't weld, however. Whenever I trigger it (even with the primary active) it seems to go into an error state and just displays "P".

The triac is triggered via an optotriac, MOC3023. That one doesn't have ZC detection, so maybe they're doing that in the MCU and since it's not getting a ZC signal, it doesn't trigger? Not sure. Any guesses?

[RustyCaliper]

It's fixed!

Cleaning up the mess and spatter revealed that the device was labeled "FUSE". As I suspected, it required that fuse to operate for the secondary to get phase information, so the MCU is apparently what does ZC detection of mains. This is done py a PS2701 optocoupler that is right next to the MOC.

So what I did was cut the trace that goes to the SMPS circuitry (not sure if it was the transformer or maybe the switching IC, but not taking chances), so that's all dead now. You can easily cut this when you follow the path to the fuse and then the first left turn it takes, that's where I cut it. It runs further north to the optocoupler (the layout is weird). So gotta keep that trace intact. Put in a 250mA fuse and a new SMPS (old Nokia phone charger produces about 8VDC even though it's labeled 5VDC), routed the DC to the transformer output.

Works like a freaking charm. And since I got refunded my money and was told to scrap it, that's a free spotwelder for me :-) YAY

[aqibi2000]

Can you post a link to the seller you purchased it from

[RustyCaliper]

Hey man, no offense, but the only reason I can see why you'd want that is to buy one off them, claim it's defective because you now know they're accomodating and refund the money without wanting the device back. If so, that'd be a really shitty move, and I'm not going to support that.

My unit was honestly defective, I honestly actually did not want to repair it but BEGGED them to take it back and ship me a fixed one. I'm not going to abuse that trust.