Repairing Hantek HDG2022B switching power supply

[andrija]

Hi all, I just received a brand new Hantek AWG - the HDG2022B. The thing would not turn on out of the box but did eventually after a few power cycles. Unfortunately that issue continued, with the unit flat out stopping to work (or starting to) out of the blue when plugged in and turned on. It seems that something is wrong with the switching power supply. This thing seems to have two separate sections, one for 5V (not used) and 12V (powering only a fan) and the other one powering the main board with +-17.5V and +5V. The smaller section always works but the bigger one does not. Occasionally I hear 1 per second "clicking" sound on the bigger transformer, looking like the supply is trying to boot but failing. Voltage at that time also fluctuates from 0 to about 2-3V every second. This clicking continues for some seconds after turn off, as well. Sometimes though it just doesn't do anything at all and sits dead.

Any idea where to start looking for faults? I am not an expert on switching supplies and I'm rather scared of high voltages to randomly probe around , like any typical non-power electrical engineer. I have tried refreshing most of the joints, not easy due to non-Pb solder, but that didn't help. Poking around at all components with the fluke voltalert does not help either.

[retiredcaps]

Why not return it for warranty if it is brand new?

[andrija]

Because I already soldered some stuff onto it and shipping back would cost 1/4 of the price anyhow. That's the danger of buying some of these cheap Chinese instruments that have no local representation. If worst come to worst I could buy/build a replacement power supply, even if I have to place it externally.

[simingx]

Sounds like cold solder... try reflowing all the joints and see if it works.

[andrija]

I tried that first but you may be right, I didn't do a great job. I just tried to turn it off and when it didn't, I pressed on the transformer and it came on. It could be transformer winding itself that is not soldered well to the pins.

[andrija]

I think I managed to repair it and it wasn't quite as trivial as reflowing though you could technically argue that was the problem. It turns out that one SMD capacitor has snapped - not in half but right at one of its metal ends. After several rounds of reflow I started getting desperate and reflowing the SMD capacitors and resistors. I wouldn't have otherwise bother to do so, since its kind of obvious they are soldered (or not) given they have 2 contacts and nothing holding them to board. Well, one of them just fell off after heating up one side and I noticed its end cap on the other side was not there, just some faint metal traces. It was right on pin 1 of the luckily marked 8 pin chip (the only one on board) and datasheet for it came easily enough - DC converter controller. It just so happens that the capacitor on pin 1 controls some auxiliary delay used to prevent false triggering on power up / down and glitches. So the occasional "trying to ramp up once a second" issue was a clue, just like I suggested. I replaced the cheapo 4.7nF ceramic with a (relatively) very expensive PPS 6.8nF 2% since that was the only thing in 1206 package I could find that's reasonably close although as per datasheet 0.1uF would be ok too. And so far, the power supply has been working.