HY3005F-3 PSU Fail

[sharvs111]

So a little bit ago I decided to upgrade to a dual channel PSU and bought a 3005F-3. As far as I can tell, it appears to be identical to the one jamesp15 did a teardown of here but where it should say "Mastech" on the front, it says "Dr. Meter" (might be part of the problem). You can imagine my disappointment when after only a few months it decided to not power on.

I cracked the cover and saw bits of "stuff" rattling around inside the case - never a good sign. Sure enough, one of the components appears to have failed violently (see pictures). Fortunately, the component was still intact enough to make out the "NTC" on it, so it's clear it's an inrush current limiting thermistor (in case the fact that it was in series with the primary side of the transformer wasn't enough of a clue). It should be easy enough to replace, but the remainder of the markings are completely gone, so I'm not sure what resistance/current rating it was.

So here's the question: Does anyone know what specific type of thermistor this should be? If anyone has a similar model, it should be pretty easy to open it up and read the markings off the thermistor. Or if anyone is familiar with PSU design and can recommend a part, that would be helpful too. And if anyone has a schematic showing the thermistor value, that would be really awesome! (I scoured the interwebs, but couldn't find one for this model.) A few points of info that may be relevant: 1) I'm in the US, so I'm running off 120 VAC. 2) It has a max output of 5A at 30V x2 channels = 300W. 3) It has an 8A fuse on the mains, so the thermistor obviously doesn't need to sustain more than that in steady state.

One other side issue - as you can see from the pics, one of the caps (C3 - an X capacitor) was torched pretty bad as well, so I'm planning to replace that too. The markings on that are also illegible, but I think I have a pretty good idea what to put there. I opened my old single channel Mastech and that had a 10nF 2KV rated cap across the mains, so I'm planning to go with that (no thermistor in that one though). But if anyone can tell me exactly what that cap's supposed to be, that would be appreciated too.

[SeanB]

Grab an old PC power supply ( non working is fine, the NTC will probably still be fine inside) and take the NTC out of it, and solder in using the full length of the leads as it came out with. C3 you can leave out, or replace with a similar one from the PC supply, use one of the Y caps in the mains side ( also blue) as a replacement.

Curse the manufacturer who used such a poor choice of component, and also who used Crapxon or Teapo caps as well in a power supply.

[Seekonk]

That may have just saved a good piece of equipment.  When you replace those items I suggest powering it up with a 100W lamp in series with the AC. That will prevent any further damage till you are sure the supply is ok.  Cold a lamp is only a couple ohms but its resistance increases 7-10 times when hot.  I've been testing unknown stuff for 40 years this way.  Saved a lot of fuses and components.  That was quite a hit.  There has to be more damage.

[sharvs111]

Seekonk, that's not a bad idea. I did try shorting past the burnt out thermistor - not the safest, I know, and I won't be doing it again, but it did power up, so I'm reasonably confident the rest of the unit is functioning. Any ideas what might have caused this in the first place? Since the thermistor is supposed to basically be a short when it's warm it's hard to imagine an internal short in the component would have caused it. The only thing I can think of is if there was a short somewhere else and the whole unit was drawing much more current than it was designed for.

[sharvs111]

As SeanB suggested, I opened up a computer PSU and looked at the thermistor in there. It was a SCK 2R58 which I couldn't seem to find for sale anywhere, but I found one with similar specs on Digikey (CL-30 from Amphenol Advanced Sensors). I just put in my order, so I'll install it when it arrives.

[wguibas]

I took mine apart because I'm having problems with the voltage fluctuating up with load.  If I set it on 14.1V it shoots up to 14.9 when the load hits 100mA then it drops down as the mA come back down. 
These things are really crap, I'm so pissed I got a knock-off of a chinese knock-off.  The one by Tekpower is built so much better.

The Blue part says 222 2KV.  All 3 on that board are labelled that.  Mine doesn't even have the big burnt NTC that yours does.  What a joke.



This is what mine is doing, click picture to see video.