Electronics > Repair
Korad KA3005P power supply faulty/repair
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tony3d:

--- Quote from: pomonabill221 on January 25, 2014, 07:03:51 am ---
--- Quote from: AllanMN on January 24, 2014, 11:06:34 pm ---The KA3005P arrived today from SRA Soldering. QUick.

Some testing... and.... the negative isn't floating. It surely IS suppose to float on this supply, right? Because if I connect negative to ground it draws over the CC setting (i.e. set to 0.1 it draws 0.2) with nothing on the positive. I noticed it when I tried to scope the output with scope ground to negative.

Allan

--- End quote ---
I checked both the positive output and the negative relative to chassis ground and they ARE floating.... Your supply may have a problem.

--- End quote ---

Yup! Mines floating as well. Call SRA Solder, speak with Doug, and tell him you want another one sent out! They are really good to deal with.
AllanMN:
Thanks Tony. I'm in touch with Doug. But this is perhaps an easy fix. I think something is touching ground and since the supply floats, that signal on ground is pushing plus and minus around from there. Here's what I see on the scope on the positive output lug (attached image):

5ms/div, 5v/div vertical, 0v is at top of scope screen so you're looking at -10v to -40v about.

I think that's the unregulated plus rail (about -43v) with pulses to the regulated voltage (set at 10, so -10v). It's inverted since I'm looking at it reversed if that makes sense (plus rail is grounded so positive out is pushed negative).

There isn't a lot that's at ground in the supply. I was about the disassemble the front panel assembly but it's plastic (not grounded metal). There's just the ground from the front panel lug coming though.

My question is where would such a signal be on the schematic, and then, circuit board (Pomona Bill). There's where to look for a short to ground I think. Careful inspection didn't reveal any solder bridges or physical damage.

An aside, learning to repair this supply would seem to be a handy trick for the future.

Allan
tony3d:

--- Quote from: AllanMN on January 28, 2014, 01:03:52 pm ---Thanks Tony. I'm in touch with Doug. But this is perhaps an easy fix. I think something is touching ground and since the supply floats, that signal on ground is pushing plus and minus around from there. Here's what I see on the scope on the positive output lug (attached image):

5ms/div, 5v/div vertical, 0v is at top of scope screen so you're looking at -10v to -40v about.

I think that's the unregulated plus rail (about -43v) with pulses to the regulated voltage (set at 10, so -10v). It's inverted since I'm looking at it reversed if that makes sense (plus rail is grounded so positive out is pushed negative).

There isn't a lot that's at ground in the supply. I was about the disassemble the front panel assembly but it's plastic (not grounded metal). There's just the ground from the front panel lug coming though.

My question is where would such a signal be on the schematic, and then, circuit board (Pomona Bill). There's where to look for a short to ground I think. Careful inspection didn't reveal any solder bridges or physical damage.

An aside, learning to repair this supply would seem to be a handy trick for the future.

Allan

--- End quote ---

Let me know how you make out. When the supply is working properly, I really like it. I like the size. Fits perfect on my small bench. Was SRA willing to exchange it?
AllanMN:
Ah. Found it. The fan wire wasn't routed through the slot in the fan housing and was then crushed when assembled, shorting to case/ground. What I saw on the scope was the PWM of the fan return,  pulled low by U12, an On Semiconductor  ULN2003A High Voltage, High Current Darlington Transistor Array. Splice or replace to repair.

A comment I saw on the video EEVblog #315 - Korad KA3005P Review/FAIL :

" The biggest problem with these 1HL brands is the counterfeit pass transistors. They can barely supply 1/3 their rated current due to a severely reduced SOA. I routinely buy them - they're a steal - and replace the pass transistors with the genuine parts. I never, EVER have problems again. I am very hard on power supplies and do everything I can to blow them up. "

Any thoughts if this mod is warranted? And details? If say, you are inclined to charge SLA batteries at 5 amps for a few hours which I would not be? Cavlovic killed his doing this I believe. Did you ever figure out how, or get a repair/replacement?

I wonder how hot the sink gets at a full dissipation. I don't have a thermocouple to check but will in a few weeks.

The overshoot isn't such a concern to me if it only happens at 3-5A load ranges as in Dave's video. Do you see that too, Bill?

All my voltage outputs are very accurate. I guess I like this PS for the money. Even if I have to fix it!

Allan
pomonabill221:
Hey Allan... GREAT that you found the problem!  Your scope picture does look like a PWM signal with AC ripple on it, although since the supply for the fan is unregulated, that could be where the ripple is coming from... just glad you found the problem.

Yes, the overshoot does change depending on the load.  I do see overshoot at even low loads (although I don't remember what the loads were and what the overshoot was.. will have to try it again).

I have (I think) the same "version" that you have as far as pass xsistors used and the assembly of the heat sink.  Don't know if they are real or fake though.
I was charging a 24V battery once, and drawing max current from the supply, and it did shut down due to over temp on the heatsink (or thermostat in the xfrmr... really don't know which tripped), but after a while of cool down (about 10 minutes), it powered back up ok.
It was running pretty hot and the thermal overtemp seemed to do it's job.

The heatsink is a little small if you are going to push it, and I may mod it with a larger heatsink (maybe even a larger fan), but for now it is working great for a cheapie!

I know alot of people whine about how cheap it is, and all it's problems, but like I have said, it ain't an HP/Agilent/Lambda/Fluke/etc., and I know that.

One thing on my supply... the FWB was NOT bolted to the heatsink and they relied on the stiff leads of the bridge!  It was NOT laying flat on the heatsink (the lead side was not touching the aluminum), so I desoldered the leads, bolted the bridge on the sink, and resoldered.  How cheap was that!
I would suggest that you check the mounting of the bridge to make sure that at least it is making good mechanical contact to the heatsink, and maybe even bolt it down... it probably runs pretty hot when under high load and will not last too long.

Here are some pics of the bridge (notice the slight space on the lead side), the xsistors and bridge (notice how the goop doesn't look like it was making contact on the bridge), and the contact area on the heatsink (only partial contact on the bridge).
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