Korad 3305P lab supply failed channel 1 (RND rebadge)

[mvno_subscriber]

Hi,

I've got an RND 320-KA3305P power supply, which I've understood is a rebadge of Korad 3305P. I've had it for almost 10 years I think. I was turning it on the other day, hadn't connected anything and then a spark-ishy crackling sounded from its inside. My immediate reaction was that this was coming from the transformer, that's how it sounded to me at least. I turned off, room was filled with odour. Tried smelling inside afterwards, seemed to be coming from one side of the transformer (same as the primary, but secondary windings come out there as well).

I tried turning it on again later, just to watch the voltage on channel 1 gradually going down to 0. After that, it was dead.

I can still set voltage and current on the display, but when cycling voltage up and down, there are relays who are supposed to click, but don't.

Measuring transformer secondaries with light load seems ok.

Measuring channel 1 with a multimeter gives -130mV.


It seems these relays are controlled by transistors on the panel board (shown in a separate image), so I assume some might be blown? I can't see any indication of damage anywhere.

I can't find a schematic, if anyone have one or have any tips it's greatly appreciated! I really miss my PSU :'(

[ebastler]

A set of schematics for the smaller KA3005P single-channel model is available here:
https://www.mikrocontroller.net/topic/528843#6906823

No idea whether they are accurate. There probably were various engineering changes over time. In any case, good luck finding -- and hopefully fixing -- the fault!

[TheDefpom]

I recently did a really MINOR repair to mine, just one of the output protection diodes failed in my case, but maybe there is something in my video that helps in some way, probably not but here it is anyway:

[mvno_subscriber]

I recently did a really MINOR repair to mine, just one of the output protection diodes failed in my case, but maybe there is something in my video that helps in some way, probably not but here it is anyway:

I did watch your video before I posted here actually! Wanted to see if this was a common issue. Well, the portection diode is not the issue here, I can say that much.


So, these four relays:



Seems like there are two for each channel. Might be three, actually, but still. They switch in and out depending on the voltage range.

These four are controlled by these four transistors on the front panel:




The transistors seem ok as far as I can tell. However, they are not switching on for channel 1. The transistor base is always 0V. So the issue is further upstream. I'll have to trace and see where the signal is supposed to come from. Worst case it's a dead IC somewhere.

I still don't know what exactly failed here, if there were shorted windings I'd think the voltages wouldn't check out? Something did go out with a bang though.. 



PS Sorry for the huge images. Not sure how to make them smaller in the post, I'm not very well versed on this forum.

[mvno_subscriber]

No power is delivered, because the relays are not clicking, because the transistors are not activated, because there is not signal from the MCU (the chip titled "KORAD1" in the lower left of last image).

Thankfully, the reason for that again is the MCU for channel 1 is not receiving power.

Which, by the way, means that there is a separate MCU / IC for each channel adjusting output, and wharever is driving the displays/knobs is a separate unit. Interesting

[TheDefpom]

I would be looking on the rear PCB where there are what looks like linear regulators etc (I never looked that closely), my guess is a burnt out trace or something under that board, or maybe a blown regulator IC.

The relays on the rear PCB will be to change transformer taps as the required voltages change, to improve the efficiency of the linear regulators.

[mvno_subscriber]

It may look like the rectifier for channel 1 is dead. This is the board in the back, containing rectification, filtering and switching between transformer secondaries:



There are three rectifiers; one for each channel. It seems like the logic uses the same rectifier as the actual power delivery, however it's heavily filtered to minimize noise (although one could wonder what badly behaving load it would take to mess up the measurement circuit).

The rectifier for channel 1 has a dead short between phases. You can see it here, together with an important relay:



Now, the relay in question seems to switch between two of the transformer secondaries. If I compare with channel 2, one of them will be connected by default (i.e. when relay is not energized). However, there's not connection here on Ch1, i.e. power doesn't seem to come through. With a second look underneath the board:



That solder joint is kind of odd. Has it melted? Makes me wonder, if the rectifier failed - did the relay perhaps end up acting as a fuse? No power is reaching that rectifier now, that's for sure. I'm crossing my fingers for this, since that could mean the transformer is unharmed and I'll only have to change a couple of "commodity" parts, or at least parts that are easier to come by. The transformer does produce correct voltage on all its secondaries, remember.

Oh well, time for bed again, be back tomorrow.

[mvno_subscriber]

As suspected, I got to confirm today that the rectifier had shorted, and the relay that fed it couldn't handle the current (when I desoldered it, the middle pin just fell out). A few images attached. I cracked open the relay for good measure, maybe you can spot any issues 

Now I've got to find suitable replacements..

[mvno_subscriber]

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All good! I used the following replacements:

https://no.mouser.com/ProductDetail/905-KBU6M - Rectifier, new model but direct replacement

https://no.mouser.com/ProductDetail/893-833H1CS12VDC - Different brand but seemingly same specs

Hoping for many more years of service! Now I can think about doing something about that pesky fan noise, which makes the PSU sound like a diesel engine...