Repair Help DOA Techpower TP3005P Power supply

[DinkyMods]

When I first powered on my new TP3005P Programmable linear power supply within seconds it let the magic smoke out.  Needless to say I was not happy and called Amazon to get a different replacement. (I went with a Siglent SPD3303C) Thankfully for my trouble they let me keep the dead Teckpower so I immediately cracked her open to see what the failure could be.  I got excited since right off I spotted a blown 2200uF filter cap just after the bridge rectifier on the controller board.  I sucked it out and replaced it with a quality 25V 2200uF cap of the same specs as what was on it before.  Sadly this didn't fix it.  As you can see in the attached picture and this is true no matter what I have set the output V to its putting out just a little over half of the set voltage to the output.  It doesn't show any clamping or protection and I have tried running the current all the way up to the max of 5A just to eliminate anything I could there.  I'm out of my depth with it comes to troubleshooting a linear power supply like this and I really don't know what to check next after ruling out all the basics I could find like blown passives.  Since I have a number of pics I had to compress them more than I would have liked.  If there is anything that a better view of would help let me know. I  really appreciate any guidance the community here could give me on what to test as I really don't know what to look for and I want to go about this properly and of course safely.  Thanks in advance for your help. 

-Daniel

[JFJ]

... I spotted a blown 2200uF filter cap just after the bridge rectifier on the controller board.  I sucked it out and replaced it with a quality 25V 2200uF cap of the same specs as what was on it before...

Have you checked the voltage across that capacitor? Given that the maximum output voltage of this linear supply is 30V, I would have expected the working voltage of a filter capacitor to be around 63V.

[Kleinstein]

Capacitors do sometimes fail, but rarely so fast. So there likely is a reason behind it. This could be something like a broken rectifier or just a rectifier soldered the wrong way. So checking the rectifier / diodes used for rectification is a good idea this can be done in circuit.

I would guess a 25 V cap to be at the auxiliary supply for the OPs or similar.

Another point to check is if the input voltage is set (or correct version) correct - 115 V  setting with 230 V line voltage could be a problem, though likely more a transformer problem.

[DinkyMods]

I diode tested the rectifier and it tests out okay. Each leg gives a value of around .615 +/-0.01 which from what I had read seems to be a typical value. 

I first guess was the failed cap on a brand new out of the box unit was shipping related since it was, to borrow Dave’s term, “flapping in the breeze”.  After I get off work today I’ll see if I can somehow get some leads on that cap with it powered up to see what voltage we have coming from the rectifier. I might have to add some temp test pigtails since it really close quarters assembled. 

I am a little suspicious of the 25V cap that came on it being incorrect as well.  I hate to admit the rookie mistake I made in replacing the cap but it may be useful data.  I wasn’t paying attention to the cap voltage when I got excited finding something I could easily swap so the first cap I used was a 20V 2200uF one hung low cap. Needless to say it got my attention when it exploded as soon as I powered it up. So after a new pair of pants I made the 2nd swap to the current cap that’s the same specs as the original.  This has me wondering though, if the 20V exploded instantly could it possibily be that the only thing keeping this 25V Nichicon cap from blowing is that it’s a good quality cap. Maybe it originally had the wrong cap on it and it might be underrated still.

As soon as I get free of work and back to playing with it I’ll post an update. Thanks for the help.

[DinkyMods]

It might be chicken dinner time.  If its not the problem its most certainly a problem.  Voltage measured at the cap coming out of the rectifier is 38.46VDC.  The 25V 2200uF cap was either a mistake or a fatal flaw.  I'm confident if I had replaced it with another one hung low cap it would have blown immediately just like the original cap did.  I don't have any 2200uF caps in my stock with a higher voltage so it will be a Monday before I can get some in. Sadly Amazon only has 50V 2200uF Nichicon caps in the auto woo woo versions so I'm going to just toss a China special in there till I know for sure thats it issue.  Thank both of you for the direction *fingers crossed* this fixes it and I have a power supply for the cost of a new cap.

[Kleinstein]

This looks really promising. A 25 V rated cap has the right to blow up at 35 V, though not all will no. Some can survive - though likely with a reduced capacitance. So they could have chosen a 1500 µV 40 V cap right from the start.

There are usually other suppliers for electronics part than just Amazon or some obscure Chinese sources.

If the 2200 µF are the main filter cap, this would be rather small and there might be a second, maybe third one in parallel.
One could likely do a first test with a smaller cap (e.g. 1000 µF or 470 µF), if you don't use the full rated output current.

[DinkyMods]

This looks really promising. A 25 V rated cap has the right to blow up at 35 V, though not all will no. Some can survive - though likely with a reduced capacitance.

There are usually other suppliers for electronics part than just Amazon or some obscure Chinese sources.

I’ve been trying to workout in my head while I wait for the parts with the 25V cap in there right now could it somehow be limiting it from reaching its set output voltage or do I need to keep looking to another problem that’s causing it. I can’t quite wrap my head around how the cap could be limiting anything.

And yeah Amazon definitely isn’t my preference for components but when i need just a couple of something & fast it’s hard to beat them.



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[DinkyMods]

I've done a bit or reversing on the controller board drawing up a schematic as I went and I'm getting much more certain that the cap failing was just a symptom of another problem.  All of the surrounding caps are 25V and lower and now that I have traced the functions a bit it appears that it wasn't a case of accidentally placing the wrong cap.  My current next place to check unless someone has some other suggestions is to try and find out the ratio the transformer should be running and also to check the function of all of those regulators.