Saturday, December 8, 2012

Lab power supply blew up

Pop!

So, I was sitting trying to figure out why an EHX Super Space Drum (which I was repairing for someone else) wasn't working.
All of a sudden, I heard a loud pop, like somebody claps their hands once, right next to your ear.
It was really loud and felt like it almost started ringing in my ear.
Of course I had no idea what it was and got really scared I had blown something up in the EHX thing I was repairing, because it wasn't mine...
Though, I suspected it was the power supply, because .. what else could it be? It sounded like a capacitor blowing up. So, I switched it off and continued my repair using my other power supply...

When the EHX thing was done I opened the suspected power supply and looked inside.
This is what I found just laying on the bottom...
Yes, it's empty.




What was that?


Where did that come from? At the time of the explosion, I was using one of the variable channels.
By the way, the power supply is called PS23023 and here's what it looks like:
Photo from http://www.velleman.eu


I looked inside, trying to find a blown capacitor.

After a little while, I found this...
Now there's your problem!
Here's a shot of the inside. The board with the failed cap is removed, laying face down on the table.
I noticed a few stains that may be electrolyte, so I cleaned a bit, but it didn't seem to be much.

Mystery solved! This was the fixed +5V board btw, not for the variable channel I was using at the time...
So, it had just blown up spontaneously.

Note the markings on the other cap: "40/085/10". I believe those markings mean that it's rated for -40C to +85C with 10% tolerance. Interestingly enough, ALL the other caps I could find in there were +105C grade. I googled around a bit and found the manufacturer for these.
I found another interesting post:

https://forum.sparkfun.com/viewtopic.php?f=14&t=30949
"A capacitor in a power supply exploded. It is 2200uf 25v. I attached a photo of the other side."
The attached photo shows a cap that looks exactly like the one I have.

So in this power supply, for some reason, they had chosen cheap Chinese crap caps rated lower than all the others, as reservoir capacitors for the fixed +5V board.
Though I think more of the caps on the board were from (the same?) Chinese manufacturers, I didn't really check all of them, but I did note they were supposedly 105C, which was good enough for me.

So...

Avoid:


- CR
- Cirony
- Ningbo Zhengming

Actually, avoid Chinese capacitors overall, if possible...

I don't know what they do; just rate them higher than they are, poor production, crappy quality control... all of the above?
Who knows. Just google "Chinese capacitors" and enjoy the results :)

Fixing it

When looking at the board, it turned out those two caps were just connected in parallel, acting as one big capacitor. Anyway, I had to fix this and I wasn't going to order another pair of no-name crap caps.
I'm going Japanese on this! My local retailer didn't have any better alternatives for 2200uF than 50V.
I decided to replace both the busted one and the survivor, since I guess it was just a matter of time before that would blow up too.

I bought two Nichicon 2200uF 50V and put there instead.
This is what it looks like now:
Check out these puppies!
There's some clearance between the caps and the transformer so it should be OK.
I've had the powersupply switched on a few hours since replacing the caps and nothing has exploded yet :)

So:
Hurrah for another successful repair!
Booo for another case of Chinese caps exploding.

3 comments:

  1. Hi Synthpeter
    I Hate that. I do a lot of Valve amp Repairs and have had that happen on a couple of occasions & i suspect that a 450 Volt cap is going to make a bit more of an impact than your wee chap.
    BTB many years ago i was the service rep in Australia for SCI EMU Korg Alesis Ensonic and worked on all the rest inc Fairlight Roland Yamaha and all tho loth to remember even electric accordions (I still shudder).
    I still have Service manuals and some parts in boxes so if ever you are desperate wont hurt to ask. davfew@gmail.com
    enjoyed reading your Blog.
    df

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  2. Hearing a loud pop during a repair is never a good sign. But at least you were able to see the problem immediately and were able to do the necessary repairs. You also now have an idea of which brands to use so that you will get a better performance from the unit. I just hope this will be the last time you’ll encounter this kind of problem. Take care!

    Kellie Taylor @ Aim Dynamics

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  3. hi,I got the same dual power supply and one of these relays is whisling (K2) on the left electronic card when i'm increasing the voltage (between ~ 12.2V and 15.8v ....)
    do you have any ideas ?
    best regards
    mb

    ReplyDelete