I need help identifying blown capacitor on power supply.

Thread Starter

Pardoe

Joined Feb 6, 2016
11
Like the title says I have a blown power supply, a large capacitor failed and the fuse didn't catch it in time, I've been able to test and identify everything except this little guy.

No markings on it, if there were markings they left with the smoke.

This power supply is for a recording console so I assume this capacitor is filtering...
 

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SLK001

Joined Nov 29, 2011
1,541
File the outer coating down to the ceramic and measure its thickness, then measure the diameter of the metalization. With these two numbers, you can find the approximate value of this capacitor.
 

SLK001

Joined Nov 29, 2011
1,541
First, take the capacitor out of the circuit and measure its capacitance. It doesn't look shorted, just "punched through". It will not be the correct original value, but it will give you a ball park starting point.

It is a single layer device. From the edge, it will look like:
|[]|

A metal layer, followed by a dielectric layer, followed by another metal layer. The thickness of the dielectric is found by filing the edge of the shell down to the dielectric layer.

Once you have the dielectric thickness, scrape the outside layer of the shell down to the plated metalization layer and measure the diameter of that metalization. I would assume (a monsterously bold assumption) that it is an NPO type dielectric, since it appears to be a snubber on the primary of the transformer. Now you have the area, the separation and the dielectric constant, so you can calculate the capacitance.
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
I appreciate your help, I'll get to it right away and bring back my findings, one day, I want to be as knowledgeable as you. Real talk.
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
I was able to identify the cap as a 221k (220pF) 1kV.... New question is, with no clue about the +\- direction, how can I go about figuring that out?

My clues are as follows

One side connects via trace to a transformer.

The other to a 100ohm 1v resistor, the resistor then traces to a dioed.

Would it be a correct assumption that the + side connects to the transformer?

Or does it matter at all on ceramic type caps?
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
Do you have any information on the recording console? Make, model etc?

Hey I wanted to get you a little info because the market for repairs on this console is more than likely about to ramp up and they won't be cheap.

The PSU PCB is designed and built by a company called "PSU Designs LTD". The board itself is has a model code of PCBM-9260072-M1.

I don't know if this helps but I would love to find a schematic one day.
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
Ok so I'm not as cool as I thought I was, I did the entire repair that ended up being 2 resistors, 3 capacitors, and a fuse, but now the fuse blows every time I turn it on, what now.
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
One thing that is a bit different that might be causing the fuse to blow is...

The original fuse was 3.15a 250Vac ceramic type, I'm testing my repair with a 3.15 250v glass type... Could this cause it to blow?

I really hoped I could figure this out but I'm losing hope.
 
I would check the main switching transistor(s), they sometimes go dead short. I cant see how the blown cap relates to this. It looks like its just a filter or snubber.
 

Thread Starter

Pardoe

Joined Feb 6, 2016
11
John,

The small cap was only one of the issues. When I opened the power supply it had damage to a large cap this small cap and another 35v cap, 2 resistors were crisp. All that has been replaced. And yet it's still blowing fuses, I was afraid it might be a transistor. I don't really have the ability to load test transistors. :(
 

RichardO

Joined May 4, 2013
2,268
The original fuse was 3.15a 250Vac ceramic type, I'm testing my repair with a 3.15 250v glass type... Could this cause it to blow?
How was the original fuse marked. You may be trying to replace a slow blow blow fuse with a fast blow fuse. If you are, the fast blow fuse will blow........ faster.
 
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