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While heating up my dead graphics card in the oven, one of the capacitors detached from the board. The board itself clearly indicates the location of the positive side. However, I have not encountered this type of capacitor before and am unsure which one of the contacts on it is the positive one.

This is the video card. The positive end is clearly indicated.

A whole graphics card A close-up of the capacitor location and positive polarity

And this is the capacitor.

Capacitor Capacitor, different angle

Sorry for low resolution images, I only have access to my webcam. The markings on the capacitor are as follows:

F
7Y2b
331
16

Google search for capacitor markings showed no usable results.

Trying to ignore the possible foolishness of using an oven as a resoldering tool, please advise on determining capacitor polarity. Which contact is the positive one?

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1  
(Not related to the question) If it's not an effect of the blurred pictures, electrolytic capacitors on that video card seeem "plagued" – Axeman Aug 24 '12 at 20:35
@Axeman, you're right about those. It appears they did bulge a little bit because of the high temperature they were exposed to. However, they did not burst and the graphics card is working now, although with some glitches and overheating. – Nikola Malešević Aug 25 '12 at 10:13
1  
Why not just replace the capacitors with ones (of the same specs) that you have a data sheet for. Then you know exactly what you've got. Capacitors are a lot cheaper and easier to replace than fried chips. – Matt B. Aug 26 '12 at 18:17
@MattB. You've got the point there. I was planning to replace them, in fact I already ordered capacitors in question. As I said, the others were common ones, with their specs printed clearly. I had problems determining the red one, though, but answers listed below were of great help. Thanks for your suggestion. – Nikola Malešević Aug 27 '12 at 19:15

2 Answers

up vote 7 down vote accepted

F 7Y2b 331 - probably equals 330 uF
16 - probably equals 16 Volt

Dextorbs picture suggests that the red stripe is negative.

Independent thought might suggest it was positive.

Try this.

I have used the following method for many years with complete success. This does not mean that it is sure to work for you but that it has a good chance of doing so. When an aluminum electrolytic capacitor is operated with correct polarity the case operates at a slightly positive voltage relative to the negative terminal.

If the capacitor is operated with reverse polarity the case voltage is substantially more positive than when polarity is correct.

A PCB with many capacitors on can be checked by measuring Vcase relative to ground for capacitors with their negative lead grounded. (Otherwise measure Vacse to v- for that cap).
Reverse polarity capacitors will have substantially higher Vcase-Vnegative. This works extremely well in practice. (I have never seen this effect commented on anywhere else).

Testing:

Use a voltage less than half of the capacitor's working voltage.
(1) Apply voltage to capacitor pins and measure voltage from -ve pin to case.
(2) Reverse voltage and repeat.

The arrangement which has the LOWER voltage on the case relative to the negative pin is the correct one.

eg say you used a 2V supply.
In orientation 1 the case was at say +0.5V relative to negative.
In orientation 2 the case was at say 0.15V relative to negative.
Orientation 2 is correct.

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Do you know if there is a standard (IPC or other, etc) for capacitor markings? – dext0rb Aug 24 '12 at 21:01
up vote 12 down vote

The black mark/stripe on the top of the can indicates the negative terminal.

enter image description here

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2  
In this case the red mark is likely the - – Paul Sullivan Aug 24 '12 at 19:40
It's hard to tell if its red or black - the lighting in the image seems to fool. Never seen a red mark on a capacitor, personally. :) – dext0rb Aug 24 '12 at 20:49
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@dext0rb , Just noticed the silkscreen on the PCB (second picture is clear) shows opposite polarity than the one on the image you got! It shows the "+" sign at the red mark. – Abdella Aug 24 '12 at 21:15
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That's exactly why I wasn't sure - the color of the cap is red. Thank you for your answer. As your answer and comments above did not sound 100% positive about the... positive side, I tested the cap using Russell's instructions. It turns out you were right, the red side is a negative one. Thanks for all your help. – Nikola Malešević Aug 25 '12 at 10:11
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FWIW, OP's capacitor appears to be an OSCON-type, which tend to be marked identically (filled-in part of the top of the can is negative). – Madmanguruman Aug 25 '12 at 11:09
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