My faulty cap.. isn't faulty

[ac/dc]

Ok, well I spent time checking all the caps (~22) on my dead PSU board, and.. I thought 6-7 of them were faulty, and needed replacing, after I used the beep test on the multi, but.. after I got this one off the board and tested it again, well you can see it's fine, not faulty at all, so.. basically it needs to be soldered back in again. So bollox.
So.. well then the question is how do you test the caps? am I testing them in the wrong place? I was testing on the underside of the board? anyone know what I'm doing wrong?

 

[Les Jones]

Can you explain how you think that you can check a capacitor using the continuity range on the meter ? You need to check the capacity value and also the ESR of the capacitor. What led you to believe that the fault was caused by a capacitor ? For members to help you you would need to post the schematic of the power supply and say what tests you have done on the power supply.

Les.

 

[rjenkinsgb]

Measured on a resistance or continuity range, a capacitor should read as open circuit with a few seconds, after it has had time to charge to whatever voltage is on the meter leads.

That does not mean it's good, as it could literally be open circuit.

To test a capacitor, you need two things:
A Capacitance meter or capacitance range on a multimeter that has that; it should read somewhere around the marked value, within the tolerance for that type of cap (often quite wide for electrolytics).

And, an ESR meter: That is "quality" test of a capacitor. A high ESR (for the type & value of cap) even with the correct capacitance value means it's failing or failed and needs replacing.

 

[Nigel Goodwin]

Measured on a resistance or continuity range, a capacitor should read as open circuit with a few seconds, after it has had time to charge to whatever voltage is on the meter leads.

That does not mean it's good, as it could literally be open circuit.

To test a capacitor, you need two things:
A Capacitance meter or capacitance range on a multimeter that has that; it should read somewhere around the marked value, within the tolerance for that type of cap (often quite wide for electrolytics).

And, an ESR meter: That is "quality" test of a capacitor. A high ESR (for the type & value of cap) even with the correct capacitance value means it's failing or failed and needs replacing.

To be fair, you just need an ESR meter - a capacitance meter is fine for checking what value a capacitor is, but not if it's any good or not. Most ESR meters usually display the capacitance anyway, and have the HUGE advantage that most capacitors can be tested in-circuit.

For any one doing service work, particularly on switch-mode PSU's, an ESR meter is essential - I used my ESR meter at work multiple times every day, and while I'd got a capacitance meter (and actually still have them both) I never found any need to use it, other than trying it out when it first arrived. I was actually given the capacitance meter,where I bought the ESR meter

I'm also a little concerned about the OP's technique - you can't test electrolytics on the ohms range of a multimeter, and certainly not in-circuit. He should be fault finding, not just randomly testing capacitors (or in fact NOT testing capacitors), and removing them.

As for refitting the capacitor he removed?, doing so without checking it's ESR is fairly pointless, and if he's got one of the correct value he may as well fit a new one, while it's out.

Perhaps if he posted a picture of the board, and explained exactly what's wrong with it, we could offer suggestions?.

 

[ac/dc]

Measured on a resistance or continuity range, a capacitor should read as open circuit with a few seconds, after it has had time to charge to whatever voltage is on the meter leads.

That does not mean it's good, as it could literally be open circuit.

To test a capacitor, you need two things:
A Capacitance meter or capacitance range on a multimeter that has that; it should read somewhere around the marked value, within the tolerance for that type of cap (often quite wide for electrolytics).

And, an ESR meter: That is "quality" test of a capacitor. A high ESR (for the type & value of cap) even with the correct capacitance value means it's failing or failed and needs replacing.

Ok, thanks, the meter I'm using in the pic doesn't have capacitance, unfortunately. And I don't have an esr meter, so.. hmm, will have to look into those I think.

 

[ac/dc]

Can you explain how you think that you can check a capacitor using the continuity range on the meter ? You need to check the capacity value and also the ESR of the capacitor. What led you to believe that the fault was caused by a capacitor ? For members to help you you would need to post the schematic of the power supply and say what tests you have done on the power supply.

Les.

Well, for testing I was methodically testing each component with the beep/continuity test, so I'd started at the ac input part, so I'd done the fuse/rectifier/diode part, then I'd moved onto the schottky diode, and then the capacitors.. re a schematic, well it's an old psu that's come from a pc case, so.. well I don't have a diagram like that. But, well arn't most psus going to be fairly similar to each other?

 

[Les Jones]

Just testing components is a very poor method of fixing faults. First you can't test them reliably in circuit and removing each one to test is very time consuming. (Also doing that is possibly going to create more faults.)
As you now tell us that it is a PC power supply that will be a switch mode design. For a fault on this typeof power supply I would first check if you have 300 or so volts DC on the primary side.
As a beginer I would suggest that you start with repairing simpler items. Switch mode power supplies can be difficult to fault find. (And dangerous if you do not fully understand them.)
PC power supplies are quite cheap so it is normally not worth the time tracing the fault.

Les.

 

[ramondo]

Do you really want to know how to repair electronic devices?? If Yes then look on YOUTUBE for electronic repairing for beginners and expect to devote at least six months to begin to understand what you are doing. Many DIY test equipment items can be had from china in kit form (working but basic), move up as needed. Most defective items are suspected "In Circuit" and verified "out of circuit" You will have to solder on almost every thing you work on, Get good at it. Locate some scrap circuit boards and practice. GOOD LUCK!

 

[ac/dc]

Do you really want to know how to repair electronic devices?? If Yes then look on YOUTUBE for electronic repairing for beginners and expect to devote at least six months to begin to understand what you are doing. Many DIY test equipment items can be had from china in kit form (working but basic), move up as needed. Most defective items are suspected "In Circuit" and verified "out of circuit" You will have to solder on almost every thing you work on, Get good at it. Locate some scrap circuit boards and practice. GOOD LUCK!

Ok, thanks for the advice.

 

[ac/dc]

btw the yt vid I was using to show me, was this:

he's basically using the beep test for many of the components on the board, testing 'in circuit'. So that's why I was doing it that way.

 

[Nigel Goodwin]

btw the yt vid I was using to show me, was this:

he's basically using the beep test for many of the components on the board, testing 'in circuit'. So that's why I was doing it that way.

Sorry, but he doesn't have a clue what he's doing either, and no idea how to use a multimeter. There's a HUGE amount of utter rubbish on YouTube, and load's of complete fake stuff.