Testing electrolytic capacitors

hidinginthebush

Hi all,

I'm working on fixing a guitar effects pedal, going through the circuit board compoents, it seems there could be 2 blown electrolytic caps. When hooked up to my digital multimeter, they both read about 50K ohms, they should read infinite, right? (The other electro caps on the board do). The caps are 10pF and 470pF.

Papa_Bear

If they're bulging in any way then they are most likely leaking current when in circuit and heating up as a result. Better off replacing to be sure.

Check out this free ebook on testing components - it's very handy!

http://www.talkingelectronics.com/te_interactive_index.html

hidinginthebush

Thanks for that, replacing them is the way I was thinking alright.

Out of interest, what would usually cause these to go?

DublinDilbert

10pf and 470pf will not be electrolytic caps, more than likely a ceramic cap. Can you post a pic.

Electrolytics go for all different reasons, may have been poor quality to begin with or poor design stressing them.

Unlikely to be pushing large amounts of power in this application, so doubt they were stressed.

If I had to guess I'd say you've some bad solder somewhere, possibly on a larger component.

hidinginthebush

Thanks for that! Here's a pic of the board (possible culprits circled):



Looking at the back of the pcb, there's not really any bad solder joints as far as I can see. And the caps don't look too deformed / leaking from what i can see. When connected to a power supply the "on" led lights up, so the power supply isn't the issue.

Edit: I don't think they're electrolytic caps after, just plain old disk caps

DublinDilbert

Why do you think they are faulty? There probably other resistors / components on the circuit board that will affect any readings you try take directly.

Can you get a schematic?

Check pots, audio connectors... examine all solder joints with a magnifying glass, may be small cracks or dry (dull) joints...

hidinginthebush

Sure thing, here's a schematic:



To be honest, I though it was the capacitors at first as they were not showing infinite resistance, though now looking at the schematic and digging around, they're in parallel with resistors, so the resistance I was getting there was probably related to those. The fact that there were no obvious bad solder joints led me to start poking around and checking each component, and when I saw the capacitors weren't going to infinity I thought that was the problem.

For what it's worth, I've checked each resistor and diode, and they all seem to be fine too. Though it seems Q2 is part of the long black (semiconductor, I think?) at the bottom of my picture (the one with the BA15218 on it)

whizbang

Whats wrong with it ?

Does the check light do anything. Do you have all the power supply voltages correct?