Determine if electrolytic caps are dried up??

[fastline]

I am still fighting a potential audio amp problem. Very odd that the thing went from a clicking noise in the speaker, to working fine, to now making a loud hum at the speaker. The finger is more or less pointed to a termination or cabling problem but I cannot ignore the amp right now.

the caps "look" minty but wondering if there are ways to tell if caps are dried up? Do I need to remove from circuit? If so, will a simple capacitance test tell the story? There is 8 of them and would rather not pull on a whim. The amp is 10yo but was running great and not run very hard. These caps are all 820uf, 400V IIRC.

 

[Neal]

Try jumping the suspect caps one at a time with a known good cap and see if hum goes away.

 

[Nigel Goodwin]

the caps "look" minty but wondering if there are ways to tell if caps are dried up? Do I need to remove from circuit? If so, will a simple capacitance test tell the story? There is 8 of them and would rather not pull on a whim. The amp is 10yo but was running great and not run very hard. These caps are all 820uf, 400V IIRC.

You really need an ESR meter to test electrolytic capacitors - however, what kind of amp is it? - that's a really unlikely sounding value for either transistor or valve amps, and would be HUGE.

 

[fastline]

I apologize, they are 820uf, 200V. There are 6. I found a toasted resistor by one of them but that still did not fix my problem.

 

[Nigel Goodwin]

I apologize, they are 820uf, 200V. There are 6. I found a toasted resistor by one of them but that still did not fix my problem.

That still sounds an unlikely value - and you're still telling us nothing at all about the amp?.

 

[fastline]

schematic is in my other post with values. this is a 1300watt velodyne sub amp

 

[Nigel Goodwin]

As it's a switching amplifier (class-D) it makes it quite unusual - but the only part that works at 50/60Hz (or at least 100/120Hz) is the bridge, and those six capacitors - which really seem FAR too small in value for a high power amplifier.

Do you have a scope to check for ripple on them, bearing in mind that the amplifier looks to be (rather dangerously) live to the mains.

But any sort of failure in the amp, that leads to the amp drawing too much current, will cause a loud 100/120Hz hum - with those small capacitors, it won't take much to put lots of hum on the supply rails.

 

[fastline]

Thanks! I think you are referring to the way there is little isolation from PS to mains? I did notice that and might be why these amps are notorious for this type of failure yet the mfg will not enlighten me of what is actually failing in them.

I do have an analog scope but concerned about actually powering the amp too long. Might toast something else in that process.

I thought only 2 caps were being used for the PS and others were on the output stage? Is that wrong?

I too agree regarding the caps. Actually, the entire rating of this amp. 1300 watts yet runs all day on 18ga wire and 15a circuit. They are 1300 watt rms, and 3000 peak. Dunno but I have my Rotel main amp that is 400wpc/2 at 8ohms and it weighs 100 lbs and built 15X larger than this 1300watt amp!!! I have half a mind to go back with the largest caps I can physically fit in the PS stage. Not sure on the others.

 

[fastline]

I removed 6 of the 6 filter caps and tested them with a cap meter. All tested over 900uf and rated at 820uf. The resistance across them is 40m ohms. Should I do more testing with the caps? In my experience, caps usually show some sign of distress with meter testing or visual if bad, but......

 

[fastline]

I decided to charge them slightly with my diode tester which goes to 1.5V. They are all holding 1.1-1.3V right now. I am curious if I can charge more in them to see if I can determine if they are good. The OEM indicates they are a problem but because if the serious PITA of installing the board, I really want to know I have the problem found before ordering parts.