lm1875 filter capacitor problem

[arisoaris]

Hi!
i m building one small stereo chipamp with lm1875 at +-15Volts.
i ve sticked to the datasheets schematic, and i m using 4700μF onboard filter capacitors, one for the V+ and one for the V-

The problem is that when i power the amp from my bench psu the voltage at the positive rail drops at 10volts (from 15) and the capacitor starts to get hot.
I checked for shorts and everything seems ok. Even more strange is that when i bypass the capacitor the voltage doesnt drop...
I thought it might be a bad capacitor, so i changed it, but the same happened with the new capacitor..
I dont think there is a problem with the layout or the construction, since this is the second channel board i m building and the first one made with the same components plays ok..
Any ideas??

Thanks

[Andy5112405]

Check polarity.

Just because the PCB says it should be one way round the silk screening may be wrong.

Simply measure across caps with a meter and ascertain which lead is +ve and which is -ve.

Also check voltage rating of caps. They need to be at least 16V but preferably 25V to allow for mains supply fluctuations.

[arisoaris]

Thanks for the answer ..

i am prety sure that the polarity is right. i have put the capacitor the same way at the other channel and it plays ok, without any voltage drop. The capacitor is rated for 25volts .
Is there any chance that the marks on the capacitor are wrong ??? (+ for - and - for +)
can a faulty chip cause such a problem ? (given the fact that it has no problem when i bypass the filter capacitor)
its quite strange...i ll go check with another capacitor but i think smthing else is wrong

when i cut off the power supply the voltage at the positive rail drops immediately (meaning that no capacitor has been charged) while at the negative it drops slowly..

[Andy5112405]

What do you mean by "Bypass the Filter Capacitor" ? The filter cap is across V+ and 0V so it can't be bypassed by shorting it.

It's unlikely that the capacitor has been marked incorrectly especially as you have already swapped it.

[Andy5112405]

Is your PSU OK ?

Have you got it connected correctly ?

Is there any AC on the +15V output ?

[AndrewT]

Hi,
charge up the suspect capacitor through a 10k resistor from a 25Vdc supply.
Measure the leakage current once it's fully charged.

You could change the charging resistor to 100k to give the cap time to reform but that takes quite a long time. Allow 24hrs.

[arisoaris]

i bypass the capacitor by removing it from the board, no short between supply and ground ...
i couldnt find around another 4700u capacitor and i used 2 2200u in parallel.. and guess what .. no voltage drop! everything works good...so i guess i had a bad capacitor
i m pretty annoyed from the fact that i ordered 6 capacitors of wich 3 of them dont work... i have ordered them from www.futurlec.com and i dont think i ll order again from them.
The fun fact is that while building the boards i was picking the good capacitors by chance... i mean from the six capacitors i had the three were not working, but the first three i used were working good. lucky pick :P

@AndrewT: u say i can fix the capacitors by charging them ?

Thanks

[chlorofille]

Hi arisoaris,

Can you post some pics of the defective caps?
What brand are they ?
Thanks

[AndrewT]

you can check them by charging them at a low current.
See how the voltage rises. Does it stop rising early?
Does the capacitor reach the supply voltage?
What is the voltage drop across the 10k or 100k charging resistor?
Now work out the current that is leaking through the capacitor.
Does it meet specification for that supply voltage?

[Andy5112405]

As AndrewT is trying to tell you.

These may be NOS (New Old Stock) capacitors. They MAY respond to REFORMING. That is charge them slowly through a resistor.

They may have dried out or even corroded internally in which case they are ready for the bin.

At least you now know what the problem is.