Baffeled by parallel caps in power supply

[Thorerik]

I am working on a Pyle 1400 power amplifier. Can't find a schematic or service info, just the operator manual which is worthless.

The problem is in the power supply board. Turned it on and a few minutes later the magic smoke was released. Opened it up and saw that one of the 4700uf 50v caps had vented. Popped it out and while looking at it I saw that electrically in parallel with this bad cap was another of the exact same value. Of course I know this doubles the capacitance for a total of 9400uf. I had a 5700uf cap 50v and thought I could substitute it so I gave it a try. It smoked that 5700 cap too, leaving the other 4700uf in parallel just fine. I checked it too, ESR is nothing and it shows the proper labeled capacitance 4700uf caps are in the mail. I am reluctant to put in a new cap and watch it smoke too. I don't understand what is going on. It seems to me that the other cap should smoke too.

Pulled the bad out and left just the good one in. The amp operates just fine without this cap. No hum or noise and I can listen to it over my speakers at moderate levels. I am sure that if I started to really push it the need for this other cap would be very evident.

Can anyone clue me in on what is going on? Just to be clear the other two caps for the other side of the PPS are OK.

[budm]

Re: Baffeled by parallel caps in power supply

Correct polarity is observed?
Do you have pictures of what we are dealing with?

[Thorerik]

Re: Baffeled by parallel caps in power supply

Polarity is correct, clearly marked on circuit board. Even still that would not explain why the first cap blew. This is one of those bizarre things that happens in electronics. It would help if I could see a schematic for it but I have not been able to find it. As far as a picture it wouldn't tell much. There are two parallel circuit traces where these caps attach, pos to pos, neg to neg. Electrically they are attached to the same point.

It also doesn't explain why you can remove the bad cap, leave the other in the circuit and the amp works OK.

I thought that maybe the DC voltage across these caps was more than 50v, I checked the secondaries of the transformer and they are putting out 46.5 volts on each side of the centertap. I checked the DC voltage on the suspect caps and it was about +/- 65 volts. The other side of the power supply seems just OK with this.

[budm]

Re: Baffeled by parallel caps in power supply

46.5VAC x 1.414 = so the DCV on the filter cap will be about 65VDC.
So they use 50V rating on 65VDC power rail, the old cap may have more head room or higher working Voltage than printed on the cap. To me that is a poorly built product, and the Ac is probably not regulated either so it can get even worst.

" It also doesn't explain why you can remove the bad cap, leave the other in the circuit and the amp works OK." it will function as long as you are not trying to push the output to the maximum published specification of the amplifier. Lower capacitance = more ripple Voltage.
Learn more here:
http://www.skillbank.co.uk/psu/smoothing.htm
https://electronics.stackexchange.co...ce-calculation