Problem: two caps in ACDC unit?!?

[mashkina]

Hi, I got a buzz-interference problem with a guitar amplifier. After realizing the buzz was 100 Hz and thus related to the AC power, I detected a failure of "one" of the caps of the diode bridge rectifier that powers the mother board (this mother board has a analog preamps and a DSP to make effects on the input guitar signal). See attached diagram. C7 is the dead cap, but ...

1. why two caps instead of just one?

2. THE MOST IMPORTANT QUESTION OF THIS THREAD: Is the polarization of C7 right? Shouldn't C7 be upside down instead?

3. The pin 2 goes also to the mother board. For what reason? Since this ACDC part will likely power the analog circuits of the mother board, why two different voltage levels?!?

Is somebody familiar with this simple ACDC schema?

Thanks a lot.


More INFO: C7 and C8 are both 1mF/35V and the pins A and B from the transformer are labeled as A=0V, B=30V.

[Ldanielrosa]

Is the transformer center tapped?

[mashkina]

Thanks for the reply.

NO, it is NOT center-tapped.

The transformer has three different independent voltage convertors for different parts of the system.

The mother board gets power from the 9V and the 30V parts.
- The dead condensator corresponds to the 30V part. This affects (I guess) the analog stuff in the mother board such as preamps and causes the buzz interference.
- The 9V powers (I guess) the digital circuits (DSP and so on), which all work properly.

NONE of these parts is center-tapped.

(just to be fully descriptive, I mentioned a third convertor. This is 62V and feeds the power amplifier for the loudspeaker, and that IS center-tapped, but that doesn't touch the mother board obviously).

[umasankar]

The power supply is likely to be dual power supply,supplying both positive and negative volts to the amplifier circuit.In that case it must provide positive and negative voltages  to the circuit.Then the polarisation is wrong.I think the amplifier uses some operational amplifiers which may require double voltages.point 1 in your supply must be positive.Point 3 is negative.point 2 is ground.Hence the positive of the cap should be connected to pin 1.Its negative must be connected to point 2.Similarly,the other cap's negative must be connected to the point 3 since it is a negative supply line.This cap's positive must be connected to point 2.

I hope you understand the principle

[mashkina]

Thanks a lot. I understand clearly your suggestion.

That was my initial guess but I needed confirmation. You know, this dead cap was placed like that in the factory (manual operator?) and the manufacturer does not want to give me the right answer in its forum, while implicitily suggesting they did something wrong.

The only thing that I still try to understand is the way the cap died: it was slowly, the buzz grew day by day, slowly. By reading a bit on ELKOs, if the inverse current/voltage they are applied is low, they survive and behave like shortcuts. Since the other cap C8 may take over the whole voltage, the C7 was nothing but a useless piece. Then, it started losing properties and finally died.

I wil replace it (with the right polarization) and keep on jamming...