Hello, I would like to get some help from you guys.

Just opened up a case for an amp i'm trying to troubleshoot.

The power rails are registering +9.6v and -8.3v

I have several questions:

1) the voltage difference of 1.3v between vcc and -vcc could be causing the trouble with the noise and hum by affecting an opamp using it like a NE5532?

2) before rebuilding the power supply for this mixer and preamp, I want to understand which part can cause the voltage difference?
a) the coil has a central tap wrong?
b) the rectifier (diode bridge) is fried or cheap quality?
c) the capacitors (3300uF 25v) are damaged and should be replaced?

3) should I throw away all the psu parts and build a rectifier from a new coil, new rectifier bridge and new caps?

thank you for your help.

 

Give us something to work with; post a schematic.

 

Give us something to work with; post a schematic.

this is the circuit diagram for the basic psu im troubleshooting.

 

this is the circuit diagram for the basic psu im troubleshooting.

Where is the bad 3300uF cap? Which diodes are bad?

Do you have an oscilloscope? Troubleshooting hum problems without one is a guessing game.

 

Where is the bad 3300uF cap? Which diodes are bad?

Do you have an oscilloscope? Troubleshooting hum problems without one is a guessing game.

Hello, thank you for your reply. I dont want to know what casues a hum.
I want to know like the post title:

What causes +VCC=9.6v and -VCC= -8.3V in my PSU?

a) the coil has a central tap wrong?
b) the rectifier (diode bridge) is fried or cheap quality?
c) the capacitors (3300uF 25v) are damaged and should be replaced?

thank you

 

I assume a junction tie point with ground at C1 and C2 and transformer center tap? I would start at the beginning and measure from transformer center tap to each AC side of the transformer, less the bridge in circuit. Then measure the voltage drops across each diode, they should be identical.

Ron

 

a) the coil has a central tap wrong?

Unlikely.

b) the rectifier (diode bridge) is fried or cheap quality?

Possible.

c) the capacitors (3300uF 25v) are damaged and should be replaced?

Possible.

It could also be that neither supply was 9V; can't tell because you don't show the regulators. The imbalance could also be the design; unbalanced load on the supplies. Or something is drawing excessive current.

BTW, that schematic you posted is crap. You don't get 9V of filtered DC from 9VAC secondaries. The "hump" methodology of indicating no connects is used inconsistently, so it would appear that the negative output from the bridge rectifier is shorted to ground...

 

but you understood it right?

 

but you understood it right?

Yes. That's why I was able to point out the issues. Wouldn't have bothered reading it if it was more complex.

 

this got solved.

I tested the AC terminals with the tap and the voltage was stable +10 and -10.
So I just replace the diode bridge and the caps into a new board, and the voltage is perfectly balanced.

thank you all