Amplifier Op Amp

[jheatac]

I am looking at a car audio amplifier.

I seem to have all the appropriate power rails but am not getting an audio signal through the unit.

In looking at U5 (U5 on board) (U5B on schematic) I get a signal to it, but not from it. I have +12 and -12. R80, R187 and R188 seem close enough to spec.

My scope is acting up so I am sending a 1khz signal in and following it through with an audio tracer. The input side of R18 stays at 1khz. When power is applied, the U5 side of R18 sounds like a capacitor is charging as the unit comes on and then the signal is lost.

Could it be a faulty capacitor around U5? Is U5 faulty? or is something down the line from "radio" back feeding or loading the circuit?

If it were through hole, I would remove and check the capacitors. I am less adept with surface mount.

[Circlotron]

You won't measure any reasonable amount of signal at pin 6 of the opamp because it is a virtual ground point. Whenever the signal from R18 goes positive for example, the feedback signal from R80 goes negative by just enough to *almost* completely counteract R18 signal. The entire open loop gain of the opamp gets into action to make this happen. Because the open loop gain is very very high, the size of the signal at pin 6 is extremely small. So if your opamp has an open loop gain of 1 million and has an output of 1V rms then pin 6 will have a signal of 1uV.

[magic]

Check if the ±12V power supplies to the chip are OK.
Check if the DC on both pins 5,6 is GND.
Check if there is no short from pin 7 to pin 6.
Check if the "radio" node isn't shorted.
Check continuity of solder joints.

[Audiorepair]

If U5 were faulty it would almost certainly have DC on its output pin 7.
You should be able to easily measure this with a mutllimeter.

This is assuming it has the correct power supplies on pins 4 & 8.