Peavey 400BJ

[Jazz P Bass]

-15mv's on the output?

I see nothing wrong with that.

How much current is it pulling from the mains at idle?
Does the amp pass a sine wave?

Last I recall, all that was wrong was a little hum.

[lowell]

I only have ~1.2v across the speaker jack. Not sure where you saw that number. So here we go:

Nickb!! Thank you! Upon checking these, I notice pins 6/7 were reading dead short. I removed the op-amp from its socket and still same result, but open on the socket pins. Replaced the 4558 and main rails are +/- 51.8v. Spot on symmetrically!

Voltages before fixing this were:

Pins
4 -15v
8 +15v
5 1.55v
6 .12v
7 .12v

Post fix:

5 0v
6 0v
7 .18v

So, U1A, receives negative feedback on pin5 from the PA. I'm assuming this is what was causing the offset. Can you please explain in more detail how this was so? My guess is that the base voltage of Q100 was pulled down thru U1 Pin6 resistor R115.

Finally, where does signal enter the drivers' bases? Can't figure that out?

[Jazz P Bass]

From post #29: "Also of note is that there are -15mv on the output PNP emitters."

Not sure what you are asking about "where does signal enter the drivers' bases?"

It enters at the base pins?

The signal originates at U1 pin 7.

[lowell]

Yes 15mv. Not 15v. Vernacular.

Yes curious how signal gets to the driver bases from U1 pin 7. I'd think the diodes after U1 would clip the positive going cycle of the waveform. Doesn't the positive side of the PA want to receive positive going signal?

[nickb]

I only have ~1.2v across the speaker jack. Not sure where you saw that number. So here we go:

I inferred the 15V as in an earlier measurement one supply was 15V different from the other wrt chassis. Assuming the supplies are actually the same then that implies 15V across the speaker. Since you measure only 1.2V then what do the supply rails actually measure wrt the supply center tap?

Nickb!! Thank you! Upon checking these, I notice pins 6/7 were reading dead short. I removed the op-amp from its socket and still same result, but open on the socket pins. Replaced the 4558 and main rails are +/- 51.8v. Spot on symmetrically!

Voltages before fixing this were:

Pins
4 -15v
8 +15v
5 1.55v
6 .12v
7 .12v

Post fix:

5 0v
6 0v
7 .18v

So, U1A, receives negative feedback on pin5 from the PA. I'm assuming this is what was causing the offset. Can you please explain in more detail how this was so? My guess is that the base voltage of Q100 was pulled down thru U1 Pin6 resistor R115.

Finally, where does signal enter the drivers' bases? Can't figure that out?

Yay!

If the opamp was busted then the feedback was busted. There's not much more to say.

The opamp in a sense doesn't drive the bases at all. It helps to think in terms of currents. The drive current comes from R101 thru R137. When, for example, the opamp output goes lower, it 'steals' current from Q100's base so the output will also go lower.