Old Peavey KB 300 Farts w/compressor indicator

[groovyfingers]

Unwanted DC offset of 3.42v on pin 1 of U5, the second op amp from the end of the preamp board with the cable connector from the power amp board. My schematic calls it U5. The layout that comes with the schematic shows U5 pin 1 is common with a CR13 and CR14 4148 diode, and a R65 4.7K resistor. Problem lies here?

The other op amps read very low on pins 1 and 7, the highest reading -.071v, while the rest read lower, most in the .001 range.

I checked between the tip and end of the plug on a cord plugged into the preamp out and power amp in like you asked. No DC voltage detected, read .001-.000 on preamp, and .002 on power amp.

[groovyfingers]

Here's the U5 schematic view of CR13, CR14, and R65:

[Jazz P Bass]

Measure the input pins of u5.

There should not be any voltage there.

The odd pin 1 voltage may be coming from the transistors if one is leaking.

You may have to remove them to prove it.

[Enzo]

But U5 only drives the reverb pan, it should have no effect on the preamp signal otherwise. And is the reverb pan connected as you take these readings?

OK so no offsets on U6, try a different IC in U6 socket just for science.

[groovyfingers]

No the reverb pan is not connected. I will swap another op amp from another spot. For U6 that is.

[Enzo]

COnnect the reverb pan and see if that U5 offset disappears.

[groovyfingers]

Swapped U5 with U6 and still sounds the same, in all three inputs.

[groovyfingers]

Hooked up reverb pan and offset is gone. A non-problem.

Tried pulling U2 on power amp board, still sounds bad with U2 not installed. This is the one that handles the compression.

Do these op amps have a way to clock them? I mean where is pin 1? Can you be clocked 180 out and it won't matter?

What causes the compression light to light if U2 is not installed? How does it know it needs to signal compression happening, when the device to do that is not there?

Swapped U6 with another op amp from the preamp board, still the same. If low volulme and compressor does not engage, sounds good. Any increase in voulme and the amber compressor light is on and the good sound leaves. Ugly compressed sound of a bass guitar. Buzz-saw like.

[Enzo]

Won't matter??? Like it won;t matter if you hook up your car battery backwards?

ICs always have a way to identify pin 1. Some of them have a notch out of the end where pin 1 is. The notch is centered, but we know with the notch up, pin 1 is upper left. Others have a small round dot or dimple right next to pin 1, the dimple is right in the corner. it is HIGHLY important to put it in right, otherwise the power supplies are reversed, and the IC quickly is destroyed.

U2 DOES the compressing, it does not cause it to be done. If you follow. other parts of the circuit tell U2 when to act.

Look at the power amp section of your schematic. The upper left area of it. Three of the four op amps in U1 do this. U1a does the detecting, U1b then turns the error signal into a DC voltage, which it then feeds to U1c which drives the LED. Clearly it cal still drive the LED even with U2 removed. U1b also drives transistor Q1 which in turn controls U2. With no U2, there is nothign for Q1 to control.