OT protection diodes SR-2873?

[Enzo]

Look directly between the right side ends of the two cathodes on the horizontal ground line that passes between them. See the black dot for a junction with nothing there? That is for the vertical line yours is missing. Sometimes lines get obscured at some magnifications. Try enlarging the view to see if the line appears. If I open the file full screen, that line is invisible. It doesn't pop into view until I get to 150% size on screen.

[gui_tarzan]

Yeah, I see that. For whatever reason, the schematic I got originally doesn't show the line no matter how big I make it so I'll just replace it with the one Jazz uploaded.

[gui_tarzan]

Whoa! With the Q9 out I have lots of volume. So I guess it's time for a new one, eh?

All I need now is the part number for the cable that goes between P900/P901 and P1000/P1001. I can't find it on the parts list.

[Jazz P Bass]

As always, call Peavey.

[Enzo]

Or make one, it is just a little cable. Tack solder wires to the connector pins even.

[gui_tarzan]

I could do that, I'd like to find the proper cable though. If I can't get a number from Peavey I'll do that. Who'd have thought a little transistor could cause this much grief? lol

[Enzo]

yeah? Toss one into your hiking boot before a days walk...

[g1]

One last question, you said:

If I plug my guitar into the amp, then take the "send" into another amp I get sound. Really bad sound, but sound nonetheless.

Has this changed? And if so, can anyone explain how the FET could have had anything to do with this?

[gui_tarzan]

I'll do that tomorrow. The FET is used as a switch that is held closed by 15v to complete the bypass when nothing is plugged in according to Peavey. I don't know what else it does, I'm sure Enzo does.

[g1]

Yes, it's a muting Fet. But it should not have affected the sound at the preamp out, even in it's failed state, as R125 isolates Q9 from the loop jacks. Near as I can tell anyway.

[Enzo]

Maybe.

Look at the schematic. Though it is drawn in a different place, Q9 is directly paralleled with Q5. So one bad JFET can affect two circuits. Q9 was causing trouble, and clipping it out seems to solve that problem. But it could be that Q9 was OK as a part, but was being made to act improperly by Q5. I often mention that I change them both whenever I have one of them bad. Just to be sure.

Both of them are simple mutes, they are not in the signal path, they just ground it off. Whenever you switch channels, a pulse is sent to one of those little triacs, TR1, TR2. And they briefly trigger the two JFETs ON for a moment to ground off the signal path. That prevents loud noises as channels switch. Normally +15v from R106 holds them both OFF. Now if one fails, like a gate short to source, that will ground off the +15 for both JFETs.

So in that way I suppose we could have Q9 removed but the preamp still affected by a bad Q5.

[g1]

That's kind of what I thought too, but he measured 14.9V at Q9 gate, so I figured it wasn't affecting the gate voltage line.
Maybe it was just a testing glitch.

[gui_tarzan]

Just spoke with Peavey, they don't have the cable anymore so I'll just hard-wire it which they also suggested. I'm going to replace both Q5 & Q9 and test the other parts around Q5 just to see if anything else is wonky.

I appreciate all the help guys, once I get this far I'll let you know if anything else is amiss.

[Enzo]

You can also wire your cable and then test the amp with Q5 and Q9 removed. They are there to kill POTENTIAL noises when channels switch. But not every amp will make such noises. Yours might not. If it works OK without the two transistors, you might just leave them out and be on your way.

[gui_tarzan]

FIXED! Wow. I have a lot better understanding of how these pieces work together now, once again I can't thank you guys enough.

I made the cable and I pulled Q5 out as well and it has full volume again. I can't turn on any of the effects, does it need the foot switch to control them? The only effect that works is the echo. I can change the effects with the push switch on the front but none of them work and neither does the effects volume but you can see the lights working.