CRATE BV amp Ch1 prob- "Crunchy" Noises.

[Jazz P Bass]

Change out V2 tube with another one.

[Sea Chief]

Done that JazzP- same.

[g1]

Try grounding pin 7 of V2 (or short across R28 or C66).

[Sea Chief]

Shorted across R28/ did that g-one: same.

[g1]

I'll assume when you say "same", you mean the noise is still there. So the problem would seem to be after V2 pin7 but before the master.
Try connecting C20 to ground, for now it doesn't matter which side of C20. If the noise stops I'll be suspecting R33.

[Sea Chief]

Yes same= noise still there (both channels then.. gawd what an ass I am. Im not worthy etc etc).

Ok will do the C20. After I see what its where on the thingy.

[Sea Chief]

Done C20 to ground (@R35 side): same.

[g1]

Short from R37 to ground (Q5 side of R37).

[Sea Chief]

Short from R37 to ground (Q5 side of R37).

Done- noise still there.

[g1]

Just to be sure, double check at R38, short MV side to ground.

[Sea Chief]

Just to be sure, double check at R38, short MV side to ground.

Done again- noise gone/ quiet (bar a hum which I guess sort of a 'normal hum').

[Jazz P Bass]

I am wondering if Q6 jfet is bad.
What is that ABC designator on the gate?

SC, can you see if there is any voltage on the drain of Q6?
Where it connects to the MV pot.
Ch1 & then Ch2 engaged.

[g1]

When he shorted the top of the master/R38 to ground the noise quit. Wouldn't this rule out Q6?
And shorting Q5 out did not kill the noise so I think that rules out Q5 also?

EDIT: I see your point, if the master was up, Q6 could still be noisy and grounding the top of the master would kill the noise.
SeaChief, could you ground the bottom of the master where it meets Q6, then turn up the master, noise still there?

[Enzo]

And one other obscure possibility:

All those JFETs must be turned OFF with a gate voltage to be in clean channel. What if the gate voltage is noisy? We can ground off any one element and the rest are still there to inject noise. yes, far fetched, but I'd be scoping that control line or listening to it with a tracer, just to rule it out.


In fact, all those controls lines are driven by Q10 up top of page. To get to the 18v on the gate that TP12 asks for it has to be off, which means its base is pulled low against pullup R42. And it does this through the shunt contact on the footswitch jack and the channel panel switch. Certainly no rare bird a dirty FS contact. For channel 2, all they have to do is open, so that would eliminate noise on the other channel. By shorting across D7 or by grounding R52 we establish channel 1 without the switch or jack. If the noise stays, this theory is not valid. If it kills the noise, we are hot on it.

[Sea Chief]

SeaChief, could you ground the bottom of the master where it meets Q6, then turn up the master, noise still there?

Done this: noise still there.

Much appreciated all your help chaps.