CRATE BV amp Ch1 prob- "Crunchy" Noises.

[g1]

For testing purposes, just remove Q4 for now.

[Jazz P Bass]

Not too sure what is going down here but it appears to me that the bottom end of V2 A is suspect.
5 parts.

[Sea Chief]

For testing purposes, just remove Q4 for now.

Done: removed Q4. Am I replacing C17 leg back in?

[g1]

If you can leave C17 disconnected until the fault is 99.9% verified, that would be best.

[Jazz P Bass]

Done: removed Q4. Am I replacing C17 leg back in?

Q4 removed.
And the results are?????????

[g1]

R16 shorted: ch1 noise remains. R28 shroted: ch1 noise remains.

That test was done Sept.18. Shorting R16 grounds V2 pin2. Shorting R28 grounds V2 pin7.

Today you did basically the same test but got the opposite result:

Done: lifting C17 leg/ V2 pin2 to gnd: noise ceases.
& the lifting C17 leg/ V2 pin7 to gnd: noise ceases.

I'm assuming the noise disappears when the pins are grounded, but is there when they are not grounded? Or has the noise just disappeared now.
The first test (Sept.18) indicated the problem was after V2 pin2.
Todays results indicate the problem is before V2 pin2.
Only one set of results can be true.

[Sea Chief]

I know it may sound as if Im doing this badly, but I hve bee really thouragh with every test. I check twice each connection, nothing adjacent is getting involved, turn on and listen for a good few seconds, immediatly write results here.

As to why results might contradict- Im not sure. (the test just before/ C17 w'leg up/ 1st time ch1 noise, 2nd time -I redid it to doublecheck- it was quiet like ch2).

Ok where am I at right now: Ive got C17 leg up and Q4 removed. I have a new Q4 on its way. JazzP when you say 'and results are??' I not clear on what Im testing to answer this (ie just no shorts/ jumpers/ no tests at all, just turn ap on like it is?).

[Enzo]

We are connecting the circuit in a different manner to see what it does. Shorting across some part might reveal one thing, disconnecting a part might also reveal something. C17 and R26 form a local feedback circuit. it has a tonal affect, but is not necessary for the basic function of amplifying, so by just disconnecting one of those parts, that little circuit section disappears. It can no longer be involved with our noise. More importantly, it cannot sneak noise around the circuit through the back door, so to speak.


Those JFETs are acting as switches. Q4 basically turns C16 on and off. Turn on the JFET and C16 is in the circuit. That increases the stage gain, so that is the dive channel condition. If we simply remove Q4, then it cannot turn on and at that particular spot the amp is now stuck in the clean channel mode - the mode with the noise. So if Q4 was turning off for clean channel but not completely, so it was making noises, we can listen without Q4 and see if the noise remains or not.


So YES, fire it up without Q4 and see if the noise remains.

[Sea Chief]

Thanks for explanation Enzo- Im back on board with what Im doing now then. I was getting adrfit a bit (tho was trying to follow the pattern of tests & always look on schematic b4 I do s'thing to see the idea behind it 1st).

So YES, fire it up without Q4 and see if the noise remains.

Done this: noise ceases (lovely & quiet too).

[g1]

So, to be clear, the noise was still there and C17 is disconnected. Removing Q4 got rid of the noise and C17 is still disconnected.
This indicates Q4 is a prime suspect, but we want to make sure it is not C16, which is switched in and out by Q4.
With Q4 removed, put your jumper across R30. This will mimic switching C16 into the circuit, but without Q4.

[Sea Chief]

So, to be clear, the noise was still there and C17 is disconnected. Removing Q4 got rid of the noise and C17 is still disconnected.
This indicates Q4 is a prime suspect, but we want to make sure it is not C16, which is switched in and out by Q4.
With Q4 removed, put your jumper across R30. This will mimic switching C16 into the circuit, but without Q4.

Ok- understood! and done the jumper across R30: so with C17 leg still cocked, Q4 out: noise ceases (lovely quiet).

[Jazz P Bass]

That Jfet is most likely at fault.
Past tests, when C16 was jumpered produced odd noises. (this was with Q4 jfet in circuit.)
Mosfets & Jfets fail in odd, leaky junction ways.
The junctions are not like a bipolar transistor.

[Sea Chief]

Terrific. I know its not a dead cert, yet, but Im holding out hope on this dud Q4 then. Thanks hugely you guys.

Got one of those ^ ebay J176 coming. Now do different makes of J176 have possibly different leg denminations? (ie the drain/ emitter iirrc? on the schematic tho its D G S) and if I have to guess putting in (ie it might not have its D G S markings on) can I do damage if wrong way round?

[g1]

If it's a 176 it will have the same leg configuration.
If you can't wait (I don't think I could in your shoes), you could steal Q1 for the time being and install it in Q4 position.
Q1 is for the reverb circuit, so it will not work if you try this.

[Sea Chief]

I think I'll wait.. the tracks are so thin I didnt hesitate to snip that Q4's legs off b4 unsoldering, so Icould only remove Q1 like so. My knobly knees will be knocking resoldering in the new B'stard, such dericate tracks.. waited longtime so far so no major prob. Also I got its 'replacement' (prob no2) a vox ac15 today.. hey guess what? issues with that too (dont fret- cab only thankfk/ amp is good!). Goddammit. I told you lot not to use ebay for GTR amps, then wtf do I do again already??