CRATE BV amp Ch1 prob- "Crunchy" Noises.

[Jazz P Bass]

So it's a V2 thing.
One thought I had was is Q6 jfet working?
The bottom of C16 should measure zero ohms (well the jfet on resistance, anyway) to ground when Ch 1 is engaged.
100K when Ch 2 is on.

Edit Note: I have the logic backwards.

[g1]

JazzP, I think you mean Q4 not Q6 as you mentioned C16.
This can be tested by jumpering Q4/C16 junction to ground to see if the noise remains.

[Sea Chief]

So it's a V2 thing.
One thought I had was is Q6 jfet working?
The bottom of C16 should measure zero ohms (well the jfet on resistance, anyway) to ground when Ch 1 is engaged.
100K when Ch 2 is on.

JazzP, I think you mean Q4 not Q6 as you mentioned C16.
This can be tested by jumpering Q4/C16 junction to ground to see if the noise remains.

[Im sure Q4 was meant]

Done: tested at C16 neg side to gnd and got some real nasty hum on ch1 (measuered 94k ohms) and then a pop/switch to a near-quiet but some intermittant crunchy noise 'leaking in' on ch2 (0.01k ohms). hmmm.

[g1]

You will really have to differentiate between hum, various noises, and the "crunchy" noise you are trying to eliminate.
It is not clear from the above post whether it is still there or not. Without knowing this it is impossible for any of us to help.

[Sea Chief]

You will really have to differentiate between hum, various noises, and the "crunchy" noise you are trying to eliminate.
It is not clear from the above post whether it is still there or not. Without knowing this it is impossible for any of us to help.

Yes I understand. Very difficult to describe other than a hum noise either loud or 'normal', and the ubiquitous crunchy uinterferrance noise (loud/ normal or slight).

Ok Just did the gnd test at Jundction Q4/C16:

ch1 is very quiet indeed, but with occasonal pulsing bad hum.
ch2 is exactly the same.

(I just did ch1 before tho and -did-get the crunchy noise!? so something 'isnt stable'? in this vicinity at all it seems to me.. Q4??).

thanks alot chaps- you will be rewarded in paradise for this (& if not with 20 virgins, then defo the freckle-faced podgy one ~for an hour). SC

[Jazz P Bass]

On top of that I have the logic backwards.

[Enzo]

When we connect little wires and things to ground off the signal path, it is entirely possible, even likely that it will introduce new noises. When we do these tests ALL we care about it the effect it has or does not have on your crunchy noise. So if shorting something makes the amp hum, ignore the hum, just focus on that it does to the noise we are chasing, your crunchy noise.

[g1]

What really seems to be confounding things is the parallel path around V2b (C17 & R26).
I think it would help to disconnect it till the fault is found.
SC, could you disconnect one leg of C17 or R26.
And while you're in there, the socket for V2 should probably be resoldered.
After that, could you repeat the following tests:
1) jumper V2 pin2 to ground
2) jumper V2 pin7 to ground

[Jazz P Bass]

By grounding the bottom of C16 (bypassing Q4) you have no noise (other than hum)?
Does this provide suspicion that Q4 is bad?

[g1]

Ok Just did the gnd test at Jundction Q4/C16:

ch1 is very quiet indeed, but with occasonal pulsing bad hum.
ch2 is exactly the same.

When you say "ch.2 is exactly the same" do you mean the same as before, or same as Ch.1?
I think JazzP took it to mean the opposite of what I did.
Please try to be very precise with your answers so there is no room for interpretation.

[Sea Chief]

By grounding the bottom of C16 (bypassing Q4) you have no noise (other than hum)?
Does this provide suspicion that Q4 is bad?

Yes- this is what I found on my repeat test of this btm of C16 to gnd.

(Hum only, albeit pulsing quite loudly now and then.. but not my nasty crunchy noise).

[Sea Chief]

When you say "ch.2 is exactly the same" do you mean the same as before, or same as Ch.1?
I think JazzP took it to mean the opposite of what I did.
Please try to be very precise with your answers so there is no room for interpretation.

Sorry..

Both channels exactly the same g-one (C16 to gnd = crunchy noise awol.. but hum + occasional loud pulsing hum too).

I understand lingo a little confusing there- apologies! I will be as clear as I can.

[Jazz P Bass]

Take the jfet out.

[Sea Chief]

What really seems to be confounding things is the parallel path around V2b (C17 & R26).
I think it would help to disconnect it till the fault is found.
SC, could you disconnect one leg of C17 or R26.
And while you're in there, the socket for V2 should probably be resoldered.
After that, could you repeat the following tests:
1) jumper V2 pin2 to ground
2) jumper V2 pin7 to ground

V2 sockets resoldered a 2nd time (I did this 2 days ago in fact whilst doing a cap, carefully too).

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

Proper quiet too (both channels tested) best Ive had, just a very quiet 'on' hum, nr inaudible.. fwiw.

[Sea Chief]

Take the jfet out.

Replace Q4? (or take out for test purposes).

If so will a 'J176' (all the info I see on the schematic for these JFETs) do/ is it as uncomplicated to source the B'stard as this.. J176 TRANSISTOR x 1 pc | eBay

.. or do I have to find the precise Voltage/ or try and read the minute letters under the J176 ('A' I think it has after the 6?), which are so small I cant see.

thanks- Chief.