#124 started

[martin manning]

If you mean when you ground the 0.02uF PI cap (same as grounding the return jack tip lug) makes it go quiet, I think that's it then. The jacks are just not providing a solid ground. Removing the anodizing AND using a star washer is best for anything that has to maintain a ground connection.

[larsvictor]

Marcus,

grounding the PI entrance (connection blue wire to 0.02uF Cap) cancelled the hum, right ?
Grounding lug 2 of the MV potentiometer (or just turning it all the way down) doesn´t have an effect on the hum.
IMO the source of the hum must be between those parts.

ground the tip of the send jack - that should have no effect.
ground the tip of the return jack - if that cancelles the hum the switching contact is bad.

Edit: Martin was faster

[martin manning]

Quite right Lars, let's not forget about that! See if a patch cable from send to return kills the hum.

[larsvictor]

Confusing...

but not like when I ground out the PI 0.1uf node which kills it stone dead.

Really that one ???
Martin asked you to ground at the PI entrance cap (0.02uF)

[norburybrook]

patch cable makes no difference to hum



grounding either node kills the hum completely.


Marcus

[martin manning]

But grounding the MV wiper does not stop the hum?

[norburybrook]

no.

M

[larsvictor]

grounding either node kills the hum completely.

you mean at the PI caps ? or the jack tips ?

[norburybrook]

PI caps, I'm grounding at the 1M Resistor node..is that the correct end?



edit. I think martin wanted me to check the other end.....

grounding the jack end of the 0.02uf has no effect on the hum

[larsvictor]

Nope. The other terminal of the cap.

[norburybrook]

see above

[martin manning]

Ok, lets get on the same page. Which is the first point where the hum is not stopped?

[larsvictor]

Ok, then the issue is somewhere in the PI circuit (you mentioned that pulling V3 makes the amp quiet).

Is the NFB reconnected ?

Can you measure the V3 voltages again:

pin 1 and 6
pin 3 and 8,
junction of the 4 resistors,
junction of 24k and 390R/4k7

What happens if you ground the 0,1 PI cap on the 390R terminal ?

[larsvictor]

Martin,

I´m afraid at all of the points 1-7...
Marcus grounded the grid (pin7) of the PI.

Lars

[larsvictor]

Just to clarify that we´re all looking at the same schematic.