Dr. Z Mini - horrible static/crackling sound when tapped

[gui_tarzan]

It doesn't matter where you tap or how lightly, this thing makes all sorts of noise. I tightened everything down thinking it was a bad ground somewhere but I can even grab the chassis tight with my fingers and it does it. Even moving the wires around makes it happen, no matter which wire it is. Tapping on every component makes the crackling noise, every connection, everywhere. I don't see any iffy solder joints, I've cleaned and tightened the tube sockets, swapped tubes..

I admit it, I'm stumped.

[martin manning]

What about a microphonic, leaky cap? Bright or treble positions would be likely sources, especially if they are micas.

[gui_tarzan]

No micas, no ceramics either. It doesn't sound microphonic, it sounds like you've got a speaker or guitar cord part-way in, that nasty crackling sound. Even like I said, gripping the board hard results in a static-like hiss. There are only four non-filter/bias caps in this, there just aren't that many parts.

I hesitate to post a gut shot since Dr. Z is a member, even though he hasn't posted in a couple of years.

[martin manning]

That could be a leaky cap. What controls affect the noise? Can it be stopped by pulling a particular tube?

[gui_tarzan]

No crackle with either tube pulled, but with the EL84 only it's quiet so I'll check the pre-amp cap coming off the volume control. Turning the volume control doesn't change the crackle volume, it's loud all the time.

[martin manning]

So the problem is downstream of the volume control, and probably something connected to the power rail or a tube plate. Could be a plate resistor?

[gui_tarzan]

All the resistors checked out so I looked at the sockets and noticed the EL84 socket was not as tight as it should be. So I re-tightened everything there and touched up the connections, no more noise. Sweet!

[M Fowler]

Check that crimped on ground connector.

[martin manning]

Great!

[JMFahey]

You can ground different signal points along the way from input jack to speaker, to roughly find *where* it's generated, then look closely in and around the suspect spot.
Reducing the search area is good.

As a side note, the problem might be outside the chassis, such as a speaker with frayed tinsel wires, speaker connections and even power cable or poor outlet/extension cord.

You might even have *something else* plugged in the same power line sparking or something and getting into your amp.

Hope you find it soon.

[Stevem]

If it has a standby switch then jump it out for starters!
Pull the tubes out of it one by one untill it stops so you can better pin down if it's from a gain stage or the power output / supply section!

[gui_tarzan]

No standby switch, shoot there isn't even a tone control on it. As I said earlier, I did all of what you suggested but thanks! I've run it all morning and played it several times at low, medium and high volume and no strange noises.