SOLVED: Crack Troubleshooting - Picking up external noise

[dheffer]

Hello!

A couple of weeks ago I assembled the stock Crack.  Everything worked great - sounded perfect, voltages and impedance in range.

Just put in the speedball upgrade.  Sounds fantastic - again, everything in range. 

The problem is that now I am picking up on some outside noise, meaning that if I gently tap my finger on the metal chassis, the volume knob, the headphone plug, the tubes, etc. I can hear a very loud "pop" tap through the headphones.  EDIT: Same if I plug in/out headphones.  Again, if I don't touch anything, it sounds perfect. 

I would assume that it is some sort of a grounding issue flowing through/to the headphone jack.  I've tried re-soldering about anything I could reach, but it hasn't made a difference.  Noise exhibited whether a source is plugged in or not.  I've also tried multiple sets of headphones.

Anyone have any advice on how to troubleshoot, or where to focus my re-soldering?  Any chance it's one of the tubes?  Happy to post any pictures that might be helpful, or my any of my readings (although they are in the listed ranges/tolerances).

Thanks in advance!
Daniel

[Chris65]

Best thing to do - don't tap on the amp!

[dheffer]

Best thing to do - don't tap on the amp!

I didn't really notice how bad it was until someone touched the input tube while I was listening... my ears were ringing after that!   

I should add that plugging in/out the headphones results in a ton of noise through the headphones as well.  I like to turn the amp on, then plug in the headphones, but the noise is worrisome.  Also not sure if its damaging the headphones themselves.

[Doc B.]

You could try different tubes to see if you can find one that is less microphonic.

[dheffer]

You could try different tubes to see if you can find one that is less microphonic.

Thanks, wasn't sure if the stock tubes could be the issue or not.  I knew that some external noise could come through, but had no idea it could be this much, especially just from something simple as plugging in a pair of headphones.

Is it just as likely to be the 12AU7 as the 6080?

[Doc B.]

Why don't you tap on each tube? That would be a good way to determine which one is microphonic.

[dheffer]

Noticed the sticky on grounding (http://bottlehead.com/smf/index.php?topic=4812.0) and saw that the center pin of the 9-pin socket (12AU7 tube) had a reading of 1ohm.  Re-soldered and it is now correctly reading 0ohm.  Fired it back up, and I no longer get the horrible feedback when I plug the headphones in and out.  Still have the tapping sound when touching the chassis, but its much better.

Ran through more resistance checks (and voltage checks), and I'm now reading 0ohm at both RCA inputs, and also where the white/red wires enter into the potentiometer (bottom and top connectors on the far right).  These previously were ~95K ohms each.  The black (ground) correctly checks in at 0ohm.

I've tried re-soldering these four points (two rca's, two on potentiometer) but neither channel has been fixed.  My questions is - how can I get this fixed?  What is the source of the resistance to the L and R channels?  Is it normal for the amp to sound normal without resistance?  Sorry if these are dumb questions 

Thanks for your help so far!

[fullheadofnothing]

0Ω on a jack is usually caused by solder dripping down from the center pin of the jack and bridging the center to the ground.

[Doc B.]

It's playing music and the inputs measure 0 ohms? That's not possible, because for the inputs to measure 0 ohms they would have to be shorted out somehow. So there is an error in the measurement, unless you are not able to play music.

[dheffer]

0Ω on a jack is usually caused by solder dripping down from the center pin of the jack and bridging the center to the ground.

This is it - bingo!  Trying to figure out how to get the small amount of solder left out of there.  I think it was fine before, but in trying to solve my problem before, I likely re-heated the solder and it dripped down.  Might just have to find a replacement.

[dheffer]

OK - got the RCA jacks cleaned up, everything checks out.  Sounds great and I don't have any horrible loud noises when plugging in/out the headphones.

Still getting a good bit of external noise picked up (even from tapping the table), but I'm fairly certain it's just from the 6080 tube.  Tapping the 12AU7 directly is silent.  I'll look at getting a replacement to see if that helps, but at least now I can barely hear it when the music is playing, and even then I'm not getting the ear piercing electrical pitch.  Overall, I'm happy to have everything pretty much perfect 

Thanks everyone for your help today - learned a lot in trouble shooting.