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Old February 22nd, 2012, 07:47 AM   #1 (permalink)
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Help - broke my Normal 1 input on my 5E3.

I was in my new Cox 5E3 for the first time just to loosen up the holding nuts so that I could jig the chassis a little bit straighter. It was crooked maybe from shipping. It had been working 100% (and sounded great too), but something about using a very small socket wrench to loosen the nut on the right side, I must have bumped the normal 1 plug. I thought I was being pretty light handed, and nothing LOOKS buggered - everything looks as it should be. But when I plug into Normal 1 it's just a big bassey loud hum with very little or no guitar signal coming through. I can get it to stop and play good but pushing the input cable jack this way and that, so it must be a bad connection somewhere. The cable is a few days old and is fine in all other inputs. Volume control is still responsive with the loud hum. Did I short an input to ground somehow?
...

Well, I feel stupid enough having trying to fix a cosmetic problem and creating a big audio problem. If anyone has some good advice of things to check I'd appreciate it. I have a couple of biggish gigs coming up soon and I can't fully tear into the amp and risk not having it useable.
Thanks guys. This is the best forum ever, and one of these days I'm going to start returning favors.

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Old February 22nd, 2012, 08:01 AM   #2 (permalink)
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Some shots. Everything looks fine. What am I missing? What in the world did I do?
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Old February 22nd, 2012, 12:22 PM   #3 (permalink)
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Use bright #1 until you can replace the jack. (I'd replace them with switchcraft jacks)

Below the mounting screw (to the right in that pic)... on the circuit board is the cathode resistor lead touching the input grid resistors?

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Old February 22nd, 2012, 12:27 PM   #4 (permalink)
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The hum mean that is not making contact to the sleeve of the input jack.

shorting input to ground mutes the signal and is a way some amps standby works
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Old February 22nd, 2012, 04:23 PM   #5 (permalink)
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Can anyone confirm that based on the common cliff jack picture here that the upper left is the 'switch' lug, lower left is the 'tip', the two lugs on the right are both ground/sleeve?

That'll help me do some volt meter tests tonight.
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Old February 22nd, 2012, 04:25 PM   #6 (permalink)
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The hum mean that is not making contact to the sleeve of the input jack.

shorting input to ground mutes the signal and is a way some amps standby works
Yeah. So maybe test cable ground (from other end) to amp ground and look for something higher than ~0?
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Old February 22nd, 2012, 04:26 PM   #7 (permalink)
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Quote:
Originally Posted by teleamp View Post
Use bright #1 until you can replace the jack. (I'd replace them with switchcraft jacks)

Below the mounting screw (to the right in that pic)... on the circuit board is the cathode resistor lead touching the input grid resistors?

That's a good eye. I'll check that out, but I think that would affect both hi and lo, where as here the Normal/Lo position works 100%.
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Old February 22nd, 2012, 04:51 PM   #8 (permalink)
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Quote:
Originally Posted by nicer View Post
Can anyone confirm that based on the common cliff jack picture here that the upper left is the 'switch' lug, lower left is the 'tip', the two lugs on the right are both ground/sleeve?

That'll help me do some volt meter tests tonight.
Based on that pic, I think your right.

I'm trying to figure out what you tweaked to make the ground have an open. You might try reflowing the ground wire connection on normal 1 jack (bottom right lug)... that's my best guess.

Your right about my other observation, it is probably an optical illusion and would mute the amp if they were touching.

If it is the same hum as when you touch the loose end of a cable thats plugged in, then reflow that one joint. If it's not the ground connection, the jack is probably broken.
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Old February 22nd, 2012, 10:33 PM   #9 (permalink)
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Hey! Good news. Got it fixed.

I tested resistance to ground on a plugged in cable to the chassis, and sure enough it was open when plugged into Normal Hi.

So the first 'tongue' that slides on the cable plug didn't have ground when the cable plug lifts it off the switch.

So I followed the tongue around to the lower right lug as in the drawing.

The ground connection through the middle had a lot of flux around the resistor lead going through it. I just squeezed a solder iron in there and gave it another shot of solder.

You had it dead right teleamp. Thanks.

Worked like a charm. Must have been a weak joint I bumped loose.
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Old February 22nd, 2012, 10:35 PM   #10 (permalink)
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As an upside now I understand how the hi/low channel works. Hi get 68k || 68k in parallel, Lo gets 68k straight, before getting to the tube. When both are plugged in both get 68k. Leo was clever, no doubt about that. Now if I only I could figure out what that 1Meg to ground is doing there.
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