ground noise issue PROBLEM

[rockstah]

maybe you guys can give me some food for thought as i am stumped on this one.

its a 100w 2203 spec amp

with the amp on it hums like aground issue. with nothing plugged in if i move my hand close to the input jack it hums louder, if while i'm doing that i touch the chassis the noise goes away. why would an amp hum like this? its picking up noise when you put your hand close to the jack? huh?

i redid the input jacks and redid both shielded wires. into the first stage and back from the volume into the second stage.

i have never had this problem and have built many 100w amps.

any food for thought?

[Gibsonman63]

It should be a grounding jack. Is it possible that the contact that grounds it isn't making contact when you unplug the cable?

[rockstah]

i just replaced both jacks. same issue. with a cord plugged in i have continuity to ground from sleeve to the chassis.

[Firestorm]

i just replaced both jacks. same issue. with a cord plugged in i have continuity to ground from sleeve to the chassis.

With nothing plugged in, do you have continuity to ground from the first preamp grid?

Does the amp hum only with nothing plugged in ? Or all the time?

[rockstah]

i just replaced both jacks. same issue. with a cord plugged in i have continuity to ground from sleeve to the chassis.

With nothing plugged in, do you have continuity to ground from the first preamp grid?

Does the amp hum only with nothing plugged in ? Or all the time?

touching the grid of v1b, cascaded gain stage, on v1b, input, i have 67.5k to ground.

on the second stage, v1a, touching grid with a meter with the pot on 10 i have 825k. on zero i have continuity to ground.

[Firestorm]

On the second stage, with volume on 10 you should have about 320K from the grid to ground (parallel 1M pot and 470K resistor). Check the function of the grounding tab on the low-sensitivity jack.

[philo43]

The tip of your input jack should read ground with no plug inserted. If it doesn't, there's your problem. Does the hum go away when you plug in a guitar?

[rockstah]

this is a total bozo thing i know... ( why didn't you try the amp without the variac?)

so in my trouble shooting i decided to check the old cord in these 69 to see if the ground pin had continuity. it did. turns out my brand new variac is defective in that the ground pin in the plug inlet did not have a connection. so i tried another variac and vola, problem resolved!

thanks for all the help!

Mark

[Structo]

I hate when that happens!

I thought I had blown an output transformer once.
Couldn't get a thing out of it.
On a whim I checked the speaker cable, bad.

[Drumslinger]

Yes, it can be frustrating at times.

[philo43]

I spent a whole day tracking down a bad hum problem before noticing the fine print on the guitar cable I was using..."speaker use only". I now store them separately from my guitar cables.