Dangerous friend's amp...AC leaking through guitar...

[massygt6]

Hi,
a friend of mine brought me a D clone wich has a serious problem...it is DANGEROUS to play, 'cause it's leaking 105VAC through the input jack, and therefore, to the guitar and to the hands...
He bought this amp used so I don't know nothing about this amp, I've not opened it yet.
The typical situation is when you have your hands on the strings of the guitar and touch something in the room that is grounded...you get electrocutioned...I've measured about 105 VAC between the strings and some random ground points in the room, such as audio devices, other amp's chassis, etc etc
I'm seriously scared to check this amp, any suggestion where to start to look?
thanks...

[wicker]

There's probalby DC on chassis, strat from checking all points where elements are connected with chassis, maybe some resistor is down. Check all caps, that's what I would strat from. And do it with amp turned off.

[massygt6]

thanks for the answer but is not DC...its AC, measuerd and tested with a good multimeter...AC is very strange...maybe a bad ground in the power section before rectification?

[Gibsonman63]

It sounds to me that there is an issue with the chassis ground. Any voltage on the chassis should go to ground if the ground is properly connected... that's what it is there for!

I agree with the previous post. I would run through the power supply with a meter before powering up.

[massygt6]

What drives me mad it's that the amp seems to have all plastic jacks, from the outside they seems to be the type used by marshall, so if they're all plastics and the chassis can't transfer voltage on the input jack, where the hell is this AC picked up????

[Structo]

What kind of D clone is this amp?

[massygt6]

From the few things that has been told me, it's a build from scratch, is not a kit amp, not a ceriatone, not any known brand, simply an amp bought from a guy, built from various spareparts.
It should be a low plate version, can't say anything else 'till I will open up the amp.

[erwin_ve]

Is there a 1-N-2 switch connected? That's a anti hum cap connected to the primaries of the PT to ground. If the cap fails there is a half cycle wall voltage on the chassis and therefore also on the guitar(common ground!).

[massygt6]

Is there a 1-N-2 switch connected? That's a anti hum cap connected to the primaries of the PT to ground. If the cap fails there is a half cycle wall voltage on the chassis and therefore also on the guitar(common ground!).

good point, but no...no ground switch on the back! gosh!!!!

[erwin_ve]

Is there a 1-N-2 switch connected? That's a anti hum cap connected to the primaries of the PT to ground. If the cap fails there is a half cycle wall voltage on the chassis and therefore also on the guitar(common ground!).

good point, but no...no ground switch on the back! gosh!!!!

Have you been inside; check the primaries of the PT to ground.

[wicker]

If it's AC, then we're closer a little, check main switch, maybe there's problem, if not, I would look at everything between rectifier and transformer, starting from PT. This is classic fender PT ? Maybe wires are damaged.

[Gibsonman63]

If it's AC, then we're closer a little, check main switch, maybe there's problem, if not, I would look at everything between rectifier and transformer, starting from PT. This is classic fender PT ? Maybe wires are damaged.

Good point. One of the things that gets pointed out a lot when reviewing build pictures are grommets for transformer wiring. I would assume that the builder didn't stop building with an amp putting out dangerous voltage to the instrument. The damaged wiring concept would account for a previously working amp now misbehaving.

[massygt6]

all good suggestion, thanks...I will surely know more about it this evening when I will open up the amp...
thanks guys, I appreciate.

[wjdunham]

This won't solve your problem but may be a clue- but isn't the chassis ground supposed to sink any current such that the fuse will blow if a loose wire with some B+ on in touches the chassis? If a loose wire is barely touching the chassis, or a leaky switch (a high resistive connection maybe?) then there may not be enough current flowing to blow the fuse. Just thinking out loud here. The point being, if there is indeed a third wire chassis ground as there should be, and the fuse isn't blowing, then you've got a fairly subtle leakage somewhere.
Check to make sure the powder coat or anodizing is ground away very well where the power and standby switches ground to the chassis. Post a pic of the guts after you open it up, maybe someone can spot something for you.
Good luck,
Bill

[Guitarman18]

Might be worth checking to see if the mains plug is wired correctly.

Also check that the correct value fuse is in there. It may be that it kept blowing fuses, so it has been swapped out for the wrong value!


wjdunham wrote:

If a loose wire is barely touching the chassis, or a leaky switch (a high resistive connection maybe?) then there may not be enough current flowing to blow the fuse.