Gibson Les Paul Jr amp

[bsco]

Question.....I have this small Gibson amp..a Gibson Les Paul Jr.....made back in the late 50's or early 60's......it came in with a two wire non polarized power cord...which somebody had wired in.......the fuse holder was stuffed with tinfoil and had a 14A fuse installed....I fixed all that up and cleaned it up...it is in hard shape.... there is about 60VAC on the chassis when you measure from third prong in the AC outlet to the metal chassis when the unit is turned on....switch the power cord around and it reads about 30VAC......any thoughts on this??? there is not supposed to be any AC on the chassis.....I also connected up a proper 3 wire power cord but same thing....and a polarized two wire one as well......same thing.....
this does not have the safety or Death cap in there....any thoughts???

[Leo_Gnardo]

Possibility 1:The ground pin on the AC plug isn't firmly connected to the chassis. Because, if it was, you wouldn't be reading any voltage between chassis & ground. Do you read zero ohms between AC plug ground pin and chassis?

Possiblity B) The problem might not be the amp - a miswired AC receptacle where the ground isn't connected, or the connection to the building's ground stake is broken, a loose, corroded ground strap connection inside the AC service box, any of those could also be the problem.

Do you read any connection between the AC plug and chassis with your ohm meter? That fuse situation sounds like somebody may have tried to "cure" a failed power transformer by sticking in foil plus a huge fuse. Ya know, 'cause that always fixes things... There may be an unintended connection now between the PT primary and transformer frame.

[bsco]

Thank you Leo.....the ground pin of the AC plug is connected to the chassis..so that looks fine...it reads good......I will have to check my ground connection to the grounding rod in the basement....maybe that is the problem...I didn't think of that actually....The amp plays fine...I just don';t want to touch the guitar strings and another piece of gear and light up.....lol.....I also used a polarized AC cord as well....same thing....that fuse issue was something that struck me as being an issue...the 14A fuse was shorter that the regular standard fuse....1 and 1/4 ins...so they had tin foil jammed down inside the fuse holder so the other fuse would work...I have one of those outlet testers here...I'll check the outlets in the basement and see if they are ok....Thanks for the info.....
Cheers

[Mick Bailey]

Does your property have an earth leakage trip?

It strikes me that there cold be a problem with the earthing to your home. Because the earth impedance is low, any 'floating' voltage on the amp chassis should be negligible. If the current draw is high enough then any earth leakage trip should operate. Eliminate this possibility first.

I have an earth-leakage test box and all it is inside is a 1500 Ohm resistor and parallel cap that intercepts the earth connection. The ends of the resistor connect to a pair of terminal posts so that the voltage drop can be measured when the amp is off and when on. Ohms law gives the current leakage. Very useful and it can show up leakage in old rubber cables.

Another check I do is to wire the live and neutral mains plug pins together and clip an insulation tester between those pair and the earth pin. The amp needs to be in operating mode (but NOT connected to the mains). The reading should be very high resistance (50M Ohm) or open. I test at 250v and 500v (240v mains here). This checks whether there is leakage from the PT primary to ground, or whether something in the mains supply side is leaking, such as a charred fuse holder, bad mains switch insulation, bad cable etc.

[g1]

there is about 60VAC on the chassis when you measure from third prong in the AC outlet to the metal chassis when the unit is turned on

the ground pin of the AC plug is connected to the chassis..so that looks fine...it reads good

These statements can not both be true (with 3rd prong of plug connected to wall outlet).
Perhaps it reads good resistance wise but does not have continuity powered up.
If there is voltage between the 2 ends of the 'green' conductor, it must be open.

[Leo_Gnardo]

What g1 said ^^^. Might be a dodgy connection to the ground pin in the AC plug. I've had that happen both in moulded plugs and occasionally found the green screw in a replacement plug not tightened down. Or the wire broken inside the AC cable, usually within a couple inches of either end. Be careful, we don't want to see you light up either, unless it's your barbeque you're lighting up.

Anyway there's something hinky going on and I'm sure you'll find it. btw that 14A fuse is probably an SFE 1 inch fuse meant for automotive use. Oversize fuse and foil, I wonder the guy who put those there might be wearing both belt and suspenders. When he does something wrong, he goes all the way.

[Enzo]

Question arises, you measure 60vAC on the chassis, WITH RESPECT TO WHERE?

[bsco]

Question arises, you measure 60vAC on the chassis, WITH RESPECT TO WHERE?

with respect to the ground pin in my power bar.....

[bsco]

These statements can not both be true (with 3rd prong of plug connected to wall outlet).
Perhaps it reads good resistance wise but does not have continuity powered up.
If there is voltage between the 2 ends of the 'green' conductor, it must be open.

I will check this out later today and see what I can figure out.......as I have a couple of keyboards to finish up as well......I have an outlet tester so I'll check some of the outlets in the basement to see if anything is not right.......I will also remove the connection from the grounding rod and clean it up and then re-connect it.......but I will do that with the house power shut down......I will check the power cord and power bar that I am using as well......and will let you know what I find......or don't find..........won't be doing anything tomorrow as it is my wedding annaversary....14 GLORIOUS years??? !!!!.......my how time flies......

[bsco]

Does your property have an earth leakage trip?

It strikes me that there cold be a problem with the earthing to your home. Because the earth impedance is low, any 'floating' voltage on the amp chassis should be negligible. If the current draw is high enough then any earth leakage trip should operate. Eliminate this possibility first.

I have an earth-leakage test box and all it is inside is a 1500 Ohm resistor and parallel cap that intercepts the earth connection. The ends of the resistor connect to a pair of terminal posts so that the voltage drop can be measured when the amp is off and when on. Ohms law gives the current leakage. Very useful and it can show up leakage in old rubber cables.

Another check I do is to wire the live and neutral mains plug pins together and clip an insulation tester between those pair and the earth pin. The amp needs to be in operating mode (but NOT connected to the mains). The reading should be very high resistance (50M Ohm) or open. I test at 250v and 500v (240v mains here). This checks whether there is leakage from the PT primary to ground, or whether something in the mains supply side is leaking, such as a charred fuse holder, bad mains switch insulation, bad cable etc.

What is the value of you cap and wattage of your resistor?? I do not have an insulation tester but I can make up the Resistor/Cap network.....

[bsco]

I just checked the power bars which I use on the bench... a couple of them has loose connector fingers inside...when I put the outlet tester in these and wiggle it slightly, the lights on the tester flicker and show up an open ground when the tester is moved while being plugged into the power bar...I checked a few of them and I have a couple which has this problem....the rest of the power bars test fine.....and these are the two which I use on the the side of my bench the most....so I think this might be the problem........I will still clean up that ground rod connection as well........I'll let you know how I make out.......Thanks to all for the advice and comments......
Cheers

[Leo_Gnardo]

I just checked the power bars which I use on the bench... a couple of them has loose connector fingers inside...when I put the outlet tester in these and wiggle it slightly, the lights on the tester flicker and show up an open ground when the tester is moved while being plugged into the power bar.

Sometimes I've been able to fix this by prying open the power bar and soldering the connectors to their bus wires.

[bsco]

Sometimes I've been able to fix this by prying open the power bar and soldering the connectors to their bus wires.

I am going to clean up that ground rod connection first...then I am going to hook-up another new power bar that I have here just to see what happens.. ...maybe that is the problem.......I'll let you know......
Cheers

[Mick Bailey]

The leakage tester uses a 1500 Ohm 1W resistor paralleled with a 400v 0.15uf polyester cap. Set your meter to VAC.

It's surprising what high leakage there sometimes is with new equipment, and how low it can be on old stuff. Usually on 240v leakage is in microamps. Some manufacturers state as much as 5mA for leakage. At that level it doesn't take too many pieces of equipment to add up to a nuisance trip current if they're connected to the same supply.

[bsco]

Ok Mick..Thank you very much....
Cheers