Metal chassis laptop (MACBOOK) and ground

[tepsamps]

Hi !
I recently bought a pair of speakers for computer with an amp inside. I bought from Creative Labs...
The MACBOOK is not grounded nore the speakers amp. Only a 2 pins plug.

The MACBOOK has got an aluminium chassis...

So when i touch the strings of my guitar plugged into my grounded guitar amp with one hand AND the chassis of the MACBOOK with the other hand, speakers plugged but even switched off, i get an electrical shock quite powerfull, anyway very annoying...

Where can it come from...How can i measure something with a voltmeter?

Thanks

[olddawg]

Does it do it when the Mac is running off of the battery?

[jantoman]

The original MacBook AC adapter comes with an isolated plastic wallmount plug AND with a grounded cable.

[tepsamps]

The original MacBook AC adapter comes with an isolated plastic wallmount plug AND with a grounded cable.
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I don't have this one. Mine is without the ground...On a PC i have, the mains plug is grounded to the transformer, but from the transformer to the computer, it's 2 wires only.

[tepsamps]

Does it do it when the Mac is running off of the battery?

I have to test, and so to maybe get an electrical shock :-) But i will try...

[jantoman]

Please post a picture of your adapter, maybe it's possible to modify it.
Anyway you can find a cheap compatible adapter on eBay. What's your MacBook identifier (Axxxx)?

[tepsamps]

I have to test, and so to maybe get an electrical shock :-) But i will try...

So it does not do it if the Mac is on battery only, no electric shock but the contact is not pleasant...

[Mick Bailey]

Take your DMM and measure the voltage between the Mac chassis and mains earth. Either an earthed water pipe or radiator. I think you'll see around 80v. That's what you're feeling. The source is high impedance and there's very little current flow. Some people feel it, others don't. It depends on your skin resistance.

Do not try to modify the PSU. The shock is most likely because of EMC suppression capacitors that link the power supply secondary to the primary. Otherwise the SMPS will radiate noise from the output lead.

The problem here is that it's difficult to discriminate between a normal or fault condition. I have seen situations where the shock is caused by a reverse-polariy mains socket. In the case of a Digitech harmonizer where the singer was getting a shock when coming into contact with the mic it was more complicated. The PSU had the suppression caps wired so that the mains plug had to have the correct polarity. Here in the UK all mains plugs have an earth pin as it operates shutters and if missing will not allow a plug to be inserted. The DIgitech PSU had a plastic dummy pin as it only needs a 2-prong connection, but its essential that the polarity is correct. The plastic pin had broken off and another band member figured out if he inserted the plug upside down he could open the shutter with a screwdriver to get it to plug in. Now there was a cap that should have been connected to neutral now connected to live, and the leakage current was giving a shock.

[tepsamps]

I have measured 3 VAC only, 0 VDC but i swear that the shock seems to be a lot more. I did not anderstand Either an earthed water pipe or radiator.

But this reading is the same if i unplug every thing from the Mac, so it's not what i feel, and my wife has the same shock, i've got dry hand, and she has not...
I'm thinking about putting some tape on the chassis and cover

[Mick Bailey]

The measurement needs to be taken between the chassis and the mains earth (ground) connection - I think this is called la conducteur de terre. In many countries the water supply to homes and the central heating pipework is earthed.

[Chuck H]

The measurement needs to be taken between the chassis and the mains earth (ground) connection - I think this is called la conducteur de terre. In many countries the water supply to homes and the central heating pipework is earthed.

And never trust that the ground hole on an AC socket is actually grounded. I've seen way too many DIY, homeowner, landlord and even creative professional repairs that render it moot as a safety ground. For those that still gig this is especially bad at dive bars and clubs.

[tepsamps]

The measurement needs to be taken between the chassis and the mains earth (ground) connection - I think this is called la conducteur de terre. In many countries the water supply to homes and the central heating pipework is earthed.

So i measured correctly...Ground is "la terre" you're right ! :-)

[Helmholtz]

The DMM may need to have high input impedance AND wide bandwidth because of the high switching frequencies of SMPS.

You may try to safety-ground your computer chassis using an extra wire connected to mains earth/ground.

[tepsamps]

The DMM may need to have high input impedance AND wide bandwidth because of the high switching frequencies of SMPS.

You may try to safety-ground your computer chassis using an extra wire connected to mains earth/ground.

The problem dos not come from the speakers but from the macbook when connected to the mains.

I changed the macsafe wire from the mains to the adaptor for a 3 pins outlet. It did not change any thing. Then i put the macbook to ground with an usb plug connected to a ground outlet. It blows my 30mA differential fuse on the electric board of the house!!! Only if the mac is connected to the mains.
So there is a big problem of safety, no ?
The mac is obsolete so no apple support, but i would like to fix it...

[Helmholtz]

The problem does not come from the speakers but from the macbook when connected to the mains.

So the culprit should be the macbook's power supply.

So there is a big problem of safety, no ?

From what you describe above this looks like a potential safety issue.