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Topic: Hot Chassis Oscilloscope - need help (Read 2548 times) previous topic - next topic
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Hot Chassis Oscilloscope - need help

Howdy
I'm hoping someone here has some electronic knowledge and can help me with an oscilloscope I just got shocked by.

When I measure the voltage coming off the interior chassis, I get about 6 volts AC. If I plug the power cord in the other way, I get about 25 volts AC.

I understand that the first step is to replace the power cord with one that cannot be plugged in to allow 25vac on the chassis.
What next?

What else can I do to make this scope less 'live'? With the external case on, it is relatively safe, but certain screws still produce that 5 volts and I wonder if I shouldn't try to prevent that.

Thank yall

ps. If anyone knows another more general-electronics or vintage equipment forum I could post to, feel free to share.
pps. I'm not sure if this qualifies as 'hot chassis'...but the chassis is hot with 5vac so I am assuming that is correct. Obviously I have some learning to do.

Re: Hot Chassis Oscilloscope - need help

Reply #1 –
This is caused by capacitive coupling between the primary windings of the power transformer and its core. This should be a relatively small current and should be conducted through the ground wire of the power cord to the ground of the power outlet. If you are able to plug it in both ways then it says that the scope does not have a proper ground and is too dangerous to use safely.

Re: Hot Chassis Oscilloscope - need help

Reply #2 –
Quote from: pdq on
This is caused by capacitive coupling between the primary windings of the power transformer and its core. This should be a relatively small current and should be conducted through the ground wire of the power cord to the ground of the power outlet. If you are able to plug it in both ways then it says that the scope does not have a proper ground and is too dangerous to use safely.
Thank you for that. I am planning on installing a 3-prong power socket and grounding the chassis as well.

Can I clarify that you are saying the 7 volts AC coming off the interior chassis is, in effect, normal operation?

Re: Hot Chassis Oscilloscope - need help

Reply #3 –
Quote from: tapedeck on
Can I clarify that you are saying the 7 volts AC coming off the interior chassis is, in effect, normal operation?
If the chassis isn't grounded, this is to be expected, due to the capacitive coupling pdq mentioned.

This voltage will break down pretty quickly when loaded. One usually measures the resulting ground current. The maximum current for safe operation is part of the safety regulations. Those regulations stipulate that some gear must be grounded, i.e. may not safely be operated when not grounded. Check the manual.

If the chassis is grounded, the ground wire carries this current, and the voltage disappears. If you still measure such a substantial voltage on a chassis of a grounded device, then there's a grounding problem, which can pose a danger and needs to be rectified.

Re: Hot Chassis Oscilloscope - need help

Reply #4 –
I find it interesting that the OP says he got shocked... 6 or even 25 VAC over a typical multimeter input impedance (certainly >=1 Megohm at 50-60 Hz?) is nothing that should make for a shocking experience. (Does that even light up a phase tester?) Maybe it was the DUT or some static discharge?

Measurement equipment tied to earth obviously is the safest for the operator, but it does have its limits when dealing with DUTs that cannot or should not be floated (say, you're dealing with an audio amp connected to an earth-referenced source = instant ground loop) or when you just plain don't have an isolation transformer available. If it's floating, I would really prefer it to have a mains transformer with a shield winding for minimum coupling.

On my scope (always been floating and well-insulated by the looks of it, shield winding) I can measure about 22/52V~ on 230V~50Hz, and with yours truly in parallel (6-7 megohms) that drops to less than half, so my UT71C must be at ~10 Megohms or so, and we're talking on the order of 2-5 µA of current (equivalent to <100 pF of coupling), well below even the strictest medical requirements.

BTW, a "hot chassis" traditionally was something not mains-isolated at all, like an AC/DC set (where the chassis would in fact go hot depending on how it was plugged in). Yes, a certain rather well-known band probably got their name because typical guitar practice amps in the '60s and '70s were not exactly the definition of electrical safety. (These would have a mains polarity switch which absolutely had to be set correctly.)