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This morning, I was working with my scope, and I touched the CH1 BNC socket - ground side.
BZZZZ.

I thought I was dreaming.

I measured the voltage to AC Ground - 113VAC!!!

So something's leaking somewhere.

This is a 3 wire AC system - Live, Neutral, Ground.

I immediately checked my Signal Generator, on the same outlet - just fine, so it's definitely the scope.

Before I start poking around, does anyone have some fast pointers, to zero in on?

Menahem

Are you sure the scope is properly grounded? Measure it rather than assuning it's OK because another peice of kit doesn't give any trouble. Just stray capacitance can cause an ungrounded chassis to feel a bit lively and measure at half mains voltage (I'm assuming you're in a 220V/230V/240V country). However there's little current available as you'll find if you try a lamp between the scope ground and actual ground.

As far as I can see, there aren't any "Y" capacitors between live or neutral and ground. But C1020 and C1051, both 2200p, are from ground to parts of the power supply that are ahead of the main isolating transformer.

Correct on the 230VAC assumption!

I've checked that it is "grounded" - there are Y capacitors in the sealed EMI-filter unit - right at the AC input, and I'm beginning to suspect those of failure.

All the caps are new, so those 2.2nF's are not suspect.

The 113VAC voltage exists, even if the machine is switched off.

Shalom Menachem. I've just looked at your profile and note you're in Jerusalem.I've been amused when visiting Israel to find some high current applainces connected using our old British BS546 15 amp plugs and sockets.And landline phones using the same connector as in the UK, though with different pinout as I discovered the hard way.

I've certainly had a Schaffner mains inlet filter fail badly. It was on a Tek 1755A Vectorscope. It got hot enough to melt solder before an RCD tripped.

The question still remains, is the chassis just floating at half mains or is there real current available? Easily tested with a lamp or resistor to true ground or neutral. If there are Y caps in a mains input filter then loss of ground will definitely cause the chassis to float at half mains. Once you're sure that it's just floating rather than with real current available then I'm pretty certain you've got a missing ground. In the scope itself, in the mains lead or at your power outlet.

Let's gather some solid evidence:

The bond wire in the IEC style power cord goes
directly to the chassis by way of the mains power
line filter module.

It is a belt and suspenders system. The filter
module is in a metal can that has a metal mounting
flange that is screw mounted to the scope's chassis.

Inside of the can is a strap that goes from the bond
wire pin of the socket directly to a terminal on the
back of the can. The terminal is riveted to the
can.

The ground terminal on the back of the can has a green/
yellow wire soldered to it. The green/yellow wire has
a crimped on closed ring lug that is affixed with a nut
to a riveted threaded stud on the rear panel of the scope.

The rear panel of the scope is screwed to the chassis
using multiple screws.

The BNC connectors on the 2465 are directly (mechanically)
connected to the chassis at the front panel beneath the
bezel ring. Each has its own torx screw.

Assuming that the scope is the problem is a poor idea.
First do some measurements.

Connect a good low ohm meter to the offending BNC connector's
shield connection. Measure ohms to the front panel ground
banana socket. Measure ohms to the bond pin on the rear
IEC power connector.

If either of these measurements is not very near zero ohms,
you have a problem within your scope. Most likely inside of
the rear power filter... usually a burned out strap due to
a very, very high current path from the filter bond pin to
the filter hot mains lead... or possibly a manufacturing
defect.

By very near zero, I mean less than 1/4 ohm.

Next, unplug your power cord from the wall outlet, and
plug it into the IEC connector on the back of your scope.
Measure from the bond wire pin of the cord to the
front panel ground banana socket.

If it is not very near zero ohms, you have a bad power cord.

By very near zero, I mean less than 1/2 ohm.

If these grounds are correct, your problem is likely in your
bench power, or your house grounding system.

-Chuck Harris

M Yachad wrote:

Chuck, thanks for laying out, step by step, what I was summarising.

The 113V reading is about what you would expect using a DVM with high (10M) input resistance with the scope's ground disconnected. It's simply the pair of Y caps in the filter acting as a potential divider. I don't know the value of the Y caps but from memory the maximum current that's allowed to flow from live to ground is about 250uA for a Class II (double insulated) appliance, somewhat more for a Class I (grounded) appliance. Even 250uA will give you a tingle, I've felt this many times when handling kit that isn't grounded.

If 1 of the Y caps in the filter was faulty I would expect the voltage reading to be either close to zero or close to full mains. The fact that it's close to half mains strongly suggests that the filter is OK and the scope's ground is missing. Also if one of the y caps was faulty I would expect a 30mA RCD (aka GFI) to trip. Assuming your lab is protected by one of these, and I really hope it is.

I have seen so many of the IEC type power cords, made in China,
with failed wires that I am immediately suspicious of them.

I have taken a few apart, and was appalled at how crappy the
hidden connections were. Not solder, not crimp, but wires pressed
into "V" shaped nocks in the stamped brass (?) that forms the
socket and plug pins...

I just found a #14ga stranded copper wire, with a couple of
funny capsule shaped bulges in it. It had no conductivity
from end to end... So, I cut open one of the bulges, and it
was entirely filled with a copper salt. Someone probably spattered
some acid (flux?) on the bulk wire reel of the wire coating machine...
The acid slowly etched away the copper metal, and the fuse was lit,
waiting to catch some future user.

I have never seen anything like that with USA built wire.

I have seen other IEC cords where the internal insulation
turned to goo. The first sign was green goo leaking out of
the socket side of IEC cords that I left hanging from my cord
storage comb.

There seems to be a feature of human nature to always look
for the most difficult and expensive to fix failure when
seeing symptoms of simple problems. I see it in others, and
I am constantly fighting it in myself.

-Chuck Harris

ppppenguin wrote:

Gents

Thanks so much for all your clear pointers.

Yes, we all suspect the worst, when a problem appears.
Our diagnosing brain starts whirring away, tunneling through all the circuits, even in our sleep!

Glad to report that my scope is 100% OK - there was nothing wrong with it.
A simple ohm-check between the IEC earth pin and the chassis, confirmed that I did not have to open even a single screw.
The power cable was also OK.

The problem was a loose earth wire in the (Chinese) power strip.
Par for the course.

Menahem