Apparently trivial issue gives me the headaches - PLEASE help

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in short: when connecting audio chain to earthed equipment something goes.

in detail:

I have an audio chain made of laptop (running on mains-powered SMPS), USB DAC (transformer supply), USB HDD (SMPS), amp (SMPS) and, obviously, speakers.

the apartment I live in only has earthed outlets in the kitchen and near the bathroom. old building, different regulations back then.

a while ago I thought it was a good idea to earth my equipment. I made an extension cord running to the kitchen. I was in the kitchen when I connected it to the outlet, only to hear a loud bang. I went shaking to the living room, hoping that it wasn't the speakers ($$$) that went.

upon investigation it turned that it was the amplifier. the input stage (transformer powered) got fried. it's an input switching and volume control front-end, controlled by a MCU. all this is powered by separate transformers, the amplifier modules themselves (UCD180) are powered by a SMPS.
the MCU controlling the front-end essentially burned (part of it basically exploded). everything directly connected to its outputs burned too. the amp has indicator LEDs in series with current limiting resistors and all those resistors burned. strange thing, they looked intact upon inspection, that possibly indicating a very large and short voltage spike.

well, I fixed the amp, all working.

until one day when I thought about connecting the audio chain to the TV to watch a movie. small spark upon connection, TV shut down. TV was OK fortunately, front end of amp partially burned again. not so bad as the first time, I got away with only replacing the MCU controlling the input stage.

needless to say, the audio chain works perfectly if not earthed or not connected to TV.

what those 2 situations had in common was that the chain got earthed. first time through earthed outlet, second time by TV cable ground. I measured 0V from TV cable screen to mains null and 230V to hot.

still, I have no idea WHY this happened. I measured resistance from amp ground to mains hot and cold. nothing measurable. voltage from ground to hot and cold is ~mains/2, likely due to Y caps inside the amp's SMPS. that stands for all the SPMS's in the audio chain, the same mains/2 voltage from ground to hot/cold was measured for each and one of them.

I have no idea what to check next, please help.
 
Could be one or more of your power sockets is miswired. Also, remember that neutral and ground are not necessarily the same (in the UK - not sure about where you are!) and should not be connected together (except where the power company may have done this deliberately).
here hot and cold are not differentiated in any way, the plug is revertible.
still this would not explain the problem I'm having as currently the audio chain (PC, amp, HDD and DAC) is not earthed.

I have never imagined such a thing making me feel helpless.
 
what those 2 situations had in common was that the chain got earthed. first time through earthed outlet, second time by TV cable ground. I measured 0V from TV cable screen to mains null and 230V to hot.

still, I have no idea WHY this happened. I measured resistance from amp ground to mains hot and cold. nothing measurable. voltage from ground to hot and cold is ~mains/2, likely due to Y caps inside the amp's SMPS. that stands for all the SPMS's in the audio chain, the same mains/2 voltage from ground to hot/cold was measured for each and one of them.

What ground, in each case? It seems from what you write that one set of equipment has its ground at 125V with respect to the other. Did you measure the voltage between the ground levels of machines on the two different supplies? Sorry if you've already said so, but I got confused.

You refer to hot and cold, but say later that there is no difference. That also suggests some confusion about ground reference.

Whatever, there may be some danger of electrocution if you don't find out what's wrong.
 
What ground, in each case? It seems from what you write that one set of equipment has its ground at 125V with respect to the other. Did you measure the voltage between the ground levels of machines on the two different supplies? Sorry if you've already said so, but I got confused.
I meant that there is ~120 V (that is mains voltage (230V) divided by 2) between ground of each SMPS-powered device in my chain and earth (or mains cold). that is most likely because of Y caps (small current).

You refer to hot and cold, but say later that there is no difference. That also suggests some confusion about ground reference.
your profile shows the UK flag so I'm assuming you live there. AFAIK the UK power outlets are three-pronged and are not revertible. where I live the power outlet is revertible, each of the wires going my gear receives either hot or cold, depending on how I insert the plug.
here's a pic in case you don't know ahat I'm talking about:
An externally hosted image should be here but it was not working when we last tested it.


Whatever, there may be some danger of electrocution if you don't find out what's wrong.
I know, this is why I posted this.

anyway, since there is no measurable current flowing from ground of every piece of gear except the TV (which is earthed by means of the coaxial cable) to mains plug. I don't know where your confusion comes from. I'm pretty confused myself.
 
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Loads of appliances here don't use the mains earth. Leads terminating in "figure of eight" sockets don't care which way they're plugged in. Nevertheless, one wire is live, "hot", and the other neutral, "cold". The neutral is earthed at the local distribution transformer, and may be at some low voltage by the time it gets to the consumer. That's not the bit that confuses me.

On the equipment powered by the the two-pin mains, you say there is 1/2 mains voltage between ground and each of the supply wires. Is this ground the signal ground of each machine?

Are the two phases on the two pin system the same as, and connected to, the two non-earth wires in the three pin supply?

Is the TV ground earthed?

Once again, it seems from what you write that one ground is at 1/2 mains potential, and the other is earthed. Connecting the two grounds together means the one at 1/2 mains voltage must be dragged down to earth. That requires a surge of current, depending on several things we don't know.

I could imagine a worst case, where one system is centre-earthed and the other singe ended, and you connect them together and electrocute half of Bulgaria...

Measure the voltage difference between the grounds of the two systems. Current between the two when connected together may be momentary, but large.

If I'm totally barking up the wrong tree, ignore me. I've never measured where grounds in non-earthed machines float to...nowhere special, I rather assumed...

:confused: Consider rewiring your house :(
 
I think the source of the confusion is the fact that I mentioned having mains/2 relative to earth on all grounds. I don't even know why I wrote that, it's irrelevant as it's because of a very small current passing through caps. each and every SMPS out there behaves like that because of 2 small caps going from ground to hot and cold.
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that would only explain failure if those caps were defective.

and yes, the TV ground is earthed by the coaxial TV cable going to it. it is earthed by the provider for good reason.
 
HI.

I had a similar problem, except my amplifiers did not go "poof". I had lots of HUM.
I finally wired in a huge (2kVA) isolation transformer, that solved the problem. Since then I have remodeled the house and ripped all the old wiring out. The wiring was done by a very good crew that followed all the most current regulations. Since the rewiring I have not had any HUM problems at all.

Moral is that old wiring might not have followed the code and might have mixed up and miswired neutral and ground connections. Depending on the load in your house or the neighbors houses you may have had volts of AC between the various grounds. A central isolation transformer is the general solution, rewiring the house is the more expensive solution.

Hans J Weedon.
 
Hi Push-Pull.

May I then suggest that you one of your pieces of Audio-Video is wired wrong, or has a short inside from line to chassis-ground. A second suggestion is that very old AC units had no line-transformer but used direct off the line AC to power the unit. At least in the US very cheap equipment made as late as 1955 had that, causing no end of interconnect problems.

This was especially so with really cheap radios and TVs. Those were illegal in most of Europe since 1935 or later. NEMCO, SEMCO and DEMCO ratings made sure of that. Those agencies were among the first to outlaw direct connections to the line insisting on an isolation transformer.

Hans J Weedon.
 
My money's on transformer primary/secondary insulation breakdown.

What's bugging me is how can there be no difference between the two power leads, and how an earthed system has just two pins. The story has lots of seemingly contradictory detail. A language thing perhaps.

Two-pin mains plugs here in the UK are reversible. Three pin, earthed plugs are not. This is handy because adding the earth forces an electrical distinction between live and neutral.

For insulation test purposes, it shouldn't matter, but whether the fault manifests itself, and how and when, depends not only on the presence of an earth connection, but also on which way round the power lines are, surely?
 
PlasticIsGood said:
This is handy because adding the earth forces an electrical distinction between live and neutral.
The distinction is already present, as UK mains supplies are not floating: the neutral is earthed at the substation or local transformer, and things in your house will be approximately earthed even without an earth connection. Hence the voltage between neutral and your house (and you) is, except under fault condition, much smaller than between live and your house.

The OP's problem does sound like insulation breakdown somewhere.
 
Hi Push-Pull.

May I then suggest that you one of your pieces of Audio-Video is wired wrong, or has a short inside from line to chassis-ground. A second suggestion is that very old AC units had no line-transformer but used direct off the line AC to power the unit. At least in the US very cheap equipment made as late as 1955 had that, causing no end of interconnect problems.

This was especially so with really cheap radios and TVs. Those were illegal in most of Europe since 1935 or later. NEMCO, SEMCO and DEMCO ratings made sure of that. Those agencies were among the first to outlaw direct connections to the line insisting on an isolation transformer.

Hans J Weedon.
I know what you're talking about, I once took apart an old TV. the "power supply" if I may call it that was basically a diode connected directly to mains, high voltage electrolytic caps and some very large resistors used to drop voltage. and now the phones tell you to unplug the 5W SMPS when the battery is full in order to save energy :D

but it's not the case, there's no such thing in my system, the TV set is a SONY LCD screen.


My money's on transformer primary/secondary insulation breakdown.

What's bugging me is how can there be no difference between the two power leads, and how an earthed system has just two pins. The story has lots of seemingly contradictory detail. A language thing perhaps.
I never wrote that an earthed system has two pins. the earth came into picture either when connecting the system to an earthed outlet or when connecting the system to the TV set (the ground of the TV cable is earthed).

I reread my posts and I can't find any contradictory details, sorry.

I'm too almost sure it's breakdown of some component, either one of the low power transformers powering the additional circuitry inside the amp or the amp's SMPS. I'll do some more tests today and report back.
 
happened again.
secondary laptop was temporary unavailable so I had to use the laptop which is part of the "offending" system to watch a movie.
out of caution I disconnected the amp (supposed to be the faulty component) from the rest of the system. laptop only had external hard drive connected to it, absolutely nothing else. the amp remained powered though by means of the same cord extender.
I connected the TV to the laptop with a VGA cable and...
amplifier goes again. laptop fine, TV fine, external HDD fine. amp dead.
I repeat, amp was only powered from the same outlet but wasn't connected to the laptop. normally the electrical connection is by means of the DAC (laptop->DAC->amp). now the laptop->DAC amp connection wasn't there (USB cable unplugged from laptop and no other electrical connection present).
took amp apart, the digital part that controls the front-end went, as usual. AND the transformer providing power to it was VERY hot. I initially thought it was because of the 7805 regulator that shorted. but no, that wasn't the case. the heating is obviously related to the mishap but not a consequence of it.
I must confess I feel like an illiterate person trying to read Shakespeare. I'd have never imagined that such a thing can go unsolved for such a long while.
 
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