Strange event in my workshop?

[kalee99]

Weird doings in my den?

Well I just thought I had better write this down somewhere before they cart me off.
I have been on quite a while recently with a Marconiphone 871. A huge set with motor tuning, magic eye and 2 short waves. All in all a very fancy pre war set.
So all was going well as I rebuilt the capacitor box this took care of a lot of shorts on the HT rail.
Next I replaced the few wax paper jobbies that remained and decided to ramp the set up on my variac. At 200 Volts I was hearing some static when a sudden cloud of smoke and a loud bang indicated that all was not well. It was not too bad. I had missed the mains filter capacitor tucked away in a dark corner of the chassis.
So in with a modern replacement & up to mains voltage, plugging in my workshop aerial the set burst into life & was lively on all wavebands. Feeling proper chuffed I decided to carry out a full alignment which went along fine.
I decided to put the chassis on soak test while I gave some thought as to what to do about the cabinet.
This is when problems set in. I replaced the perished 2 core mains flex with a modern reproduction woven 3 core flex, taking the earth lead to the chassis. Connected this to my variac and as soon as I connected the earth to the mains earth the set fell silent. No RF signals at all? Gram setting still nice & loud so it was an RF problem.
I went round this for a whole day looking for open circuit coils and any capacitor I may have missed. Aerial in loud signals, Earth connected, no signals. I went to bed to sleep on it.
Next day feeling refreshed I remembered the Sherlock Homes saying that if you take away all the innocent items, then whatever remains, however unlikely will be the cause.
So I removed the aerial and earth and stuck a long screwdriver in the aerial socket. Set played ok on just 6” of metal. Next I reattached the mains earth and what a surprise, the signal instead of disappearing actually got louder.
So I started to investigate my workshop aerial and to my utter amazement it had 100Ω to the earth of the mains.
My workshop aerial has been in use here since we moved in over 30 years ago and it is a long wire up in the loft, well away from any electrical fittings. It comes down through the ceiling in the corner of my room where it splits off one way for the radio in my bedroom and the other way to my workbench.
So I separated it at the junction and did an earth test on the ends. Happily the wire from the loft was over 20MΩ same going to my bedroom radio.
The 10 ft. bit of wire to my workbench that just runs along the wall behind my wooden shelving was the guilty party with 100Ω to earth on either end.
It’s just a bit of insulated wire & short of pulling out all the wooden shelving I can make no sense of it at all. I have however cropped the wire off at both ends and put a new wire, cloths line fashion along the front of my shelving from the incoming aerial to the bench.
The Marconiphone is now playing very well on all wavebands but I still don’t know the cause of the mystery 100Ω wire? I suppose I never will know the cause unless I pull out the shelving, but this is beyond me now as I am too old to be doing such feats of athleticism. Any one any ideas as to the cause of the mystery?
Paul.

[teetoon]

Hi Paul. I love a good mystery. It's difficult to say without seeing the crime scene but are the shelves open backed ? I was wondering if something on a shelf, maybe a piece of test equipment or similar had got pushed back and pierced the wire insulation ? Who knows how the 100 ohm reading comes about but with the meter connected, try moving things gently on the shelves to see if it changes.

We really need a picture and it would be nice to see the radio too after all your hard work.

David.

[John10b]

Interesting
1.When did you last use aerial successfully?
2.When did you put up shelving?
3.Have you put up new lighting or any other mods to workshop?
4.Have you suffered from any vermin?
5.Have you had any water ingress ?
Cheers
John

[Brigham]

You could earth one side of the Variac, and connect the other to the stub of the aerial.
Ramping up the voltage should make the location of the leakage apparent!

[Heatercathodeshort]

Originally Posted by John10b

Interesting
1.When did you last use aerial successfully?
2.When did you put up shelving?
3.Have you put up new lighting or any other mods to workshop?
4.Have you suffered from any vermin?
5.Have you had any water ingress ?
Cheers
John

My thoughts exactly!

Whenever a customer came into the shop, particularly on a Monday morning complaining of some odd electrical fault, fuse blowing etc, my first question would be, WHAT HAVE YOU DONE?

The reply would often be NOTHING..After some detailed questions it would turn out they had a new carpet laid, new light fitting and the list goes on.

It could well be vermin urine. Not so uncommon as one might think.

The mystery will remain until you pull that cupboard out. John.

[The Philpott]

Mice was my first thought...but not quite sure how they would manage this unless the aerial has been nibbled, mouse (inside cavity wall?) has died from indeterminate cause, now the aerial is earthed to a little corpse inside the wall, which in turn is earthed to a steel conduit. (sorry everyone, i hope you have had your breakfast)

Too convoluted..

(edit- yes, mouse piddle would manage to drop it to 100 ohms, no doubt about that)

[ex seismic]

Kalee99, if it is any consolation my two outside aerials have come out, or rather come down, in sympathy. I've been away for a week and came home to find rope and wire draped over the car. There's a big Black Pine in front of the house that used to have one end of a doublet attached half way up and a vertical wire going all the way up. The top of the vertical wire (single core insulated) has some very neat cuts showing bright copper so they are fresh. Admittedly a lot of the assorted ropes have obvious UV damage but it has all been up for about 10 years. I blame the squirrels for the cuts, little beggars.

[David Simpson]

Paul, is your ceiling &/or walling clad in foil-backed plasterboard ? Or are your ceiling joists &/or wall stud-framing made with treated timber ? such as "protimized". i.e. impregnated with a copper based solution. Cracked or chaffed wiring insulation might produce a low resistance if in contact. Especially if there is a bit of dampness somewhere.

Regards, David

[kalee99]

Originally Posted by Heatercathodeshort

My thoughts exactly!

Whenever a customer came into the shop, particularly on a Monday morning complaining of some odd electrical fault, fuse blowing etc, my first question would be, WHAT HAVE YOU DONE?

The reply would often be NOTHING..After some detailed questions it would turn out they had a new carpet laid, new light fitting and the list goes on.

It could well be vermin urine. Not so uncommon as one might think.

The mystery will remain until you pull that cupboard out. John.

Hi,
Its not a cupboard its free standing wooden shelving & Mrs. kalee99 is non to happy about vermin being mentioned My work room is in the house. We have no mice or rodents as our ever vigilant Jack Russle Millie would make short work of them. No water ingress either?
I will post photos soon.

Nothing has changed with the layout of the room for over 20 years. The Aerial was working last time I had a set on the bench which was before last Christmas. Don't do a lot now due to my state of health.
The answer is indeed to pull out the shelving but I am no longer fit enough for such work. It did cross my mind to connect the wire to the Variac live & wind it up just to see what would go bang, but decided against it as I like living here and the prospect of burning down my little abode was not a happy one

[dseymo1]

If it were longer, you could 'ping' it with a pulse and time the reflection. <10ft is a bit too close for that though!

[John10b]

No offence intended regarding vermin, it was only a suggestion.
Hope you get to the bottom of it soon.
Cheers
John

[kalee99]

Non taken. I could have understood it if it had been in my outdoor workshop because Little Millie dog has "cleaned", that up of all pests. She is one tough little cookie.
Paul.

[Skywave]

Kalee99: "The 10 ft. bit of wire to my workbench runs along the wall behind my wooden shelving: that was the guilty party with 100Ω to earth on either end."

I wonder if, at one point in time, at your workbench, you gave that wire a little tug - possibly not intentional - and as a consequence, behind the wooden shelving, it's sleeving was penetrated by something sharp which, in turn, was connected to earth. I obviously don't know the constructional details of your wooden shelving, but suchlike often uses metal nails and / or screws to hold it together. And metal brackets also spring to mind.

You say "100Ω to earth on either end." It might be informative to measure the resistance of that wire from one end to the other end without making any connection to earth. From what you have said, you should get a reading of 200 Ω. Plus, of course, the "100Ω to earth on either end" implies that the short to earth is approx. 5 ft. from either end, since the total length is 10 ft.

Al.

[Station X]

Surely the resistance of 10 foot of wire is negligible and the 100 ohm earth fault could be at any point on the wire?

I think that anything which has penetrated the wire will need to be in pretty good contact with something else which is earthed. A mere nail in the wall would presumably have a high resistance to earth unless the wall was very wet?

I wonder if the aerial wire has somehow come into contact with a mains cable? Is there any voltage to earth on it?

[Maarten]

Could it be a screw?

[Refugee]

It could be stuck under the foot of an instrument that is plugged in to a power socket so presenting the resistance of an old rubber foot to the meter.

[kalee99]

Originally Posted by Refugee

It could be stuck under the foot of an instrument that is plugged in to a power socket so presenting the resistance of an old rubber foot to the meter.

Yes it could, except nothing on the shelves is plugged in. I just use them for storage and display. Like the idea though. The 10Ω I was getting was very constant and did not move about at all when I tugged on the wire. I just got fed up and cropped the wire off as close to the shelves as possible at each end.
Paul.

[cheerfulcharlie]

If it happened recently, could it be something to do with the hot spell we have been experiencing?

[kalee99]

Hi,

Yes could be. Nothing would surprise me. Has not been as hot as this for a long time. I remember the hot one in the 1970s and someone frying an egg on a car bonnet
Paul.

[Station X]

Unless the shelving is dismantled, which clearly isn't going to happen, we're not going to get answer to this and can only speculate as to the cause of the problem.

A bit of testing might help:-

What's the resistance from one end of the wire to the other?
Is there a voltage on the wire wrt Mains earth?
Does a Megger "break down" the fault?