09-07-2018, 11:57 PM
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.
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.
Good gap between the shelving units and the wall. Photos will follow shortly.
In general we don't sense anything bad in the house and enjoy living here. Just now and then odd things tend to happen.
