15-07-2018, 06:04 PM
The cable system I worked on based on the Isle of Dogs was a Switched Star network.
Instead of a broadband signal being fed to the set top box in the subscriber's house, all of the intelligence was in the street and all signals were converted to the same VHF frequency for feeding to the home. The set top box was essentially a simple VHF-UHF up-converter and the electronics to communicate with the street cabinet.
I was sent to investigate a fault which had eluded the service techs - intermittent random channel changing and interference in a block of flats. The line card in the street had been changed and the set top box but the problem still existed.
I went to the subscriber's flat and found the TV switched to BBC1 where the cricket or tennis was being shown - I can't remember which.
Anyway, the picture quality was perfect so the only thing to do was sit and wait. I gave the subscriber my mobile number, warned them that they would lose their picture but it would come back after a few seconds. I returned to my van, disconnected the sub's drop cable and inserted a directional coupler into the feed. I then reconnected the drop and ran a cable from the DC to my van and connected it to a 14" Grundig with multi-band tuner, so it tuned directly to the 64MHz output signal from the sub's card. I placed the set on top of a crate of various stuff, pointing into the van, closed the back door and sat down in the front where I now had an excellent view of the TV in the rear view mirror!
After some while, the programme changed to the French channel TV5! A few second later and it went back to BBC1. Another few seconds and it went back to TV5 again! The one thing I noticed, though, was that the picture was perfect on both channels, which didn't quite fit the fault description. Then my mobile rang - it was the sub.
I switched of the TV and disconnected my cable, then proceeded to the sub's flat. I was greeted with a dire picture bearing all the hallmarks of co-channel interference. Changing channels was difficult but worked after a few attempts - and then would revert to TV5!
Back at the cabinet I started going through the often badly written labels on all the other drop cables. All those that I could read were for the terraced houses that lined the street. I got on the radio and asked how many subs did we have in the block of flats? Two - and I was given the other flat number and a foreign sounding name.
I went to the other flat, explained who I was and asked if they were watching TV5. Yes! An elderly relative was staying with them who was a native French speaker and she was watching the cookery programme!
So, what next? I went down in the lift and, while I was mulling this over - the problem was almost certainly in the block somewhere - turned the wrong way and found myself leaving by the back door instead of the front. I started to turn round and was greeted by a fantastic sight. The green duct emerged beside the doorway with a very large chop bone wedged in the top of it! The installers should have extended the duct up a couple of feet to where the sub's drop cables entered the building - but they hadn't (surprise, surprise!).
A rat had obviously found the bone and attempted to take it home, but it got wedged. The bone was picked absolutely clean but so were the drop cables! The PVC sheath had gone, as had the outer braid and the inner dielectric. The only thing that remained was the 1.02mm copper plated steel centre conductor!
So, the two conductors, stripped naked, ran parallel for about four inches and a quarter of an inch apart. Not a long distance for the low VHF frequencies to migrate to the other conductor! When changing channels, the request went back to the cabinet on a 10.7MHz carrier, which was also induced into the other drop cable and, if successful, managed to change channels on both cards!
I removed the bone and pulled some slack out of the duct, then spread the bare conductors as far apart as I could. I checked on both subs and success! Both watching there own programmes with only the slightest trace of CCI.
So, tidy up and return to base and arrange for an install crew to re-pull both drops and fit the missing piece of duct.
(If you think this was OT, remember that it wasn't me that introduced rats into the conversation in the first place!
)
Instead of a broadband signal being fed to the set top box in the subscriber's house, all of the intelligence was in the street and all signals were converted to the same VHF frequency for feeding to the home. The set top box was essentially a simple VHF-UHF up-converter and the electronics to communicate with the street cabinet.
I was sent to investigate a fault which had eluded the service techs - intermittent random channel changing and interference in a block of flats. The line card in the street had been changed and the set top box but the problem still existed.
I went to the subscriber's flat and found the TV switched to BBC1 where the cricket or tennis was being shown - I can't remember which.
Anyway, the picture quality was perfect so the only thing to do was sit and wait. I gave the subscriber my mobile number, warned them that they would lose their picture but it would come back after a few seconds. I returned to my van, disconnected the sub's drop cable and inserted a directional coupler into the feed. I then reconnected the drop and ran a cable from the DC to my van and connected it to a 14" Grundig with multi-band tuner, so it tuned directly to the 64MHz output signal from the sub's card. I placed the set on top of a crate of various stuff, pointing into the van, closed the back door and sat down in the front where I now had an excellent view of the TV in the rear view mirror!
After some while, the programme changed to the French channel TV5! A few second later and it went back to BBC1. Another few seconds and it went back to TV5 again! The one thing I noticed, though, was that the picture was perfect on both channels, which didn't quite fit the fault description. Then my mobile rang - it was the sub.
I switched of the TV and disconnected my cable, then proceeded to the sub's flat. I was greeted with a dire picture bearing all the hallmarks of co-channel interference. Changing channels was difficult but worked after a few attempts - and then would revert to TV5!
Back at the cabinet I started going through the often badly written labels on all the other drop cables. All those that I could read were for the terraced houses that lined the street. I got on the radio and asked how many subs did we have in the block of flats? Two - and I was given the other flat number and a foreign sounding name.
I went to the other flat, explained who I was and asked if they were watching TV5. Yes! An elderly relative was staying with them who was a native French speaker and she was watching the cookery programme!
So, what next? I went down in the lift and, while I was mulling this over - the problem was almost certainly in the block somewhere - turned the wrong way and found myself leaving by the back door instead of the front. I started to turn round and was greeted by a fantastic sight. The green duct emerged beside the doorway with a very large chop bone wedged in the top of it! The installers should have extended the duct up a couple of feet to where the sub's drop cables entered the building - but they hadn't (surprise, surprise!).
A rat had obviously found the bone and attempted to take it home, but it got wedged. The bone was picked absolutely clean but so were the drop cables! The PVC sheath had gone, as had the outer braid and the inner dielectric. The only thing that remained was the 1.02mm copper plated steel centre conductor!
So, the two conductors, stripped naked, ran parallel for about four inches and a quarter of an inch apart. Not a long distance for the low VHF frequencies to migrate to the other conductor! When changing channels, the request went back to the cabinet on a 10.7MHz carrier, which was also induced into the other drop cable and, if successful, managed to change channels on both cards!
I removed the bone and pulled some slack out of the duct, then spread the bare conductors as far apart as I could. I checked on both subs and success! Both watching there own programmes with only the slightest trace of CCI.
So, tidy up and return to base and arrange for an install crew to re-pull both drops and fit the missing piece of duct.
(If you think this was OT, remember that it wasn't me that introduced rats into the conversation in the first place!
)
