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Burned centre pin at LNB connector.

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Aug 7th, 2016 at 1:55pm  
In response to an email from someone with a burned centre pin of 75ohm cable F connector at the LNB connector I wrote the following reply.  Maybe someone will find it of interest and save me having to write it all out again....

Poor contact between the centre wire and the spring contacts in the LNB socket cause heating and arcing. That, with oxygen from the air and water vapour causes the burning of the centre pin, and the spring contacts. The problem gets worse because the spring contacts are heated and loose their spring pressure further.

The cure is probably a new LNB and new cable and most important a carefully made connection.  The centre wire should be bright and clean and stick out about 1.5mm proud of the rim of the F type plug.  The tip of the wire should be filed to a rounded point so it slips easily into the spring contacts. It must not get stuck and “push back” the cable, the insulation and the sheath as the plug is screwed in. You should be able to feel that the wire has gone smoothly in and is being gripped by the springs. A smear of electronic grade silicone grease should be put on the centre wire. This lubricates the process of pushing in and also seals, from oxygen and moisture, the microscopic area where the wire and springs actually touch.  The final joint should be covered in waterproof tape.

75 ohm coax cables do not have a specified diameter for the centre wire.  Dimensions vary, while the ratio of inner to outer diameters should stay the same to make the 75ohm, assuming the same dielectric.  If you initially use a cable with a large diameter wire and later change to a new cable with thinner wire then the springs inside the LNB won’t grip properly as they have been stressed by the previous larger diameter wire.

Sometimes I have ‘repaired’ an LNB with burnt/corroded springs by taking a short length (say 3 inches) of coax inner wire, coating it in switch cleaner fluid and repeatedly inserting and removing it into the LNB socket perhaps 20 to 40 times in an attempt to clean the spring contacts. Then use a largish diameter wire for the connection.

If moisture has got in the cable it is likely that corrosion has occurred for a long way, possibly several yards down inside the braid/sheath.  Some cheap coax cables use an aluminium coated plastic mylar film as the RF outer conductor, plus a few sparse copper wires braid to carry the DC supply current. The aluminium film is extremely thin and turns to white powder in just a few days if the end of the cable is exposed to moist air at the seaside.  During the day the cable gets hot and air in the cable escapes at the end. During the night it cools and sucks in cold moist damp air which condenses inside and causes corrosion.

Best regards, Eric
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