Unusual Combo Quad LNB issue

Ger Roe

I have an interesting issue at the moment.

I am using a universal Combo Quad LNB currently feeding three tv's with FTA satellite and local Saorview reception. Two of the TV's are using the combo signal from the LNB/antenna with a Sat/DTT splitter output at each. The third TV takes the DTT signal from a different Saorview transmitter and so uses the satellite combo LNB output, but a different and separate Saorview antenna and cable (LNB signal is not split out at the TV end).

The issue is that the third TV that is only using the LNB combined signal for satellite reception, has recently been losing specific channels randomly where the signal level suddenly drops off to nothing. Channels affected include BBC News HD, BBC1 wales HD, Channel 5 HD, Ch5 +1 SD, CNN HD and also some other SD channels like Food Network. Basically it affects only some specific HD and SD channels.

When this happens, other satellite channels remain perfectly fine, with normal 70-80% signal showing. The thing is that if I turn on any one of the other two TV's and select any satellite channel, the missing channels re-appear on the problem TV. If I have more than one TV viewing a satellite channel at any time, there is never an issue with the disappearing channels - they remain perfectly stable.

I did have a similar issue before where it seemed to have been caused by corrosion of an F-type connector on the affected TV port of the LNB. I can only suspect that it is this issue again, but haven't had a chance to check the dish yet.

Would anyone know how the problem seems to only affect certain channels/frequencies and yet can be solved by effectively turning on another separate LNB port?. Could it be a bad screen connection caused by a loose/corroded LNB connection - is the screen shared across all LNB ports? If it is a screen connection issue, why does it only affect certain channels/frequencies?

I am a bit baffled trying to understand the issue, but I should get a chance to take a look at the dish tomorrow.

dxhound2005

I think every channel you mentioned is on a Horizontal transponder, you could Google about that. All Channel 5 are on one transponder. Is it just 5 HD and +1 which are being affected?

https://www.lyngsat.com/muxes/Astra-2F_UK_11305-H.html

The Cush

An internal LNB fault would be my first guess. I'm not really a fan of those combi LNBs but if they're a solution to a problem then go for it.

Before considering changing the LNB do the basics first, checking cables/connections, especially the cable to the third TV. LNB feedhorn cover condition.

Has anything changed in the setup since the fault appeared?

Any internal terrestrial aerial power supply switched on within the TVs

Ger Roe

https://www.boards.ie/discussion/comment/123243946#Comment_123243946

Thanks Cush.

There has been no change to setup and I have checked the cable run and connection, except at the dish. There is no power going to the DTT aerial. As dxhound has spotted, whatever the problem is, it seems to affect horizontal polarisation only.

LNB inspection tomorrow will hopefully give some more clues. It is the restoration of signal by turning on any other receiver that is puzzling me at the moment. I would think that if the direct cable or connection is faulty, it would remain faulty under all conditions? I was thinking it might be frequency related as an intermittent fault indication, but it is certainly polarity that is the common factor.

Antenna

resistance to the DC current due to corroson on connectors ?

the satellite IF signal can largely pass through the corrosion due to capacitance but not DC current.

All the DC power sent up from all receivers that are on being 'pooled' to operate the whole LNB, so less demand for current from the problem receiver when others are on, so more likely to work properly if it suffers from significant DC resistance to the LNB. The LNB gets a higher voltage (18V) for Horizontal as opposed to around 13V for vertical. When more current is drawn (only the one problem receiver on) , there is more voltage drop across resistance, so it ends up (at the LNB) closer to the lower voltage for Vertical, and the LNB stays in Vertical mode (when receiver wants Horizontal).

Ger Roe

https://www.boards.ie/discussion/comment/123251280#Comment_123251280

Spot on Antenna …. that was it.

A corroded shield connection was the issue. A new connector and an application of self amalgamating tape has the dodgy cable run operational again.