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Old 24-06-2006, 03:08 PM   #16
twometerdish
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Quote:
Originally Posted by farmsky
No spider up the spout?
No spiders or insects inside the feedhorn.
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Old 24-06-2006, 11:13 PM   #17
tytower
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When I bought my dish second hand I played with the zinwell lnb that came with it and could get nothing . I bought a new one and away it went.

When I took the old one apart to have a look I found the backing plate is sealed with a formed " O " ring . Inside is a circuit board which in my case seemed dirty and perhaps a little water damaged.

I sprayed it with kero first I think and scrubbed it with a tooth brush then hosed it out.
Then I drenched it in WD 40 and let it drain . A little rubber grease on the " O " ring and plate back on .

When tested it was as good as the new one and is still up there working.
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Old 26-06-2006, 03:37 AM   #18
bassett
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yep,, there solid state, no reason why a good wash would hurt them,, I do the same thing with remote controls, then leave the bits out in the sun, to dry.
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Old 26-06-2006, 05:07 AM   #19
twometerdish
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The LNB is covered with three layers to protect it from the rain and sun. The LNB is wrapped with 2-3 plastic covers for the first layer. Then a thermocoal cover as second layer. Then an Aluminum box as third layer. Because of this I thought that the LNB would be free from dust. Would it be a risk by opening the lnb? I know some basics in electronics but not an expert.
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Old 26-06-2006, 06:10 AM   #20
satpalchat
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My LNB is some getting fail to get H polarity from Asiasat3, then it become okay, working fine. lot of reasons it may happen. some technical reasons which can be the cause of weekening LNB.

Understanding Polarity
Digital signals are transmitted from the satellites on either Vertical (V) or Horizontal (H) polarity for linear feeds, or on Right (R) and Left (L) polarity for circular feeds. Standard big dishes are most likely to have Feedhorn that can receive linear (H/V) polarity. Other system such as Dish Network and Direct TV use circular polarity.
If you have a big dish, most manufacturer of feed horns such as Chaparral and ADL provide what is called a "Teflon Slab" that can be added to the feed horn which would allow it to receive both linear and circular polarity signals. This will cause a loss of about 1 dB in signal level on the linear polarity signals.
In order to receive the digital TV signal, you must have the feed horn set to the correct polarity. For LNBFs, the polarity is controlled automatically by a voltage transmitted form the receiver to the LNBF via the coax cable. The receiver will send the LNBF 13 volts for horizontal polarity, and 18 volts for vertical polarity. For standard LNBs, the polarity is controlled by a motorized motor. In this case, odd channels represent one polarity, and even channels represent the other polarity. Standard satellites have the even channels set the polarizer to horizontal polarity, and the odd channels set it to vertical polarity. If the Satellite Polarity is inverse, then the even channel set it to the vertical polarity, and the odd channels set it to the horizontal polarity.
If you are unsure of which channel represents which polarity, simply set the digital receiver to display the signal strength meter, then switching between odd and even channels on the analog receiver will generate a marked difference in signal strength.
Polarity can also play an important role in whether you are able to receive the digital signal at all. Digital signals are not as forgiving as analog signals, it is either you receive it or you do not. If the signal meter shows a high signal (over 85% on the Sat Cruiser), and you are still unable to receive the digital signal, then adjusting the SKEW will almost always help.
Most analog receivers have auto tune feature, it will automatically find the best dish position and the best polarity for the analog signal. Be forewarned that this is not always the best setting for digital signals. However, in most cases it does produce the best setting for the reception of digital signals.
If you are able to receive only the odd or even channels on a given satellite, then your polarizer is defective and need replacement. The polarizer is a motor located on your feedhorn which switches the unit to receive the vertical or the horizontal polarity. Since every time you switch channels the motor moves, eventually it will wear out and will need replacement. It is connected by three wires from the back of your analog receiver to the dish (the wires are normally (Red '5VDC', White 'pulse', and Black 'ground'). Before replacing the polarizer make sure to turn the receiver's power off. It would be prudent to remove the power cable from the electric socket since most receivers continue to provide power to the dish even when the unit is off.


Polarisation Problems for analogue satellite TV
A common question is "I get only horizontal..." or "I get only vertically polarised channels".
Even worse is "I get only some channels".
You need to understand a few very simple concepts:
The LNB on your dish is a Low Noise Block downconverter. The reason for the name is this:
Low Noise because it must introduce very little electrical noise to the very weak signal which is captured by the dish and focussed on the LNB. If the LNB produces electrical noise then the signal will be blotted out. What is electrical noise? OK, imagine you have a guitar amplifier and you turn the volume right up without a guitar connected. You hear a hissing noise. That is the electrical noise which is produced by all the little transistors inside the amplifier. You can't get rid of it but you can reduce it by careful design so it does not swamp out the sound of the guitar.
Well, at very high frequencies (so called "microwave" frequencies) it is quite difficult to reduce the electrical noise. Even a warm brick wall produces "noise". The LNB is designed to produce bery little noise.
Why "Block downconverter"? Well, the LNB captures the microwave signals from ALL the available satellite channels then converts them in one single block of frequencies to a lower block of frequencies that will travel down the coaxial cable. (Microwaves won't go down cable - only down pipes - which makes plumbing a bit difficult!)
The LNB also amplifies all the frequencies to compensate for losses in the coaxial cable. The amount of amplification is called the "gain" of the LNB. The amount of noise it produces is called - yes - the "noise figure". Easy huh? So back to polarisation....
In order to squeeze as many channels as possible into a limited frequency block (or bandwidth) the satellite transmits alternate channels in alternate polarisations. Think about your polaroid sunglasses for a moment: If you look at your car clock while wearing polarised lenses, you can't read the clock digits if you tilt your head. This is because your lenses and the filter on the clock are of opposite polarisation when you tilt your head. When the polarisations are 90 degrees different, you can't see the clock digits at all.
This is how LNB polarisation works. A satellite channel is transmitted with (say) vertical polarisation by means of a polarising "filter" on the satellite in the sky. The LNB can receive the signal perfectly if it is mounted on the dish at a certain position. However, if you rotate the LNB through 90 degrees, it receives almost no signal at all. So the first thing we realise is that there is an optimum "skew" or rotational position for the LNB.
Now, because of polarisation, we can transmit a horizontally polarised channel on a frequency very close to that of a vertically polarised signal without one affecting the other. We twist the LNB one way to receive one channel and twist it through 90 degrees to receive the other. In practise, of course, we do not want to climb a ladder to twist the LNB every time we change channel! So we need some way to switch polarisation remotely.
The old fashioned method is to put a tiny stub of wire inside the LNB feed horn. This wire can be rotated by a motor. The wire acts as a "filter" that twists the incoming signal until it matches the polarisation of the LNB. This type of control is infinitely variable - it doesn't matter how the LNB is mounted or how the incoming signal is polarised - the tiny stub can be rotated until the LNB can "see" the signal.
Another method is to wind a coil of wire around the LNB feed horn. This is called a "magnetic polariser". When an electrical current is passed through this coil of wire, it creates a magnetic field. The magnetic field can twist the incoming signal to change its polarisation inside the feed horn. The current can be varied to adjust the strength of the magnetic field and, therefore, the amount of "twist".
The method we use most now is to design the LNB with two tiny aerials inside - one at 90 degrees to the other. Once the LNB is mounted correctly, one aerial can "see" horizontally polarised signals and the other can "see" vertically polarised signals. We can switch between the two internal aerials by means of the voltage that the receiver sends up the coaxial cable to power the LNB. The convention is to use 13 volts for vertical and 17 volts for horizontal polarisation. So every modern satellite receiver can supply either 13 volts or 17 volts in order to select the correct polarisation for each channel. Something goes wrong, here, you will receive channels of only one polarisation.
What can go wrong?
The LNB can develop a fault. Bear in mind that it sits there exposed to rain, snow or hot sun all year round. This combination doesn't do your wood fence any good and it doesn't do much for an LNB !
The cable can develop a fault. A bad connection caused by corrosion can reduce the voltage that reaches the LNB so it can never get 17 volts.
The receiver can develop a fault either in the power supply or in the control circuit that "tells" the power supply what voltage to send up the cable.
What do I do?
You need to carry out tests. Measure the voltage that comes out of the receiver cable connector. If it measures approximately 13 volts for one channel and 17 volts for another, you can be reasonably sure that the receiver is OK. You can double-check by connecting your receiver to a neighbour's dish cable or by borrowing another receiver to connect to your dish cable.
If the receiver is OK, you can connect a new dish cable alongside your existing one to see if that cures the problem. If it doesn't then the LNB is probably faulty.
See how easy it is to figure it out for yourself, once you undersand what is happening!

Now I want to explain a couple more things about LNBs while you are in the mood:
Firstly, I mentioned that the rotational position or "skew" of the LNB is important so it can properly and accurately "see" horizontal and vertical polarisation.
Well, the other important thing is its distance from the dish. The dish is collecting the weak signal from the satellite and focussing it onto the LNB horn. It can do this only if the LNB is positioned accurately at the focal point. Get this wrong and you could get "sparklies" the first time a rain cloud decreases the signal.

Do hope this will give idea for newbies..........cheers
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Old 26-06-2006, 06:22 AM   #21
tytower
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Quote:
Originally Posted by twometerdish
The LNB is covered with three layers to protect it from the rain and sun. The LNB is wrapped with 2-3 plastic covers for the first layer. Then a thermocoal cover as second layer. Then an Aluminum box as third layer. Because of this I thought that the LNB would be free from dust. Would it be a risk by opening the lnb? I know some basics in electronics but not an expert.
I dont understand all these layers. If they are plastic bags that could be your problem with condensation trapped between the layers. "thermocoal" is not one I recognise or can transpose to something recogniseable. The aluminum box would be the housing .

If the housing is screwed together open it as above and dry and/or clean it then carefully screw it back together. Get rid of plastic bags if that's what you have.
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Old 26-06-2006, 11:51 AM   #22
twometerdish
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Thanks Satpalchat & all the members. I don't know its correct spelling. "Thermocoal" is a packing meterial which is used to pack many fragile items. Dew may not be possible here in this month as the maximum temperature is around 38C and minimum is around 28C. Dew point is around 23C. But I will check that. Because of this high temperature and the dish is also facing south west that there are no spiders as they would be burnt alive
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