Very Weird Signal Problem

[jampedro]

Setup: Simple - one round dish.
Legacy dual LNB with 2 straight cable lines to 2 FTA's. No switches.

When one of the sat cable lines from the LNB is disconnected from the FTA receiver the other one goes 'No Signal' - Level low and -0- Quality. Reconnect the cable to the turned off receiver and the other one comes back normal. Turn it on and both work fine on all channels. The owner swears there are no switches or splitters in the sat cables in the ceiling.

Anybody ever seen anything like that? This one's got me stumped.

[AntAltMike]

Voltage on either "F" port powers both LNB amplifiers, so the voltage from one receiver is not reaching the LNB. Could be a bad receiver or bad (high resistance)coax connection..

[FTA Michael]

I agree with AntAltMike, and my first guess would be bad coax. I'd try a test run of RG6 to replace the one from the 0 quality receiver and see whether it makes a difference.

[jampedro]

When you say bad coax, wouldn't it most likely be a break in the mesh shieding rather than the center wire? Like most likely the shield not making contact with a connector? Or rats in the attic? If this were the case, how could both receivers work fine when both are connected?

Excuse my ignorance but is it correct that the center wire carries the signal and the mesh shield carries the voltage to switch LNB polarity?

[FTA Michael]

AFAIK, the coax forms a circuit, with the center wire carrying the outbound current and the mesh returning the inbound, or vice versa. (With lots of good insulation in between so the outbound and inbound signals don't interfere with each other.) Weaken that circuit so it can't carry the voltage from the receiver, and that might match your symptoms. If the "good" circuit is intact, then the LNB is getting enough power to act on requests that limp in from the bad circuit.

Unless I'm completely wrong, in which case I hope that some real techie folks here bail me out.

[jampedro]

Where's AntAltMike when we need him?

That makes sense for the voltage but we still don't know the path of the signal.

[eakes]

The dual LNB needs to be able to be powered from either input. Internally to the LNB there will be a blocking diode from the center pin of each connector to the common power supply input for the internal amplifiers. For the condition that has been described either one of the receivers is not providing voltage to its coax line or one of the diodes in the LNB power circuit is open.

When the coax to one receiver is removed then both receivers stop working. That receiver is obviously the one that is providing power to the LNB. If you disconnect the non-powering receiver and reconnect the first receiver, then the first receiver should start working.

Once the working receiver is discovered, swap coaxes to that receiver from the other non-working receiver. If the working receiver continues to work then the problem is the non-working receiver. However, if the working receiver does not work after the coaxes are swapped, then either the coax now connected to that receiver is bad or there is a problem with the LNB.

Reconnect the original coax to the working receiver and go the antenna and swap coaxes at its input. If the receiver works after the swap then the coax to the non-working receiver is bad. If it does not work after the swap, then the problem is with the LNB.

The coax provides two functions. First, it delivers power to the LNB - voltage is applied to the center conductor and the shield serves as the return path for the DC circuit. The second is as a transmission line for the LNB signal down to the receiver. This is an unbalance transmission line - center conductor and shield. An example of a balanced transmission line would be the 300 ohm twin lead that used to be used from a TV antenna to the TV.

[AntAltMike]

Where's AntAltMike when we need him?
Working for a living.

That makes sense for the voltage but we still don't know the path of the signal.


Your receiver ordinarily puts out 13 volts when it calls for odd transponders and 18 volts when it calls for evens. The electronic relay in the LNB needs to "see" about 15.5 volts to select even transponders.

There is an amplifier in the LNB that works at lower voltages than that. I wouldn't be surprised if it continues to work down around five or six volts.

If you have a crappy electrical path on one of the conductors, then the line resistance becomes too high, and most of the voltage is dropped over its length, and the first symptom experienced is that the receiver can only call for odd transponders but not even ones, and the second will be when it drops below five volts or whatever at the LNB, and then, if it drops even further, at which point it can't sufficiently power the LNB to even produce the odds.

Fortunately, the DBS signal is so robust with respect to the needs of the receiver that even if the crappy, corroded coax connection has a resistance of 1000 ohms or more, enough signal will still make it to the receiver for the receiver to be able to tune it, so as long as the other coax is supplying LNB amplifier voltage, you will be able to get the odd transponders on the other receiver, and if the LNB is a stacked LNB, as many FTA LNBs are, then the receiver that is not delivering any voltage to the LNB will still often get enough signal to tune all of the transponders.

[jampedro]

Great information. Thanks to both of you. Now I know which line to check first. I'll let you know what I find.