lnb switching voltage v\h

[4wd]

Just a finding than some may find useful if encountering something similar.

When using a Miraclebox s9, a problem was some lnb's refusing to switch to vertical, immediately suspected defective lnb, but worked fine on other receivers. After some pondering, measured the voltage, that box gave out a voltage in the very high limits of switching specs (afaik should be around 13\17 v) and some lnb's were sensitive to this.

Apart from that peculiarity it's a good receiver, excellent picture. Thought about modifyng the psu or tuner modules, but higher chances to destroy the box rather than fix anything! A cure was to connect any difficult lnb through a diseqc switch, that tiny voltage drop fixed the vertical.

 

[Lazarus]

Sounds similar to the issue with the Skybox F5 : A "fix" is sold for that and is based upon some sort of zener diode arrangement inserted into a short stub of coax.

 

[Trust]

Sounds similar to the issue with the Skybox F5 : A "fix" is sold for that and is based upon some sort of zener diode arrangement inserted into a short stub of coax.

Sometime even a silicium diode inline will do , drops 0.7 v .
Signal goes full trough .

 

[Terryl]

Just use cheap coax, or a longer one.

Some boxes I have seen have a plus or minus a half a volt on the H/V switching, others its about a volt ether way, it all depends on the load to the box.

A multi output LNB or multiple LNB's could cause a heaver then normal DC current draw on the receivers LNB power supply, this would drop the DC voltage a bit more then normal at the final end.

Some cheap coax with a copper plated steel center conductor may also have a bigger DC voltage drop at the LNB then a coax with a pure copper core.

Some cheap barrel connectors and ground block have been seen to drop the DC voltage more then others.

However there is no way to correctly measure (without some special equipment) the DC voltage and DC current draw on an LNB unless it's active, receiving a signal and sending this signal down to the receiver, to do this you need a special RF interface test box, or a specially modified satellite receiver.

Touching a DVM test lead to the LNB voltage feed or breaking the center conductor to measure the DC current will upset the circuit and you may/will get a false reading.

I use a specially modified satellite receiver to measure all this, I can measure the true DC current draw of the LNB and any attached switches and the exact DC voltage going out to the LNB.

 

[4wd]

^^^ maybe adding 50 m coax could fix the vertical for the lnb's that didn't like the Miraclebox voltage :O)
But got only 2 lnb's that refuse v switch at 14.3 v , a bunch of others are ok.

The other boxes here, all approx 13.5 v \ 18.5 v at the receiver plug. The Miraclebox has the lowest power spec of all, 400mA , so could theoretically be most subject to voltage drop upon load, but seemingly not the case.

 

[Terryl]

One way to test this is with a DC load on the output of the receiver, find out what the DC current spec's are for all attached equipment, add these up and use Ohm's law to find an appropriate resistor (or combination of resistors) to load down the LNB power supply, then measure the DC voltage and see what's up.

Don't forget to add in the DC resistance of the length of coax used.

Also dont forget to use the correct wattage resistor, I would say a 10 watt would do.