No Polarity Switching - Trouble Shooting and Solution (1 Viewer)

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Lifetime Supporter
May 23, 2013
Meadow Vista, Northern California
Had an interesting problem happen with my bench satellite FTA system and thought that sharing my experience might help another satellite hobbyist. Though I have run into problems before with attenuated 22KHz/DiSEqC signals and LNB voltage drops, I can't recall running across this problem before...

The problem was first noted yesterday when I hooked up the TBS6983 tuner card to capture a spectrum scan for a few satellites. the TBS card would not switch polarities on either the ASC1 controller or if patched to a LNBF. I tested the voltage output on the LNBF IN port of both devices and they were approximately H=18v and V=13v. Measured the LNB voltage at the patch panel and it was the same. Tested the voltage at the controller/LNBF connection and it remained at 13v no matter what polarity was selected at the TBS or the STB. This fixed 13v voltage prevented servo control or a LNBF polarity switch.

Tested with a replacement coax jumper from the patch panel to the controller and the voltage switched normally between 13 and 18v as it should when the TBS6983 or STB polarity was changed. When I removed the original coax jumper running between the patch panel and the ASC1 position I found a sharp 160 degree kink in the coax. The impedance measurement was off the chart! Putting this jumper back in-line caused both the TBS card and the STB to not change the polarity voltage and it remained at the 13v output. Straightened the coax kink and the impedance returned to normal and both devices switched polarity normally.

How did this happen? Last week I upgraded the router and LAN. While pulling new Cat6 cables behind the patch panel, evidently caught the coax causing the kink. While the controller and LNBFs switched normally with a few STB's on the test bench, several devices would not work when connected to the same kinked coax.

Lesson learned? High impedance presented at the LNB IN port can prevent a FTA receiver or tuner from switching the LNBF polarity voltage.
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Pub Member / Supporter
Dec 19, 2008
Might be interesting to test different manufacturer's RG6 products to see which ones are more likely to cause this sort of issue. I try to stick with Beldon/Commscope which I rarely have trouble with, but some off-brand cabling might be more prone to impedence bumps in bends. Also better cables usually stipulate minimum bend radius in their specs...
This also indicates how one must carefully treat co-ax when reeling out or untangling a length to install, and do so to not create kinks etc!
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