DMX741 and Polarity

[acadiancowboy]

Today I replaced my C-Band LNB on my 10' UNIMESH dish with a DMX741. Up to this point I had been using a Toshiba TRX-80 to actually move both the dish (via a SuperJack) as well as change the polarity using a Chaparral Polarotor. So when I scanned a bird using my Pansat 9200HD, I had to change transponders on the TRX-80 in order for the polarity to switch from H to V and back.

To replace the Toshiba TRX-80 as a dish mover, I've got a GBOX 3000 on order. So no questions there for now.

My question today is regarding the DMX741. The big reason I purchased this LNBF (besides the fact that it does Ku) is that it somehow switches automatically between H and V polarity, or so I thought.

Now when I do a blind scan on my Pansat, it goes through the Horizontal polarity scan, then the Vertical polarity scan. However, from the list of frequencies, it appears that the receiver is not actually switching polarities. The reason I say this is because in the V scan I will see a frequency that is one number off from a frequency found in the H scan. And consulting Lyngsat, it appears that channels on the V polarity are not coming in.

So my question is: does the Pansat somehow "switch" the polarity on the DMX741 automatically, or do I need to do something? Or do I need some additional hardware?

 

[yankee495]

It should have 13v/18v switching. The receiver should power the LNB with either 13 or 18v which switches the polarity. I am not familiar with the 9200HD but it should be able to do it. You may have to enable it on the 9200, but it should be able to switch H/V automatically with no additional hardware.

 

[Lone Gunman]

Funny, I spent the afternoon yesterday doing this too.

You should have only one coax (plus the jumper at the LNBF) connected to that 741 and the other end should be connected directly to your 9200. If you look in the antenna setup menu you will see a section there that makes the 9200 send power to the LNBF, which also controls H and V. If you're trying to get Ku through it then you have to activate the 22hz switch for that function on each Ku satellite. The only thing your Toshiba should be doing is moving the dish. Gbox or Vbox X will make this much simpler once you have it setup.

 

[acadiancowboy]

Thanks for the responses gentlemen. I took out my el-cheapo multimeter and connected a pigtail RG6 to the antenna input of the Pansat 9200HD then put it into blind scan mode. Measured the voltage on the other end of the pigtail with the meter. When it was scanning Horiziontal, the voltage on the meter read about 19V. When scanning Vertical, the voltage on the meter read about 14V.

Under Antenna Setup I'm choosing AMC3/C as my satellite and LNB is just set to Standard 5150 (there are other choices but this one is the default). I've attached a picture of my television screen showing the H and V blind scan going over the exact same frequencies and finding hits. This is proof that polarity switching is not working.

Lone Gunman, based on this, do you think I have a bad DMX741?

 

[yankee495]

He may have a DMX741U in which case it uses Diseqc switching but that is for C/Ku. I forgot about the cable, well I assumed he had it hooked up right.

Since you are getting 14/19 then the 9200 is doing its job. Can you select a sat with known good H/V TP's and scan them separately? Force it to scan only H or V?

That would tell you for sure.

By the way, is it new, used? New means "untested" and used from a friend means "well tested".


Old joke, it if was working it should still work.

 

[acadiancowboy]

Actually I have the DMX741, not the DMX741U.

I tried your suggestion and did a forced scan on V and got the same channels that I had on H again. It's certainly looking like the LNBF is the problem.

I purchased this LNBF new from Sadoun.

Just to be clear, I'm uploading the picture of the LNBF mounted on the dish. As you can see, the wire to the left goes to the receiver and there's the jumper from KU to the LNB port.

I just ordered a new AZ Box which is coming in tomorrow. I'll try that and if V still doesn't come in, I'll contact Sadoun.

 

[hd fan]

Could it be possible (long shot) that the skew is not set properly in such a way that strong H signals are getting into the V probe and superimposing on the weak horizontal signals due to cross polarisation?. More likely you set the skew at 0 at true south as it was on your Polarotor but who knows? . I hope this long shot makes sense?

 

[acadiancowboy]

Could it be possible (long shot) that the skew is not set properly in such a way that strong H signals are getting into the V probe and superimposing on the weak horizontal signals due to cross polarisation?. More likely you set the skew at 0 at true south as it was on your Polarotor but who knows? . I hope this long shot makes sense?

Hdfan,

Thanks for the suggestion. I have the zero mark on the LNB facing directly at my 6 o'clock. So I think that is fine. I'll report what I find when I hook my AVBox on Monday. Thanks much.

 

[yankee495]

ONE LAST THING. I've read where the wires in the throat of the LNB are bent. Mine were. I straighted them to be inline with the bars in there...

 

[WTguy]

Try the skew with the zero at 3 o'clock.

 

[acadiancowboy]

Well I tried rotating the LNB 90 degrees in the ring and did a scan.. Nothing came up. Just to reiterate, I followed directions and positioned the LNB so that it's 0 degree mark pointed at both 12 o'clock and 6 o'clock and performed scans on both positions with no verticals coming up.

Got my AZBox in last night and hooked it up. Then I performed a scan and found that it's behavior is the same as the PanSat: only horizontal polarity channels are found.

At this point I've tried two different boxes while rotating the LNB in several different positions and cannot obtain vertical polarity. Any other recommendations before I contact Sadoun?

 

[acadiancowboy]

An update: I called Sadoun and explained the issue I was having. He conceded that it could be a defective DMX741, and sent me one at no extra charge. I got it in today and hooked it up, and am now receiving distinct horizontal and vertical signals via blind scan. A subsequent channel scan did confirm that both horizontal and vertical polarity is now working with this replacement DMX741, so thanks to Sadoun for great customer support and helping me to solve my problem. Attached is an image from my Pansat 9200HD which shows distinct H and V scan patterns.

Thanks to everyone who chimed in to help me with my problem.

 

[Bluztrvler]

This is also a long shot, so I will keep it short. Do you still have the TRX80 connected to the lnb, possibly through a splitter or as a daisy chain? This same thing happened to me on my Pansat 3500 and my DSR 920.

Sorry just read your new post. Congratulations on getting it working!