A-B switching, Power Issue?

[upnatm]

I remember someone suggesting an A-B antenna switch, to use between
a motorized Ku-band dish and a G-Box/C-Band dish on a single STB.

I've installed the switch (an old Bakelite thing, from the 60's I think) right after
the receiver, then have the two dish systems hooked to the A/B sides of the switch.
The signals and DISEqC commands seem to pass thru very well.

My question is; "Must I cut the power to the receiver, before throwing the switch?"

Many, many threads state that when connecting or disconnecting an antenna
cable at the receiver, the power must be OFF.
Does this hold true when using a mechanical switch of this type?

The switch sounds spring loaded, and makes a good solid "snap" when thrown, but is it fast enough to avoid a problem?

Please share your experience and advice. Thanks.

 

[Skyscanner]

UP,
My opinion would be "no, you wouldn't have to shut down" When switching, but really, since you're at your receiver switching from a to b, what would it hurt to take the extra effort. I think most of the concern over disconnecting the power was related to disecq switches, and it sounds like that's not an issue in your case. LNBs seem to be a little more tolerant of that sort of thing. I suspect the real problem with not turning off the power when working with the switches comes in when you don't make a clean electrical connection or disconnect, which can cause an electronic-killing voltage spike. Sounds like your a/b switch has that taken care of. But again it's just my opinion which could be overridden by someone with more expertise than I have.
Sky

 

[Corrado]

I would verify whether or not switching the receiver on and off really turns off the LNBF power. If it doesn't, it's asking for trouble.

I think most FTA type receivers do switch off the power to the LNBF when off, in my experience.

However, I wouldn't try it with a C band or 4DTV or pizza dish receiver. Switching these types off really doesn't power them down, just the panel displays and AV outputs.

 

[updatelee]

I can't vouch for all stb but dreambox maintains lnb voltage when switched off through the menu. They do this to maintain the epg ( dreambox is mainly a euro stb where epg is used ) also for cardsharing so the card doesn't miss the keys or tier updates.

Some dvb cards, like the prof 7500 in Linux, even after the program closes the frontend and exits continues to attempt to tune untill it times out, aprox 2min.

If you have one of those elchepo signal meters put it inline and see if when the receiver is switched off the meter goes dead.

 

[B.J.]

I remember someone suggesting an A-B antenna switch, to use between
a motorized Ku-band dish and a G-Box/C-Band dish on a single STB.

I've installed the switch (an old Bakelite thing, from the 60's I think) right after
the receiver, then have the two dish systems hooked to the A/B sides of the switch.
The signals and DISEqC commands seem to pass thru very well.

My question is; "Must I cut the power to the receiver, before throwing the switch?"

Many, many threads state that when connecting or disconnecting an antenna
cable at the receiver, the power must be OFF.
Does this hold true when using a mechanical switch of this type?

The switch sounds spring loaded, and makes a good solid "snap" when thrown, but is it fast enough to avoid a problem?

Please share your experience and advice. Thanks.

Without knowing something about the switch you're using, I'd suggest that you get a VOM and test the switch, just to see exactly what it's doing. I had one friend who tried using some old RS switch somewhat similar to what you're describing, (but it may have been in reverse), and his switch essentially went up in smoke by shorting out the LNB voltage. I'm not exactly sure what he did, but I think the problem is that the switch was tending to short out some connections. I *THINK* that it was only shorting out the un-used A or B connection, which shouldn't be an issue for what you are describing since it's not shorting out the powered side, but some switches are make before break, and some break before make, and it's possible that they might short out the receiver port for a second while switching. So some quick tests with a VOM would tell you if it's ever shorting out the line to the receiver, which could be bad.

Relative to the idea of "when connecting or disconnecting an antenna
cable at the receiver, the power must be OFF", I just don't beleive this at all. Turning off a receiver every time you change a coax will avoid some potential problems, such as accidently grounding the powered center conductor, but if you're careful, this should never happen. I've been changing coax connections without turning off receivers for decades, have done it literally thousands of times, and have seldom had any problems associated with doing this. About the only issue that I've had is with my TT3200 PCI receiver, in that if I switch coax lines hot when the coaxes are actually drawing power, that it sometimes puts the 3200 in a situation where it loses sensitivity, and I have to reboot the computer to get sensitivity back, so I generally run that receiver in slaved mode, so that it's seldom powering any LNBs. But just yesterday, I even DID accidently ground the center conductor (which I just said should never happen, but I was being careless), and it didn't hurt anything. When using my Genpix with my laptop, I have 2 coax lines laying next to my recliner, one being C-band, and one being Ku, and I switch them several times a day without cutting the power to the Genpix.


It's just my opinion, but I really believe that cutting the power is a waste of time, and in some situations just isn't practical, such as with PC receivers where you'd have to go through a long reboot process to switch coax lines. Actually, since I know I'm switching hot, I "usually" tend to be a bit more careful about what I'm doing, compared to when I turn things off, I just connect lines up without thinking, and often make mistakes. I believe that if you are just careful to not short out the receiver's LNB voltage, then there shouldn't be an issue, and using a switch that is a pretty sure way not to accidently short anything out, provided that the switch isn't designed to intentionally short out connections while switching. So I think that if you test the switch to insure that it's doing what you think, that you shouldn't have an issue with it. Even if it's a make before break, and ends up having the receiver power two LNBs at once, that shouldn't be excessive as most receivers should be capable of providing at least twice the power drawn by a single LNB.
Just my opinion.

 

[crossbow]

My question is; "Must I cut the power to the receiver, before throwing the switch?"

Please share your experience and advice. Thanks.

Im doing the same between Ku with a SG2100 motor and C with a Gbox through a RCA mechanical A/B switch to a SV8000HD. I hot switch it most of the time with no problems yet, works great.

 

[upnatm]

Thanks all, for the valuable input!

So far, I've been doing like you say Skyscanner, I'm right there anyways, and the K2HD
only takes about 15 secs to reboot. The G-Box simply goes into stand-by when
the STB's power is cut.

Corrado and updatelee, what I mean by "cut the power", is to shut it off at the back of the box. Sorry for not being more concise at first.

B.J., I pulled the switch and did some VOM tests. The three connectors on the switch
share the same ground, no shorts occur where the LNB power source is concerned.
The switch is "break then make", but while the "break" is made from the receiver, the
two antenna inputs are both momentarily shorted to ground.

Now, can this be a problem for the LNBs?, I'm thinking not, but what will the G-Box think?

I just did a test to see if the G-Box had any power at it's input connector. There being
none, now makes me realize that the G-Box is waiting for voltage signals, and not providing them.
I even forced a short at the input, with no faults arising--WHEW!

It might sound as though I've answered my own questions, but should I be worried about
the two LNB lines being shorted for that brief moment? Mind you, the LNB power source is safe,
it's just that both of the LNB lines are being grounded briefly.

Thanks again for the awesome help.

 

[upnatm]

Good to Go!

Hi again,

I tried to post my latest reply a couple of hours ago, but work is being done nearby, on the internet lines.

Thanks Crossbow for the input. After hearing from you and everyone else,
I do believe I'm good to go with no troubles.

Thanks again, and Happy Sailing through the Clark Belt!