Lose some channels until cable removed/reinserted

What's the model number of your TiVo?

Who's your cable company?

(My motto is, when in doubt, blame the cable company, but occasionally it's something which isn't their fault)

So far, you only seem to be having a problem with VHF-low channels (2-6), which could indicate a problem in your cable company's signal chain somewhere back up the line that manifests itself whenever you don't have a practically perfect connection.

Or it could be a problem with the part of the tuner that handles that specific frequency band (54Mhz to 88Mhz, roughly), or, depending on the way it gets told to tune those channels, a problem with the motherboard's communication with the tuner. (There's discussion elsewhere of satellite TiVos with a bad capacitor that screws up one of its tuners, so this could be the same general sort of cause)

Or maybe it's a failing solder joint that you re-disturb plugging and unplugging the cable.

Take the cable off of the tuner input on the Tivo and, taking the tuner input between thumb and forefinger, wiggle *very* gently. Does there seem to be any play or looseness? If it has a tuner input and a tuner output directly below, see if they are stationary relative to each other, but the entire tuner seems a little loose from where it connects (at the bottom) to the motherboard. Again, do this very gently.

bking1138 said:

I should have tried this earlier -- I disconnected the cable from my Series 2 and connected it directly to the TV. After about an hour I had the same problem. Picture on lower channels went to static. 4 is the worst, but 2-6 are all various forms of bad signal.

So it is not my TiVo. :up: Anyone have a pointer to a good cable troubleshooting page?

This is Comcast, but we had some work done on the house at one point and added a splitter to install a run into another room. That combined with Comcast only covering the wire up to the house without charging a fortune means I get to troubleshoot this myself.

Would a splitter going bad manifest as a bad signal only on the lower RF freq's?

Any help would be greatly appreciated.

Click to expand...

Assuming that we're talking analog cable, analog tuner(s) in the S2, and analog tuner in a television set not new enough to have a digital tuner, then either your cable company has a problem with a low band booster amp somewhere in the chain, or something on your side of the demarcation point (the grounding block where their cable turns into your cable) is somehow rolling off the lower frequencies.

The first is more likely, since there are active components involved, and the second is less likely, because physical problems are much more likely to cause high frequency roll-off.

See if you can get hold of a pre-digital battery operated television and a short piece of coax. Disconnect the coax feeding your TiVo and hook it up to the portable. Make sure it's working on all channels 2-13. Just for grins, wait until you get the channel 2-6 problem on the portable to be absolutely certain this isn't the tv tuner and the Tivo tuner both developing the same trouble at the same time (admittedly a one-in-a-zillion chance). Disconnect the coax feeding your house from the grounding block, hook up the short piece in its place with the other end going into the portable tv. (Now you know why it needs to be battery operated and portable) If you still get the problem coming right off of the grounding block, then it's the cable company's problem to fix.

EDIT:

Before doing the above--

Analog cable channels 95-99 come right after 2-6, ascending frequency wise. Before you do any of the above, check any or all of those channels for the same problem.

Above cable channel 99 you get cable channels 14-22. If you get the troubles on 95-99, check for it on 14-22.

After 22 comes 7-13, where I assume you aren't having the problem.

unitron said:

Assuming that we're talking analog cable, analog tuner(s) in the S2, and analog tuner in a television set not new enough to have a digital tuner, then either your cable company has a problem with a low band booster amp somewhere in the chain, or something on your side of the demarcation point (the grounding block where their cable turns into your cable) is somehow rolling off the lower frequencies.

The first is more likely, since there are active components involved, and the second is less likely, because physical problems are much more likely to cause high frequency roll-off.

Click to expand...

Yes to all the analog questions. Sorry for not being specific. My stuff is older than the Series 2

As for "somewhere in the chain" at this point I have it isolated to either (a) one out of 3 out links on a 3-way splitter, or (b) the cable running from the splitter to the TiVo or Analog TV. One of the split lines goes up into a guest room, and I just checked the analog TV in there and the low channels are fine, so it is not the feed into the house.

I know you said a physical problem was more likely to affect the high freq., but could there possibly be a problem only with the low end signal on only part of a splitter? Is it possible for the cable to introduce the problem depending on where it runs? The cable run is old, and a lot of work has been done on the house since, so perhaps some piece of equipment added afterward is interfering? The only odd bit is: Why not when the cable is first plugged in, why does it grow worse over time? Is it getting out of sync? If this was telecom signalling I would be more comfortable, but cable RF I do not know much about.

I will check the other channels and see what I can find, based on your suggestions.

I guess the next step is to get the best quality splitter I can find and replace the existing 3-way with that one.

Thanks for the help! I really appreciate it.

bking1138 said:

That brings up a question: As I understand it (looking at the outside of my house), I have a 3-way splitter installed there (a different one from the one splitting the signal inside). Once went to the basement (previous owners has a big TV down there), and the 3rd was the data line to the cable modem. I insisted on only one split between the cable feeding the house and the modem since I had read that multiple splits caused interference that reduced bandwidth on the cable modem. I tested the line in the basement with a small TV and it is working too.

I am not sure if there is a grounding block unless that is what you mean. I will have to look into grounding blocks to try and understand what I should be looking for.

Definitely time to replace the splitter and perhaps rerun the cable, since those seem to be the only potential sources of the problem.

Again, thanks for taking your time to give me a hand.

Click to expand...

A grounding block looks like a one input, one output splitter with a place to screw down a grounding wire as well. Unlike a splitter, the center conductor inside the threaded things has no little coils or capacitors or resistors, it's just a straight piece of metal with plastic insulation around it.

It serves two purposes. It's a handy way to connect the cable company's coaxial cable with the property owner's, and, if installed correctly, it keeps the outer conductor of the coax at ground potential.

If a cable company is doing an installation where there will be more than one outlet, I wouldn't be surprised if they substituted, in place of the grounding block, an n way splitter (where n is the number of outlets ordered) that had a place to connect a grounding wire.

If I understand correctly, you have one cable coming in from the cable company to a 3 output splitter. One of the outputs goes directly to a cable modem. Another feeds directly into the basement. The final one goes somewhere inside where it hits a 2 output splitter and one leg off of that goes to the TiVo.

(At this point let me interject that any cable run that's not terminated by either a cable modem or some kind of tuner needs to have a 75 Ohm terminator attached. It looks like the kind of plug on the end of a coax cable--those plugs are called "F connectors"--except no cable comes out the back. You can pick 'em up at Radio Shack)

If you can physically get at that 3 outlet splitter on the outside of the house without having to break open anything, disconnect the outputs, one at a time, and connect that small TV to each one in turn, and see if you get the problem from any of those 3 outputs.

If you do, disconnect the cable company's cable from the splitter and connect it directly to the small TV. If you get the problem there, it's time to call the cable company and tell them that they have a problem. If you don't then suspect the splitter.

If you don't see the problem on either side of the 3 output splitter, go to the 2 output splitter and disconnect the cable that feeds into it and connect that cable to the small TV. If you get the problem, then there's probably something wrong with that piece of cable.

If you don't get the problem, reconnect it to the 2 output splitter, and using the aforementioned small tv and short piece of coax, test both of that splitter's outputs by connecting directly to them. If both are bad, suspect the splitter. If one is bad, but not the other, suspect the splitter.

If both are good, reconnect the cables, and go to their other ends and test there.

I'll try to answer your other reply to my reply soon.

bking1138 said:

Yes to all the analog questions. Sorry for not being specific. My stuff is older than the Series 2

As for "somewhere in the chain" at this point I have it isolated to either (a) one out of 3 out links on a 3-way splitter, or (b) the cable running from the splitter to the TiVo or Analog TV. One of the split lines goes up into a guest room, and I just checked the analog TV in there and the low channels are fine, so it is not the feed into the house.

I know you said a physical problem was more likely to affect the high freq., but could there possibly be a problem only with the low end signal on only part of a splitter? Is it possible for the cable to introduce the problem depending on where it runs? The cable run is old, and a lot of work has been done on the house since, so perhaps some piece of equipment added afterward is interfering? The only odd bit is: Why not when the cable is first plugged in, why does it grow worse over time? Is it getting out of sync? If this was telecom signalling I would be more comfortable, but cable RF I do not know much about.

I will check the other channels and see what I can find, based on your suggestions.

I guess the next step is to get the best quality splitter I can find and replace the existing 3-way with that one.

Thanks for the help! I really appreciate it.

Click to expand...

For something to cause the low frequencies to be attenuated it would either have to introduce an inductive path to ground or insert capacitance in line with the signal flow.

A capacitor is 2 conductors separated by an insulator (i.e., non-conductor).

It can be a deliberately constructed device, or just the effect of two conductors being near each other without actually touching.

You could make a crude one out of a sheet of aluminum foil with a piece of waxed paper on top and another sheet of foil on top of that.

Direct current won't flow from the conductor through the insulator to the other conductor, but any current that changes has a chance to flow through it, and the faster the change (higher the frequency), the better the chance.

There are lots of different things that can be the insulator, but two of them are a vacuum (a space with nothing in it) and air.

If you have a piece of wire, and it breaks, if at the place where it broke the ends are still really, really close together, there's a chance of a high frequency signal getting across that gap, and less chance as the frequency of that signal goes down.

So theoretically you could have a break somewhere (the size of which could be effected by expansion and contraction of the metal due to temperature change) which would let something like channel 7 pass, which has a signal that changes polarity between 174 million times per second and 180 million times per second, but would block channel 6, which only changes polarity between 80 million times per second and 88 million times per second.

But if it blocked channel 6, it would block everything below channel 6 (like 5, 4, 3, and 2), even more.

You indicated the trouble started with 4 and 5 and spread upwards and downwards from there, which is why I suspected the cable company's signal chain.

unitron said:

For something to cause the low frequencies to be attenuated it would either have to introduce an inductive path to ground or insert capacitance in line with the signal flow.

<..snip capacitor discussion..>

If you have a piece of wire, and it breaks, if at the place where it broke the ends are still really, really close together, there's a chance of a high frequency signal getting across that gap, and less chance as the frequency of that signal goes down.

So theoretically you could have a break somewhere (the size of which could be effected by expansion and contraction of the metal due to temperature change) which would let something like channel 7 pass, which has a signal that changes polarity between 174 million times per second and 180 million times per second, but would block channel 6, which only changes polarity between 80 million times per second and 88 million times per second.

But if it blocked channel 6, it would block everything below channel 6 (like 5, 4, 3, and 2), even more.

You indicated the trouble started with 4 and 5 and spread upwards and downwards from there, which is why I suspected the cable company's signal chain.

Click to expand...

If I disconnect the coax cable from the TiVo or TV and then reconnect it, the signal is good for about half an hour, then starts to get noisier and noisier until it is reduced to static after an hour or so. The sort of thing you describe sounds more like it would happen all the time (at least once I stopped fiddling with the coax cable).

I don't think it is the TV, since the TiVo always sends on Channel 3. Since I can see the TiVo images for "good" channels, I think I can rule out the TV being the culprit. Likewise, when the TV was hooked directly to Comcast, the high channels were good, only the low ones were out.

There was some signal distortion on 99, but it was a bit better than the distortion on 6.

As I said, I will try and grab the best quality splitter I can find and replace the existing one. I will also ground the splitter (hooking it up to a water pipe should be sufficient, right? I know the pipes are grounded, since we had to adjust that when we went off of well water and on to town water ) If that does not work I will also install a new coax cable in case there is a split or something in the line.

Thank you for your explanations. I don't know much about RF stuff, so it has been very interesting and helpful. I really appreciate it very much.

Edited to clarify a statement and correct some spelling

unitron has been giving you excellent advice.

Here are two other things to consider.

I had a low frequency rolloff which was evidenced by snowing pictures below channel 6. It was finally found to be a corroded center conducter on the cable at the grounding block. When you remove the connector from the grounding block, you might try scraping the center conductor to make the wire shiny again. In my case, the tech cut off the last 3 inches of the cable and put a new connector on it and my problem was solved. However, this does not explain the intermittant/progressive nature of your problem.

I had another issue that sounds more similar to your problem. My signal level would fall and the simple act of disconnecting the cable at the grounding block and reconnecting it seemed to restore the signal. The cable tech who was a good old boy who had many years of experience said that sometimes a device attached to the cable might induce stray voltages up the line. I am not sure how this worked, but somehow breaking the connection bled off the stray voltage. But the fix was on good temporarily. He suggested that if my problem persisted to try disconnecting other devices from the line one at a time to see if the problem stopped. I never got that far because my problem seemed to fix itself.

Perhaps unitron can comment on my problems/fixes.

unitron said:

Is your electrical an underground feed or from an overhead connection (coming off of a telephone pole)?

Is your cable feed underground or off of a phone pole?

Where I'm going with all this is to see if we can't ground that splitter/demarcation point to the same ground that the electrical power service entrance uses.

Click to expand...

Cable and power are both off of a pole. I double-checked the entry point, and it is a 3-way splitter and is grounded, though it is not grounded to the service, but just to a line set into the ground outside my house.

However, I may have finally resolved the issue.

Originally I had my cable hooked up to a HTPC in my office that I used for watching news in the morning and such (that was one of the 3 feeds off the splitter. I had replaced the PC, but had not disconnected the cable from the UPS (it has a cable surge protection option).

Since it had been suggested that some other equipment may have been causing the issue, I disconnected the cable from the UPS, and (at this point) that seems to have cleared up the problem! I only disconnected the cable a couple of hours ago, so I will have to check tonight to see if that is the real solution or not.

I must admit to being baffled by all this RF stuff. Hopefully that is the end of my problem, and if it is then I owe it all to the wonderful folks spending their time to share their expertise. Thank you all very much.

bking1138 said:

Cable and power are both off of a pole. I double-checked the entry point, and it is a 3-way splitter and is grounded, though it is not grounded to the service, but just to a line set into the ground outside my house.

However, I may have finally resolved the issue.

Originally I had my cable hooked up to a HTPC in my office that I used for watching news in the morning and such (that was one of the 3 feeds off the splitter. I had replaced the PC, but had not disconnected the cable from the UPS (it has a cable surge protection option).

Since it had been suggested that some other equipment may have been causing the issue, I disconnected the cable from the UPS, and (at this point) that seems to have cleared up the problem! I only disconnected the cable a couple of hours ago, so I will have to check tonight to see if that is the real solution or not.

I must admit to being baffled by all this RF stuff. Hopefully that is the end of my problem, and if it is then I owe it all to the wonderful folks spending their time to share their expertise. Thank you all very much.

Click to expand...

RF generally involves phases of the moon, reading the entrails of goats, and sacrificing your first born. Other than that, it's entirely predictable, and theory and practice are completely interchangeable and indistinguishable.



If you were to dig where that ground line disappears into the ground you probably would, and certainly should, find that it ends in a clamp attached to a ground rod, perhaps it even attaches to the same grounding electrode used by your power service entrance, or perhaps the two are bonded together.

It wouldn't be the worst idea in the world to loosen up the screw on the splitter, ease out the grounding wire, shiny it up with some steel wool or fine grit sandpaper, put it back and tighten the screw back down, but not too "manfully", as the grounding "ear" might break off if the splitter body is just diecast, or what we called "pot metal" in my junkyard days.

And if you're going to leave the UPS coax in/out out of the path, consider another surge protection measure for the cable.