DSL Extreme → Occasional hissing & crackling noise when modem is on

For the past few months, I've been dealing with occasional and random hissing and crackling noises on my phone line that disappear when the modem is turned off. There's no pattern as to when this happens. I just had Fusion activated a few weeks ago, and it's still occurring.

I just completed installing a DSL splitter at the MPOE in a last-ditch attempt to isolate any noise problems. I live in a two-bedroom apartment with two internal-wired phone lines, though I only use one of them (each line is on a separate two-pair wire bundle).

At the MPOE, which is an ancient binding-post behemoth in an enclosed shed/closet, I ran new 22 AWG hand-twisted wire from the screw terminals to a splitter (an Excelsus CP-V413WT, datasheet here). I then split the signal so that the voice part goes over one internal line, and the data goes over the other. I connected the splitter to the internal wiring using new 22 AWG wire from the splitter and UR butt splices. So the wiring is about as clean as it can get on the premises side of the MPOE.

The modem is a brand-new Comtrend CT-5072T I picked up off of eBay. I reset it to factory settings and put it in bridge mode. It works fine when there is no noise on the line (although I wish I was getting more than 8.5M down with Fusion), but once the crackling starts, the DSL throughput drops like a stone, even though I split the signal right at the MPOE.

Any assistance would be greatly appreciated.

Following up to my own post, it looks like the noise just started again -- I'm away from home at the moment, but my throughput just dropped to about 2.5M down, and my SSH tunnel to my home server temporarily disconnected.

dying caps and/or power regulator inside modem or power brick... replace it pronto, or solder new caps/regulators...

I've tried swapping three different ADSL1 modems before I purchased the new Comtrend. Same noise on the line in each case. I'm inclined to believe it's not the modem.

First, let me make sure I'm tracking with you here: Most of the time, the line works just fine; normal internet connectivity, no static on the line. Intermittently, however, you start getting static on the line, and when that happens you hear the static on the phone and the internet connection starts crashing - but the static goes away once the modem is turned off?

If that MPOE splitter is doing its job correctly, then normally the modem and phone should be isolated from each other - nothing you do with the modem should have any effect on the phone, and vice-versa. So the fact that turning the modem off clears the line is suspicious - not indicative of a faulty modem, but suggesting there may be a fault with the MPOE splitter.

That said, I've been studying your line, trying to catch it while it's running into trouble... it looks like you're experiencing occasional but noticeable spikes of low power and low attenuation, which suggests there may be a short on the line. Haven't been able to run a proper LMT while the issue is occurring, however, so not sure yet (A watched pot never boils, apparently... ).

Yes, and I agree that it shouldn't be happening if the splitter is working (and my description is accurate).

I installed the splitter on Monday night, and this morning was the first time that the problem had returned. I was rushing out the door as I noticed it, so my diagnosis of power cycling the modem to fix it may be incorrect. I know for sure that, before I put the splitter on, the noise would disappear as soon as I powered off the modem.Yup -- the erratic nature of the problem has driven me crazy!

Edit: May be happening right now. Can't SSH into my home server at the moment.

Since the problem existed before you installed the Excelsus splitter it's probably not the source of the noise problem, and since you swapped modems and the problem persists it's probably not a modem defect.

Before you installed the Excelsus splitter you were probably using a simple DSL filter on each of your phones, answering machine etc. Those filters block the higher DSL frequencies from reaching the phone but don't block the lower voice frequencies and 48 volts DC on the line from reaching the DSL modem. While the DSL modem is not affected by that, having the DC voltage reach the modem may allow the modem to draw a small amount of current from the line and the amount of current may change when the modem is on/off. Some of the dedicated DSL splitters are of a simple design and also do not block lower voice frequencies or DC from reaching the DSL modem.

The hissing and crackling noise on the line is usually caused by bad connections and partial short circuits from corrosion and/or damaged wiring insulation allowing adjacent wires to touch or be bridged to each other by corrosion. In some cases this kind of noise can come and go depending on humidity, temperature, and voltage difference between the affected conductors. If the modem can draw some current from the line because 48 volts DC is not blocked by the splitter then the small difference may trigger a change in the noise just as taking a phone off hook could.

Since the problem is intermittent it may take awhile to find since it can exist anywhere from your equipment all the way back to the CO. Phone line techs have equipment that can show how many feet away from one end of the cable a short is located, and in some cases they can clear a short caused by moisture or corrosion by application of high voltage pulses to the line, or they just swap the bad pair for a good one from the same run.