Why filters go bad?

Hi all,

After relenting and trying different filters to see if my connection issues are filter related, I am left puzzled why they fail in the first place. I had previously had the free t-piece type and subsequently more expensive sealed type at considerably more cost.

I did a BT Silent Linetest (17070, option 2) and switched between my various filters, with only a handset connected to the test socket, and was suprised to hear a difference in background noise / hum between the filters.

Two questions spring to mind.

1) Do you get what you pay for with filters?
2) If you do have more expensive ones, like I had, then what causes them to go wrong?

My ISP advised me to go for ADSL 2+ filters as 'they're better' (but I'm due 21CN later this year too).

Any views?

Pete

They're like any device with electronic components - the connections inside the components can fail or where they're soldered to their circuit board.

The old maxim about disconnecting your old dial-up modem during a thunderstorm in case it gets fried holds equally for anything connected to the line.

John.

ADSL2+ filters are generally better because the cheapo firms have not latched onto that label yet, nothing has changed in the design specification

The difference is that you can buy a cheap capacitor that does the job, but the variation among a batch will be higher than the more expensive ones. Also some cheap capacitors break down faster.

For problems with capacitors just look at the Sky digi box (Sky+/HD) forums on digital spy and these are a major point of failure.

thanks chaps

In reply to a post by MrSaffron:

For problems with capacitors just look at the Sky digi box (Sky+/HD) forums on digital spy and these are a major point of failure.

There's a slight difference between capacitors exposed to a continuous AC component and high temperatures, and one that you'd find in an ADSL filter. The capacitors in Sky boxes fail due to the conditions quoted.

I'm also mystified why a low pass filter with a cutoff frequency of about 15kHz could affect hum ? Unless it was being exposed to a fairly strong alternating field which was inducing hum into the components ?

I'd guess that many ADSL filters die because of the voltage transients which can occur in long lines, particularly those that travel above ground for fairly long distances.

Edited by deleted (Fri 05-Mar-10 16:43:18)

I recently had an Excelsus filter fail, allegedly one of the better ones. Symptoms were hiss on the phone (not the one where the modem was plugged in as it happens). Replacing with a cheap O2 filter instantly cured the noise (and my O2 modem has been far more stable since).

Being inquisitive I opened it up. No sign of physical damage, all the joints still look OK, and all the caps measured on my LC meter read correct values. So what has failed is a mystery. I have a feeling they fail due to the permanent 50V DC on the phone line coupled with the 75V AC or so ringing voltage causing capacitor breakdown, but there is nothing to validate that theory. They would not fail due to the broadband signals which are miniscule (especially the incoming signal).

Before this I was of the mind that failure of filters would be very unusual and the support suggestion to keep changing them was just because they couldn't suggest anything else. But I relent, they do fail...

In reply to a post by dsergeant:

.... Replacing with a cheap O2 filter instantly cured the noise.

I've had an ADSLNation XF-1e; reputedly one of the best available; in continuous use for 5+ years, with not a hiccup.
In fact, because I purchased 2 - just in case - I have one that is unused. I can't recall exactly what I paid, but the current price is £7.99.

That figure is not going to break many banks, and for such a key bit of equipment, why skimp on a few quid???

The components in Broadband filters are all low voltage rated, hence if there is a voltage spike on the line, lightening or even the BT guys working on the line, the components can be blown. The damage is unlikely to be visible. Normally it is an internal breakdown of the dielectrics.

Unfortunately it happens quite frequently. Hence one of the first things to check is the filter.