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Originally Posted by Monkfish
I just put a scope between my PC ground line (case) and the screen of the NTL cable (disconnected), and it shows a 50Hz 4V p-p noisy signal with 2V spikes. I didn't measure the signal power (source impedance) but would guess it was higher than for simple cross-talk pickup. In fact even just connecting the negative lead of my scope to the NTL screen was enough to produce the buzzing on my powered speakers, and that was with the PC and modem turned off!
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I guess there could be a lot of earth loops, the PC case is often a problem in itself. I know from experience of watching someone else (fortunately) carrying out a PAT test on a PC and blowing the Graphics card. It may be worth checking the point you are connecting your scope to on the PC is a good earth point, and not one of the plug in card plates. (I appreciate you probably know that already)
It would also be worth considering the point you metioned about making the measurement with the ntl cabel disconnected, I know from amateur radio experience that a HF diploe can have hundreds of volts across it when theres a thunder storm about, I used to often connect my old Avo across and be suprised. I'm just thinking what a good aerial all those customer drops could be if the cabinet is not properly earthed. I worked in the South Wales ntl franchise area, and the local electricity supplier was fussy about earths. There was a copper plate under the cabinets, earth spikes and sometimes they still had problems passing them. I understand some electricity suppliers were not so fussy and did a visual inspection, perhaps your area is not that well earthed at the cabinet.
Remember the amplifiers are run from 65V AC fed down the coax with the RF, so there will also be a certain amount getting past the line injection choke/cap filters. I have even come across distribution amplifiers with fuses in the output port feeding the customer taps, with some types of customer tap banks the RF splitter/transformer on the input would burn out, but I think some would probably keep themselves warm for years!
A bit like the 18 volts being fed through a 6dB resistive attenuator I found in a cable TV headend one day, I found it when I burned my fingers on it.
