Digi TV distribution amp issue, cap value question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 23, 2016, 03:22:48 pm
content: This digi TV aerial amp and distribution box suddenly, after several years began passing my HF ham transmissions to all the TV's in the house, freezing the pictures or totally blanking the screens. I replaced it with a new one and the problem immediately went away. Out of curiosity, and to learn, I removed the screened tin box from its plastic case and checked the PSU caps and the PSU for noise and it was fine. I then spotted a small disc ceramic cap across the TV aerial input socket and ground, marked 0.5 I assumed this should mean it is a 500pF cap. I lifted the none grounded leg and checked it out on a calibrated Fluke / Philips PM6303A cap tester and it showed 19pFCould this be the problem and it's now too low a value to sink HF frequencies to ground?Photo hopefully attached, thanks for looking.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2016, 05:24:11 pm
content: That'll be 0.5pF. Likely your meter can't read that low, or it's reading the probe capacitance.Tim

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 23, 2016, 06:55:46 pm
content: Quote from: T3sl4co1l on May 23, 2016, 05:24:11 pmAh, right, so it's not that then, thanks for the explanation.

author: SeanB
author_rank: Super Contributor
timestamp: May 23, 2016, 07:23:16 pm
content: The sudden passing of low frequencies is more because those nasty Belling Lee connections have gone high resistance on the braid somewhere, or your ground lead has rotted away. A new amplifier probably meant you had to change to screw on F connections, so you inadvertently remade all the braid connections in removing the old plugs and sockets. This is a pretty common problem on TV cables as they do tend to either wick moisture in or simply adsorb it through the outer insulation or via a cracked cable somewhere outdoors.I would suggest replacing the cable from the antenna into the box, at least that section till you are indoors, as it will be badly degraded from sun exposure, and when you strip the outer insulation the copper will be very likely green all along the cable. Just the moving could have remade the broken braid enough to work for a while, it will come back after a few wet days.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 23, 2016, 08:06:11 pm
content: Hi Sean, the aerial and all cabling is indoors, in bone dry conditions and has never been wetted at all. You are quite right, I had to fit adapters to go from the Belling Lee connectors on the cabling to the F type sockets on the new amp. But this determination led me to try refitting the old amp and sure enough,m the problem immediately returned. I again checked its internal PSU and from just over 12V AC from its little mains transformer it gives about 16V DC. This comes out of the little single package full wave bridge rectifier into a 330uF cap, then through a dropper resistor (current limiting?) of 47 Ohms into another 330uF cap and this feeds the circuitry of the amp. The output looked clean earlier, but after it had been on a while I was seeing the attached on my scope on AC setting. Could that ripple be affecting things? The two 330uF electrolytics checked out OK out of circuit. It's all pretty academic, now a new unit has fixed this, but I am stubborn and rather than bin it would like to learn from this a bit more. Thanks!