[ARC5] PS Hum in BC-453-B (solved!)

[Brian]

Hello Bill,

Here is my take on Cat-5:
For a very long time Cat-5 was not an industry-recognised cable 
categorisation. Various cable manufacturers came up with all sorts of 
4-channel balanced feed lines to handle transmission speeds in excess of 100 
MHz, without too much crosstalk. For instance, all eight wires in a simple 
rope wind, four pairs with an inner counter wind, four twisted pairs with 
all the same polarisation of twist, additional spacing between each of the 
twisted pairs, adjacent twisted pairs in opposite twist polarisation ... all 
with magnificent marketing claims for superiority.
Then one manufacturer added a shield to overcome crosstalk between so-called 
'Cat-5' cables, especially when there are many running in ducts, and 
therefore in close proximity. This eventually became an industry standard - 
Cat-5E. Of course, if the shield is thick enough at the required frequency, 
then it doesn't need to be grounded at either end. So, the usual 8-way RJ 
connectors can be used. But, because Cat-5E is so much heavier, the RJ 
connector needs to have additional back stabilisation to support the cable.

Twisted pair is very good at cancelling out both outgoing and incoming 
induced signals - that's because the twisting makes the pair as near 
balanced as possible for a cheap installation, such as heater wiring in 
mains-powered domestic radios. With careful layout of wiring so that 
low-level signal wires run at right angles to the twisted pairs carrying 
heater power, further reduction of hum can be achieved. Dressing near the 
chassis can be a plus and a minus. If there are exactly the same number of 
twists of each wire in the pair at the same distance from the chassis, then 
the twisted pair will be approximately  balanced; any imbalance and you'll 
get radiation. But if the chassis is thin the em radiation from unbalanced 
twisted cables can go straight through and interfere with other sensitive 
above-chassis circuit components, eg, audio transformers, as in tube-based 
studio-quality tape decks.

In the end, twisting pairs of wires is only of any value for attenuating 
radiation to and from wires carrying AC - it is of absolutely no use at DC, 
except to keep wiring tidy - a cosmetic-only value. And I remember an early 
chassis wireman telling me that all wiring needed to be straight and with 
sharp, right-angled bends. Oh dear. As any EMR or EMC text will attest, 
sharp bends = very tight radius coils which resonate (with capacitance to 
chassis) and radiate at very high frequencies.

The only benefit of putting capacitors on DC leads near entry to a sensitive 
receiver, is to reduce RF that may have got there by radiation from 
elsewhere, being conducted inside the chassis. Caps across battery 
terminals? Are you trying to stop RF getting into your batteries? You need 
another cup of coffee!

73 de Brian, VK2GCE.

On Friday, January 30, 2015 9:00 PM , you said:

Hi Brian,

In private discussion Roy already said he isn't sure there is a shield.
In my limited experience watching designated staff makeup some cables I
don't recall a shield, either. Maybe Roy saw some of the shielded
variety along the way. Thanks for the additional information.

I have observed that AC heater wiring can be run as 'twisted pairs' and
dressed near the metal chassis to help against hum radiation. I looked
specifically for that in the heater wiring of my command receivers to
help identify the heater strings and it ain't there! It was built to use
DC. All of my own CAT5 cables have the RJ connectors on each end and not
very many of them. A couple of the few are crossover cables for two PCs
to communicate via ethernet without benefit of a hub. I would rather not
hack what I have here so I will try twisting the DC wiring I am already
using. I'll probably get to that this weekend. I'm retired ya know so I
don't have time to do it right now. I'll put the caps across the battery
terminals, too, and perhaps extend the earth to those as well.

73,

Bill  KU8H