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

[Bill Cromwell]

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

On 01/29/2015 11:45 PM, Brian wrote:
> Hello Roy,
> Let's be very careful about broad-band, unqualified statements.
> Cat-5 cables do not have shielding. When Cat-5 cables are run in ducts 
> containing many such cables, all working at 100 MHz plus, eg, between 
> computers and servers, then  the shielding associated with Cat-5E has 
> some beneficial effect. Computers connected to Cat-5 are almost 
> totally insensitive to mains frequencies (50, 60, 100 or 120 Hz). That 
> shield on Cat-5E really only works at VHF. At mains frequencies on DC 
> lines, it is nowhere near thick enough to have any measurable effect 
> except on your bank balance and the country's Gross National Product. 
> The shield may have a slight effect at mains frequencies if you use it 
> for such low frequencies as on audio level 600 Ohm lines, say, for 
> your home lo-fi. Any good quality EE text will wise you up on skin 
> effect - even as far back as the 1st edition of Terman (1932). But 
> really, at those frequencies, twisting has the most beneficial effect. 
> At DC, twisting has no effect. Any recent text on EMR and EMC will 
> help here.
>
> 73 de Brian, VK2GCE.