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

[Bill Cromwell]

Hi Brian,

Well ya know - I am running DC inside my heater wiring from batteries. I 
don't really think I'm trying to prevent my DC from radiating. But the 
hum that I can barely hear if the antenna is attenuated (detuned) and 
the gain is turned down is AC that I would like to not 'receive' via the 
heater wiring (reverse of radiating). The heater wiring extends all the 
way to the battery terminals. Part of that wiring is haphazardly run 
through open air. The radio is over here. The batteries are over there. 
The pair of batteries for the heater is as big as the radio. The B+ pyle 
is not as large but won't fit inside the radio.

Caps across the batteries? As the batteries discharge the internal 
resistance increases. That can cause all kinds of crap like 
motorboating. The internal resistance change is how we can monitor a 
battery charge. It's why taper charges work automatically without any 
microprocessor. It can cause disturbances in operation if not accounted 
for. The caps short out any signals that might develop across the 
increasing resistance of the battery.

If I remove that Earth from the radio chassis large amounts of noise 
come crashing into my cans. How did I measure it? With my ears. No Earth 
- can't hear signals - only loud, grinding, buzzsaw noises. Earth 
connected, noise to the far background and easy to copy signals to the 
foreground. In my amateur mind that is rather conclusive. Maybe I got 
that wrong.

The OP is still very much in the conversation which is still about hum 
in the audio output. And did you notice the hum in mine is easily 
overridden by signals and even by normal band/antenna/air noise? The hum 
I have is not a real problem but it is still there and so I have pointed 
it out. Possibly a small amount (it *IS* small) is being picked up on my 
(external) wiring. It won't be hard to determine. Caps and twists - 
enter stage left.

Cat5 not being real is no surprise. The marketing people have no 
credibility any more. Instead of CAT5 maybe we should say what it really 
is. "Four twisted pairs of light gage, insulated, wire packaged inside 
an convenient,  fire retardant, outer sheath usually found carrying 
network signals". Or maybe "CAT5" will be sufficient these days. Whew 
<evil grin>. An interesting bit of history.

And finally and off topic comment. Hark! Super Bowl approacheth. Enjoy.

73,

Bill  KU8H

On 01/30/2015 07:07 AM, Brian wrote:
> 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.
>