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

[Brian]

Hello Bill,

You claim that clipping an Earth lead to the chassis always makes a difference. But you don't say what difference or how you measured it. For instance you may notice that you get:
  a.. a stronger signal or a better signal to noise ratio. That is because you have attached the other part of an (intended) dipole antenna. 
  b.. less hum on your AF signal. That may be because you have excluded a previous (unintended) ground loop.
The antenna input circuit of a Command Rx is very high impedance; connecting coaxial cable to a Command set’s antenna post will probably kill off any HF because of cable capacity - and, of course, because of massive impedance mismatch. If you must use coaxial cable to bring in your antenna signal, then use a small unun to step up the impedance to something near that of the Command set's antenna input.

In possible rebuttal to Dennis:

The likelihood of mains hum transfer from the overhead mains catenary to any length of horizontal antenna is small and calculable. However, the likelihood of antenna-borne mains hum getting through the RF amplifier, the inter-stage transformer and the IFTs, is negligible. Just look at the pass-band curve of any RF filter system. The Command Rx has 5 filter sets in series, all very well shielded for their intended frequencies of operation. Even if the out-of-band attenuation between the resonant frequency and the mains was only 100 dB, with 5 in series, the attenuation will be at least 130 dB. You would need to have golden ears to hear any hum via that path.

Please remember that the originator of this stream told us that the hum started immediately he turned on the power supply, which was well before any of the tubes had started conducting. Therefore, the hum was coming from the B+ line through the output transformer and then possibly via a leaky capacitor to the Rx chassis. All this talk of radiation and capacitive pickup is very interesting, but quite irrelevant to the problem as originally posed.

73 de Brian, VK2GCE.


On Friday, January 30, 2015 1:21 PM , Bill said: 

Hi Dennis,

The space I used to set up the batteries, the radio, all of that placed 
everything withing a very few inches of the nearest power wiring. I have 
the Earth lead clipped to the chassis and that always makes a 
difference. In the past I have always used my end fed wire right into 
the shack to the matching circuit (L-network). That isn't working for me 
any more (new QTH) and I'll be running coax (moan and groan) at least 20 
feet out of the shack and add chokes and filters and strainers, and etc. 
Maybe put some squirrel bait in the sub station once in a while to get 
some radio peace and quiet <evil grin>.

73,

Bill  KU8H


On 01/29/2015 05:24 PM, Dennis Monticelli wrote:
> I'm going to take wild guess here, Bill.
>
> Unless you are very close to a source of 60Hz field, it takes a long 
> wire to pick this energy up.  I suspect your antenna (which is 
> probably a long wire with some degree of coupling to house mains 
> wiring) may be depending up the receiver's common to supply the return 
> path to earth and that may be an ill-defined path that passes through 
> sensitive circuitry.   If this is your situation you may with to 
> earth-return the antenna right at the case of the receiver.
>
> If that doesn't do it for you then you must have a source of 60Hz very 
> nearby giving you grief.  The BC-453 is well shielded for 
> capacitive coupling but the thin non-ferrous Al case is almost 
> worthless for stopping a nearby 60Hz magnetic field from inducing 
> currents within.  Maybe your receiver is sitting on top of another box 
> positioned right over its power transformer!  BTW, oscilloscopes have 
> this problem and resort to using a high mu metal shroud around the 
> CRT.  Maybe someone on the list can fashion an ARC-5 case out of 
> mu-metal :-)
>
> Dennis AE6C