[ARC5] PS Hum in BC-453-B

[Kenneth G. Gordon]

On 30 Jan 2015 at 21:54, Brian wrote:

> The antenna input circuit of a Command Rx is very high impedance;

I thought at one time we had either calculated that, or found it in a reference, 
and that the value was 4K ohms. Am I in error?

> 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.

Have we yet come to the design of such an un-un? I think not.

I would love to see a practical example of a 4K to 50 ohm un-un.

The way I have operated is that if the front panel "Align Input" makes a 
substantial and obvious difference in signal strength (as noted by ear), then 
the match is "adequate". Am I wrong here?

> 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.

Yes, therefore, the hum "path" must be something in the AF system.

> 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

Not necessarily: in my experience such hum can come from such an unlikely 
source as the connections at the AF gain pot, if such exists. In his case, his 
hum may very well be coming from some source on the output side of his 
audio transformer.

> All this talk of radiation and capacitive pickup is very interesting,
> but quite irrelevant to the problem as originally posed.

Well, I agree with this statement...

Has Les yet determined the source of his hum?

None of my receivers exhibit noticeable hum output, even at extremely low 
audio output levels.

But, of course, I am using "Low-Fi" military headphones, either ANBH-1 or 
R-14 based 'phones.

The only time I attempted to use modern hi-fi headphones with any of my old 
receivers (in this case, a Heathkit HW-16) 60 Hz AC hum was VERY 
noticeable. When I switched back to my ANBH-1-based 'phones, there was 
none to speak of.

Ken W7EKB