[ARC5] Bypass Cap Question

[Roy Morgan]

On Nov 24, 2013, at 11:01 PM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

> It was suggested to me that I bypass all the power leads with a 0.01 uF cap.
> 
> Now, we all know that Xc = 1 / ( 2 * PI * F * C).      Since my power
> supply has about 100 uF in the filter stage, ...
> So, what's the point in adding the 0.01 uF caps to each of the power leads,

Mark,

The situation has a number of aspects that may well relate to noise/hum in the signal and to bypassing:

-Electrolytic caps are made with coils of aluminum foil - these things are NOT GOOD at rf frequencies.  In fact, some are not very good at upper audio frequencies or lower KC frequencies.  So you cannot count on them to do RF bypassing.  (I just now see that Mike has said this on the list.)  

I assume your power supply is at the end of some sort of cable and is located some distance away from the transmitter:
  - The power cable wires have some inductance and if the RF goes out the cable, it will not get bypassed at the power supply well if there are bypass caps in the supply.
  - The length of cable can act like an antenna, sending loose signals who knows where.
  - If there is stray RF on one wire of the cable, it will couple into the other wires, such as from the plate supply wire to the oscillator B+ lead.  You can see that this could cause trouble.  If you have buzzing or “hum” on the transmitter signal this could be the cause.
  - If RF is getting out into your supply cable, it might be going back into your receiver power supply line cord and B+ and causing trouble there - this can cause 120 cycle buzz in the received signal that is actually not in the transmitter output.  The RF picks up the buzz on its trip through the rectifier tube to the receiver front end.

RF will go wherever it finds a path, and there are many possible paths.

I have here a 40M ARC-5 transmitter that was operated in the 1950’s. It has a little house over the rear connector made from a tomato paste can, and separately shielded wires for the power supply connections.  I suspect there is a disc ceramic at each terminal of the rear connector (AND that the wires are soldered into the rear chassis connector!).


Roy

Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958