The AM Forum

Let me try and make this short.  

I use a Belar AMM-2B mod monitor for Globe King/Champ class radios.  I built a VERY simple power tap, which was nothing more than a 10pf cap on the coax center conductor in series with a 5K variable resistor to allow some control over output level.  This has served me well for several years.

I decided to move things around at the station and put the mod monitor at the main operating desk so I wouldn't have to look behind  me to see what was going on.  In the process, I increased the length of the cable from the tap to the monitor from 3' to 12'.  (Can't move the tap close enough to keep the 3' cable).  Max voltage from the tap decreased to well below the 5-10V required for the monitor.  

Sooo I says to myself, I'll just increase that 10pf to something bigger.  I had a 33pf so I paralled that across the 10.  Power on, plenty of voltage for the monitor.  Then I noticed a stinky aroma and using my rather prominent proboscis traced it to that 5K pot -- which can't handle the additional load.  It appeared to "burn" at the set point.  My presumption is that the added capacitance of the 12' cable is causing the problem.

Can anyone suggest a more robust and less odiferous tap design?

Thanks ... Grant/NQ5T

Problem solved.

Made a couple of RF voltage measurements and treated the coax run from the tap as the shunt leg of a capacitive voltage divider.  

Working fine.  It always helps to post something to get the brain working, finally ..  :-)

Grant