[Elecraft] normal K3 voltage drop on TX

[Malcolm via Elecraft]

Hi All

>From my time working in a Ham repair shop I can say that if a power supply dipped by more than 0.25 Volts between RX and 100w TX AT THE TERMINALS ON THE CASE OF THE POWER SUPPLY then that power supply wanted either replacement or repair.

I have just checked my own power supply for my K3 ( EP-925 or PS-30M badged linear PSU) and the voltage change (at the terminals on the case of the power supply) between RX and 100W TX  is less than 70 milli-volts.  This was measured using a both a Thurlby 1503HA (32000 count) or Fluke 8060A (4.5 digits) meters.  

The K3 is fed via 6 feet of 20 Amp cable and shows a voltage change between RX and 100w TX  (on the K3 display) of no more than 0.6 Volts.  There are no fuses in this lead but if there were I would allow an extra 0.1 volt per fuse giving a total of 0.8 volts.  If the k3 showed a difference of 1 volt or more then that would be cause for immediate investigation. 

Put in its simplest form a voltage drop of 1 volt at 20 amps means that 20 watts of power is being wasted. In the example quoted by the original author we are seeing a drop of 3 volts at 17-.5 Amps a loss of over 50 watts (the equivalent of 4 Amps at 12.5 Volts)!  

Even the cheapest digital meters have a 200 milli-volt range and you can use this to measure the voltage drop across each junction in the wiring to find where the fault lies.

I have cured many rigs that use the standard 6 pin connector of various problems simply by cleaning the power supply lead contacts throughout including the contacts between spade terminals and fuses and fuse holders and fuses.

As a matter of standard practice all my crimp contact junctions are soldered as well as being crimped using a good quality tool.

I hope this note provides adequate guidance.

Regards

Malcolm
G0MIC