[Elecraft] Notice regarding K3S current-sense resistor power rating

[Wayne Burdick]

It has come to our attention that a 50 milliohm resistor used for sensing current drain on the K3S RF board is, at least on paper, underrated for the application. The resistor is R36, used in the low-current path that supplies 12 V to everything except the KPA3 100 W PA module.

NOTE: 

    On the K3, a leaded 2 watt resistor was used.

    On the K4, a surface-mount 2 watt resistor was used.

So this issue only affects the K3S, which has a 1/2 watt resistor at this location. This was an oversight.

According to our lead repair technician, only one RF-R36 has ever had to be replaced during service, and it was in fact stressed because of a short on the board. The infrequency of failure is probably due to the conservative power rating of the 1/2 W resistor. 

I would not expect adverse effects under typical operating conditions. 

The way you would know that the resistor had been damaged is if the indicated non-100 W PA current drain had gone up unexpectedly, or had exceeded the allowed range, resulting in a HI CUR warning either in RX or TX mode. Normally a basic K3S with no options will have an RX-mode current drain in the range of ~1.0 A, while the ATU and sub-RX options can bump this up to ~1.5 A. On key-down, the indicated current can be as high as several amps. 

The only way to definitively verify a damaged resistor, if there's no HI CUR warning, would be to place an accurate external ammeter in series.

Even though we expect instances of damaged current-sense resistors to be very few, we will send a leaded, 2 watt substitute resistor to any K3S owner on request, at no charge.

If you would prefer to have us remove the SMD resistor and replace it with the leaded one, you may send your K3S back to the factory. There will be only shipping charges (no charge for parts or labor).

73,
Wayne
N6KR