If you are experiencing the following:
• HI CUR or
SPKRS
= 1 warning: HI
CUR
may
indicate a shorted left speaker channel; the K3S
will
reduce
AF
gain. SPKRS = 1
may
indicate that CONFIG:SPKRS
is
set to 2 , but a
mono external speaker
plug
is in use, shorting the right speaker channel to ground. The
K3S
automatically
sets SPKRS to
1
.
Especially, if you get this error when you turn on the
sub-receiver. Please consider that the K3s is not
measuring the current of the receiver properly. This can
be confirmed by actually measuring the current into the K3s
during receive and compare that to what is measured by the
K3s. Caution:
When doing so, make sure there is nothing connected to the "12VDC
OUT" connector on the rear
panel. If you find that the K3s is reading the receive
current higher than reality, especially
if the error is greater than ten per cent, read on.
Background:
The
Elecraft K3 and K3s measures the current going to the radio,
except for the "12VDC OUT" connector, and the current being
drawn by the 100 watt power amplifier. This is done
by measuring the voltage drop across a current sensor resistor
in series of the 12 volt feed. This measurement includes
all radio options, as well as the current drawn in
transmit by the low power RF amplifier, that is the driver
if the 100 watt amplifier is engaged, or the final
amplifier if the power power is less than about 11 watts.
The current drain of the radio is a function of what options are
installed, and what options are enabled. While Elecraft
does not publish the trigger threshold for the receive current
as observed through this sensor ( through experimentation ) is
approximately 2.2 A.
The
current sensor resistor (R36) has a value of 0.05 ohms, with a
precision of 1%. In the K3, this resistor is an axial
through hole part with a rating of 2 watts. In the
K3s, this resistor is a Stackpole CRS1206FK50L0, a surface mount
part that is rated at 1/2 watt dissipation.
The radio's current sensing circuit is shown below.
Depending on options, the low level radio circuits draws between
1.1 amps, to 2.2 amps, and the Low Power RF amplifier can draw
upwards to 2.2 A of current. Thus it is very common for
the sense resistor measuring between 3.3 A and 6 A during
transmit. A 1/2 watt resistor can handle 3.16 A. A 2
watt resistor is good for 6 A, a 3 watt resistor is good for 7.7
A. In the K3s with options can over stress the current
sense resistor. When a resistor is over stressed it drifts
high in value and can be the reason the K3s is measuring current
drain higher than reality.
Note: The radio draws nearly a constant current
as a function of input voltage, within the range of 11 to 14
volts. This is not true for the 10 watt RF power
amplifier.
Solution:
Removal
of surface mount current sense resistor and replacement with a
axial 2 watt resistor. On the K3s the current sense
resistor R36 is located on the bottom side of the main
board, on the right side, in front of the power pole
connector.
One solution is to replace R36 with an
Ohmite 12FR050 resistor. A 2 watt resistor. It is .416 inches
long, and .1 inches in diameter and could be placed between the
plated through hole near the silk screen R36 and where RFC40
lead connects, above and to the right of C225. A better
solution is to get a Ohmite 13FR050E. A 3 watt resistor
that is larger. I placed it parallel with the board
with one lead directly bent into the plated through hole near
the R36 silk screen, with the resistor going towards the
mounting screw. The the other lead was covered with
teflon tubing (or your favorite insulating material), and
connected into the plated through hole on the other side of
R36. A three watt resistor offers some headroom.
This from Wayne N6KR from Elecraft:
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