[Elecraft] K3 Rf gain tracking (main vs. sub)

[Mike Scott]

Further to the issue that my K3 RF gain on the sub Rx won't cause the RF
gain to go very low (I can still hear band noise and signals on the sub Rx
at minimum gain).

 

With the help of Guy I found the RF gain pots on the front panel schematic
and understand how they might work.

The high end of each 10K RF pot is at +5 volts, the low end is ground. The
wiper is a voltage divider which goes to a PIC on the MCU board for decoding
through an A-D converter on the PIC. The decoded pot voltage is then used
later to compute RF (really IF) gain after some translation magic happens in
the PIC and later after all the magic an analog IF gain level is created in
a DAC to bias the RF IF amp stage. At least this is my understanding with a
lot missing in between. It looks to me that there are HAGC1 and HAGC2 test
points on the two RF boards. The HAGC is the hardware AGC which is derived
from either the RF gain position Pots or H/W AGC circuitry. If I could find
these test points and measure the 0 to 3 volts expected (0 = max gain and 3V
= minimum gain) in the respective IF amplifiers I might learn that the sub
RX never goes to 3V at minimum RF gain pot setting. The 0-3 volts is biasing
a J309 IF amplifier.

 

Because it might have been possible that the ground end of the Sub Rx RF pot
was not at ground potential it was possible that the voltage signal on the
wiper was not correct and never going to zero. This might be caused by a
poor solder connection for example or just a faulty pot.

 

The RF gain pots are accessible by removing the front bottom cover plate.
Each of the RF-gain pot connections can be seen by inspecting the location.
With a volt meter and tiny hook probes I measured the wiper voltage on the
main and sub RF-gain pots at three gain settings: full on (CW), full off
(CCW) and 12 o'clock.

 

Pot         full on                   full off                  12 o'clock

Main      4.682V                 .002V                    2.36V

Sub         4.682V                 .002V     `              2.40V

 

It looks to me like both RF-gain pots are doing exactly the right thing.

 

So, now I am down to one MCU PIC A/D port is not working properly, a bad
solder connection between the wiper arm and the PIC or Wayne is not decoding
the two pots the same way in my serial 506 K3 or the main and sub Rx are
treating the decoded RF gain positions differently or some other gain
calculation is going on to modify the gain requested by the RF gain pots. I
don't know how the variable RF gain setting is accomplished once the PIC
decodes the pot position. Since there is no real current draw through the
Pot I don't believe a bad solder joint is causing a partial voltage to
arrive at the MCU PIC, it should either work or not.

 

To the person who has weird main receiver RF gain action the test I did
could help you determine if it is the Pot. As you are looking at the pot
with the bottom front cover off put a volt meter on the middle of the three
pot connections, the ones on the mic connector side of the pot, the other
side of the meter to a convenient ground. You should be working with the
most accessible or bottom-most pot. With K3 power on move the main Rx RF
gain and check to see that voltage moves from ~5v to 0v smoothly as you turn
the pot counter clockwise. I used mini hook grabbers to hook up the meter
before turning power on and verified that pot resistance to ground moved
properly when gain was moved. I didn't want to have a slip up and short
something, this verifies I was connected to the wiper arm of the pot. You
can't really test the full range of the pot in circuit with power off with
an ohm meter as the high side shows something like 900 ohms resistance to
ground with power off.

 

AE6WA

Mike Scott

Tarzana, CA