New SOTA Rig

The new Elecraft KX2 arrived last week.  So far it looks like just the thing for SOTA activations.  A battery pack was not ordered for it, so one was constructed from 3 x 18650 2500mAh cells with a protective polyfuse.  A 3 cell protection module is on order from Ebay to complete the battery pack.

An action camera case from Target ($6) is just the right size to house the radio, mic and a few small bits and pieces.

Looking forward to taking it out to the mountains.  Its quite a bit lighter than the FT-817 at 380 grams without batteries.  The display is big and easy to read compared to the squinty 817 screen.  The radio case has a few gaps here and there around connectors and joins, so it will need to be protected from the elements carefully.

So far, the radio performs nicely, the controls are well laid out and fairly intuitive.

The microphone for KX2 is pretty expensive at nearly $100 Aus.  Mine had a something rattling in it so I opened it up to have a look.  The rattling turned out to be a blob of solder.  While apart, I noticed that the braid wire from the cable to the internal connector was very tight and might break away if the curly cord is tugged.   There are some reasonable gaps between the PTT pressel and the shell sides, and it wobbles sideways a bit.  The mic. lets the radio down a bit in my opinion.

As its hard to avoid things getting wet with SOTA, so I wired up another smaller microphone.  This mic is well sealed with a gasket between the shell halves and the PTT switch completely sealed from moisture ingress.  The PTT switch strikes a rubber membrane with the switch is on the other side.  Being a smaller mic., it will pack better as well.    The schematic from this page was followed, although my mic. had a 2k resistor and 0.33 uF cap in parallel.   The level from this mic is much higher than the original, so a 10k resistor was placed in parallel on the cord side of the mic wiring.

 

 

 

 

 

The audio from the small mic seems OK, although I did add a piece of fabric behind the mic aperture behind the case to reduce popping.

The battery pack was built with 3 x 18650 2500mAh cells.  In this case tagged 18650 cells were used.   I have had good service from Samsung 18650 cells from Ebay.  There are many dodgy cells out there though.

The 3 cell protection pcb for the cells was obtained from Ebay for $4.00 delivered.  The pcb limits the charge and discharge current and voltage to safe levels for the cells.  After the cells were connected in series, double sided rubber tape was put over the exposed cell ends.  The protection pcb was stuck to one end of the pack to the rubber tape and wired up.  Silicone wire connects the power from the pcb to the DC connector.  The silicone wire is very flexible and wont melt.  Some Nitto tape holds the cells together until the heatshrink will cover the pack.  In the mean time the pack is going to get some charge and discharge cycles with the cell voltage monitored to make sure the cells stay in balance.

Note that if you make one yourself, you are doing so at your own risk.  Utmost care has to be taken to ensure no possibilities of a short circuit can occur in the wiring, during assembly and in the completed pack.  A short circuit inside the radio could cause extreme damage to the radio, not to mention a fire risk.  Photos below show the pack before heat shrink applied.

 

 

 

 

 

Just to check that the protection PCB was working, I put a dead short on the output and as expected, nothing much happened.  For another test, a potentiometer was put between the battery negative and the junction of the second and third cells.  The slider of the potentiometer was connected to the PCB input for the junction of first and second cells.  This allows a wide voltage simulation of the input to the PCB for testing how it behaves when a cell goes high (above 4.2V) or low (below 3V).

With the pack charging, the pot was slowly rotated from 4.0V to 4.3V while watching the meter on the slider.  As expected, as soon as the voltage went higher than 4.2V, the charging terminated.  Also, when then voltage went below 3V, the charging terminated.

At least I know if the cells go out of balance as the pack will not charge.  The charger used with the pack is a Hyperion RC charger.