Heat Shrink Tubing for a Fatter Pot Shaft (AF Gain Control)

Heat Shrink Tubing

by Don Cantrell, ND6T

I've read stories about people replacing the volume control/power switch that was supplied with the BITX40 simply because of the 4 mm shaft! There are  ways to adapt it to your favorite knobs.

I was in a rush to get my first BITX cased and operational so I filed that nylon shaft down to size for a knob that I had. It works, but took more effort than it should have.

A friend of mine is adept at salvaging parts from the recycling bin. I had never considered using toothpaste caps and container lids for knobs but his builds look much better than mine! Some of those lids are just perfect; fluted, colorful, and large. A bit of glue, maybe an old damaged knob for a base and you can have a one-of-a-kind creation.

My second BITX was simpler. I'm learning. I used a couple of layers of heat shrink tubing to build the shaft out to fit ¼ inch knobs.

Most of us use heatshrink tubing to insulate splices,  connector pins, and component leads in tight places. I like to layer progressive lengths and diameters to use as strain reliefs on antennas and microphone cables. It is flexible stuff, to a degree, and makes for good weather proof coverings for splices. However, it can be stiff enough to form a good hard surface to support a knob on a shaft.

Cut the shaft of the control to the desired length. The small 4 mm nylon can even be cut with flush-cut wire cutters. Then cut a length of heatshrink to match. Use the dense tubing material and the size to just slide over the shaft nice and snug. Gently heat the tubing so not to distort the nylon shaft. Let it cool before slipping on the next layer. When it cools you will note that the flat side of the shaft is still evident on the new, expanded, surface. Your ¼ inch knob now slips on with a good fit and the setscrew seats firmly and with no wiggle or wobble.

de ND6T

Pete Juliano's BITX40 Resource Page (TRIBAL KNOWLEDGE!)

You will find a lot of very useful "tribal knowledge" here:

http://www.n6qw.com/Bitx40.html

ND6T's Approach to Heat Sinking the IRF-510

Heat Sinking The PA To The Cabinet
by Don Cantrell, ND6T

I am a real advocate for keeping components cool. They last longer and are generally more stable. The heat sink provided with the BITX40 is adequate for lower power levels but bigger and better heat sinks provide for longer component lives and peace of mind. If you are going to install the transceiver in a metal box then you might consider using that enclosure as a heat sink. Whenever I have done this it has  always worked very well.

In the early stages of installation I positioned the board as close to the rear of the cabinet as was convenient but it still left about 1/4” (6mm) gap between the Power Amplifier transistor (Q15) and the back panel when the original heat sink was removed. I found a small block of aluminum that fit nicely. I drilled a hole toward one end to match the hole in the tab of that IRF510.

When the mounting spacers had been attached to the BITX board (so I could tell where the transistor was located in respect to the panel) I marked and drilled the transistor's attachment hole first. I then placed the mounting bolt through the transistor, spacer block, and rear panel to temporarily hold it before marking and drilling the four holes to the attached mounting spacers of the BITX.

When mounting stuff in cabinets a bit of extra time and planning goes a long way toward preventing mistakes (and forehead slaps with cries of “OY!”). Go slow and save big washers and hole plugs.

After the board is mounted use a shouldered nylon or ceramic washer to insulate the mounting bolt from the transistor tab which is connected to the collector. Don't let it short circuit to anything. A mica or silicone pad between the spacing block and the transistor completes the insulating job while still conducting heat. If you use a mica insulator be sure to use heat sink compound on both sides of the insulator. A silicone pad does not need it.

If you cannot find a suitable aluminum block then look for any heat conductive metal. Copper, brass, even steel. I haven't tried it but I would assume that a stack of coins (with any corrosion sanded off) would work. Considering the cost of new metal stock in the hardware stores, it might be cheaper to just use money!

My BITX output transistor now runs so cool that I cannot detect any temperature increase. Just DO NOT TOUCH THE TRANSISTOR WHILE TRANSMITTING! RF burns are worse than the damage you feel from picking up the wrong end of a soldering iron. Or so I'm told.

de ND6T

A Simple Voltage Monitor for the BITX40 (from ND6T)

                                                

BITX Voltage Monitor

by Don Cantrell, ND6T

With eight pins available for reading analog signals, the little Nano® has some enticing capabilities. More useful than an “S” meter reading, a measurement of the supply voltage is another display item that you can easily include. This will allow you to keep track of your battery status if you are portable. It could also be adapted to check on any other voltage (or voltages) that you might be concerned with. Like your PA supply if you are going to feed it with a separate higher voltage.

Looking at the wiring diagram of my VFO build you will notice a couple of resistors that are in series and placed between the +12 Volt input and ground. There is a 1.8 Kohm from ground and a 5.6 Kohm on the power feed side. Power rating is unimportant since they are going to dissipate only about 25 milliwatts. I chose these values so that it could measure up to 20 Volts without subjecting the Nano® to more than 5 volts. A safe Nano® is a happy Nano®.

Whenever measuring any voltage with the Nano®, endeavor to keep the maximum voltage to the analog pin below 5 Volts but compromise to retain the best dynamic range. The measurement involves converting the range into 1024 steps and then dividing the result by whatever number will display a reading that will be closest to what the measured voltage actually is. If you have too much range then those steps will be larger and more inaccurate.

Software implementation is wonderfully simple. Here's how to read the voltage on pin A2:

    // Read supply voltage 

    int supply = analogRead(A1);

The first line is just a comment, reminding you what the next code is about. This line is optional. The second line establishes a local variable (that I named “supply”) and specifies it to be an integer type, saving a little storage space and keeping handling simple. The “=” sign assigns the result of the “analogRead” function on port A1. If the voltage applied to that pin was 2.5 Volts, for instance, the “supply” variable is now worth 512. That's because 2.5 volts is half the maximum 5 volts and, therefore, half of the maximum 1024. Make sense?

To display that as a voltage on our LCD screen we would use this line:

            lcd.print(supply/47.7);

Why divide the supply variable by 47.7? Because that would make it correspond to the voltage that we were feeding the resistive scaling divider with. Find it by experiment. In this case you would see 10.73 on the display. Apply 10.73 volts to the top of the divider and you will see 2.5 volts on the tap where you have pin A1 attached and you will read 10.73 (or so) on the display.

Easy. Two resistors, two lines of code. You can use similar schemes to monitor any voltage or current. Rectify the voltage from a winding on a toroid that you run the antenna lead through and read output power (antenna current). Set up a VSWR bridge. Ahhh! The possibilities!

de ND6T       

A simple S Meter for the BITX 40 (from Don ND6T)

Adding an “S” meter function

 Don Cantrell, ND6T

Aside from dressing up the display (taking up room is more like it) there is little use for a signal strength indicator (“S” meter) unless it is accurate. Admittedly this implementation is not accurate, merely a nod to the notion. That said, it does work as well as a few “store-bought” rigs that I have used. Since it is simple and easy, I will pass it on for what it's worth.

Just three components, none critical. I ran a lead from the “hot” side of the volume control through a diode to an available analog port on the Nano® and paralleled a 0.1uF capacitor with a 4.7 Megohm resistor from that port to ground. The diode can be a silicon switching diode like a 1N914 or 1N4148 but a germanium like a 1N32 will work better for the lower strength levels.

The reason for the inaccuracy is primarily due to lack of the usual AGC circuitry. Normally one just measures the AGC voltage and scales it to suit. This mod is simply using the detected audio and then using a short-period peak hold circuit to feed the micro-controller.

The code is poorly done at this point but it met my low expectations. It currently resides in lines

90 through 94 in Version 1.0 of the BITX40_VFO.ino sketch. Please feel free to change it and pass it along.
de ND6T

ND6T's Suppression of a Pesky 2nd Harmonic

SECOND HARMONIC FIX

 by Don Cantrell, ND6T

No matter how I tried, I could not drop the second harmonic to make 42 dBc. Best I could get was about 39. That's not far out of FCC regulations and is probably well within the suppression required in other parts of the world. Probably it is just my particular board, the minor variations of the capacitors and inductors combining to just miss the mark. I haven't heard of anyone else reporting it but I will pass this along just in case. If you are using an external amplifier then I am certain that the filtering on that  amplifier will solve it easily. You could also use an external low-pass filter.

I ran a simulation of the filter bandpass using the AADE software. The results are shown in Figure 1(Simulation) as the upper trace. You will notice that the suppression provided at 14.4 MHz (the second harmonic) is around 26 dB. The lower trace is around 50 dB at that frequency and is the result of paralleling L7 with a 100 pF capacitor. This extra capacitor acts like a trap for the second harmonic. I find myself installing this on most of the transmitters that I design, just to save an additional filter section.

I just soldered the 100 pF disk across that inductor on the underside of the board. I don't notice any difference in operation but I am now legal. Figures 2 (Before) and 3 (After) show the before and after measurements on my spectrum analyzer.

Has anyone else experienced this problem? If so, here is a cheap and easy fix.

de ND6T

Before

After

Video of ND6T's Shuttle Tuner for BITX 40

Two days ago, Don ND6T sent us a very nice description of how he used a 10K pot to tune the AD9850 DDS in his BITX 40 (scroll down).   Don sent us a video showing how it works.  Very interesting.  Thanks Don!