ND6T's RF Gain Control (with "Tombstoning" and Feng Shui)

RF Gain Control

by Don Cantrell, ND6T

I've grown used to RF gain controls, especially on my home brewed receivers. Often it is the only gain control and serves to adjust audio output as well. When I first put the BITX on the air I called a friend who lives within shouting distance (literally!) for a test. That's when I began to miss that RF gain control on the BITX.

During my experiments with the analog VFO I installed an additional pot on the front panel that provided a precision 1 kHz tuning. The new DDS system, of course, didn't need that knob and it became available for re-purposing. What better use than a RF gain control?

My plan was to insert a 10K ohm variable resistor (rheostat) in series with the power feed to Q1, the receive RF amplifier. Original design integrity is maintained by placing it between R15 and R16. This was accomplished by “tombstoning [1]” R15 (10 ohms) on the pad next to the supply via. I attached one lead from the control to the top of the (now upright) R15 and the other lead to the (now vacated) solder pad. Done! That's it!

I now have my beloved  RF control with about 17 dB of adjustment range, and the feng shui of the front panel is restored.

de ND6T

[1] “ Tombstoning” is the term for tipping up a surface mount component. It now stands like a headstone over the site of its original position. This is the normal method of opening a circuit during trouble shooting. Very handy and one of the many reasons that surface mount construction is so much easier to work on and modify. Thanks Farhan! The BITX40 is beautifully laid out large, logical, and well-spaced. A true joy to work with. Those relatively large components make it easy compared to through-hole.

ND6T's Amazing Shuttle Tuner (DDS)

 SHUTTLE TUNING

by Don Cantrell, ND6T

As much as I love the native BITX40 VFO with it's clean signal and simple design, our rural mountain home has room temperatures that vary widely and the VCO just proved too much for me to stabilize to my taste. I caved and raided the junk box for a Direct Digital Synthesizer solution.

Finding an Arduino Nano®, an AD9850 board, and a generic I²C  1602 LCD display, I cut a slot for the display in the front panel of my Radio Shack® #270-235 cabinet that I found in a swap meet (5 ¼”W  X 3” H X 5 7/8” D) and mounted it with 4 short stand-offs.

I used a small individual-hole-plated piece of vector board to mount the Nano® and DDS board using IC sockets. A 7805 regulator was then soldered to small piece of un-etched printed circuit board and   glued to one edge. A hole was drilled through both for a support bracket. I also mounted the filter capacitors along with a couple of resistors for a voltage monitor feature. This daughter board is now bolted to the rear panel of the cabinet with an “L” bracket, therefore using that previously wasted space and also heat sinking the regulator (as if it needed it!).

Not having an encoder, I decided to use the original tuning pot in a “shuttle” arrangement. (That was what we called the control on video editing boards back in the 1960's). Instead of spinning the knob, you twist it one way or the other. In the center position, it is idle: No tuning, the frequency is stable. Turn it clockwise slightly and the frequency starts to slowly increase. Counterclockwise, the frequency decreases. As you twist farther it tunes progressively faster. No buttons, no pressing. Tune any frequency in mere seconds and in single Hertz steps when you near your target.

It's like having a power tuner. The amount of twist is like the amount of throttle. I have mine set up so that it takes a leisurely 15 seconds to tune from 7.0 to 7.3 full throttle but I can still tune down to the single Hz frequency. That's actually much faster than tuning any commercial rig when set to single Hz resolution. Turn to the 11 o'clock or 1 o'clock position and slowly scan for signals. When I get around to it, I intend to install spring loading to provide haptic feedback on the knob. In the meantime I display a tuning indicator after the frequency reading.

One big knob. No clicking, no pushing, no range switch. Best of all, no boggle like you get if your knob is not-quite-in-the-middle of a stepped value. It is one big range with the tuning speed adjusted at an exponential differential rate. If you don't like the rate or the idle zone size, just plug in the USB cord from your computer and change it to suit. Create band limits. It's an Arduino®!

Although Shuttle Tuning requires just a few lines of code, this blog isn't probably the place for it. If anyone is interested in reading my poor programming style they can email me at nd6t_6@yahoo.com  or at ND6T@ARRL.net  and I will gladly attach a copy of the simple software and answer questions.

Yesterday I ordered a second BITX40, this one with the Raduino. I can hardly wait!

de ND6T

Hacking the Bitx40 with a homebrew Si5351 with OLED Display.

Hacking an earlier version of the Bitx40 to include the Si5351 LO and an OLED Display.

I purchased my Bitx40 early in November 2016 before the "Digi" VFO was available from hfsigs.com. This hack replaces my original hack that used the AD9850 and Pro-Mini I initially installed  with a Nano/Si5351 plus Black and White OLED Display. The code for my OLED/Si5351 can be found at http://www.n6qw.com/Phase7.html  The current configuration of the Bitx40 being offered would obviate the need to build this hardware.

 

 

73's

Pete N6QW



Switch an outboard Linear Amp from your Bitx40

Adding a Linear Amp Hack to the Bitx40.

The Bitx40 is an amazing platform and VU2ESE has even included an option to select the power output level simply by changing what voltage is fed to the IRF510. This easily done using the on board power connector that supplies "juice" to the IRF510. At 12 VDC you have the typical 5+ watts (more like 7 on this end) or by upping the voltage at this connector to 24 VDC the Pout is now 20 watts. However at the higher power level you will need to increase the IRF510 heat sink size (significantly). Bill, N2CQR has done this and you can see this at his blog http://soldersmoke.blogspot.com

 

Here is an option to keep the Bitx40 at the normal power level (12 VDC) but switch in line an external linear amplifier. It takes but few parts and the modification is totally external to the Bitx40 board. I have an SB200 Linear Amp (heathkit workhorse) and with the Bitx40 as a driver the Pout is 100 watts. More than QRP but really great when 40 Meters is somewhat marginal.

 

The theory behind this hack is the panel mounted Push To Talk (PTT) line is modified to include two isolating diodes. The diodes essentially separates two functions, where one line, which connects to the PTT1 connector on the board puts the Bitx40 into the transmit mode and the second line engages a small reed relay to provide a closure to ground to turn on your external amplifier. The parts required include two 1N4007 isolating diodes (yes a bit of overkill but in the junk box) a 1N4148 "snubber" diode across the reed relay coil, a terminal strip, the 12 VDC reed relay (low current draw on the coil ~ about 10 ma) and a panel mounted RCA jack. You will need about two feet of insulated wire to make the connections.

 

The actual hardware installation part includes using a small piece of scrap PC board that serves a base for the Reed Relay. Essentially I super glued the relay upside down to the PC board and then mounted the board to my case (10X6X2 metal chassis). For the ground side of the linear amp contact closure, I simply soldered a wire to the copper PC and the other end to the reed relay. The other end of the relay has a single wire (blue) running to the rear mounted RCA jack. In my initial install of the Bitx40 I used a 5 lug terminal strip for the power connection/power switch. That only used up 3 of the terminals so the remaining two lugs were perfect for the linear amp switching hack.

 

Whether the linear amp is in line or not is controlled at my linear amp itself. An additional hack might be to include a panel mounted toggle switch at the Bitx40 that would "open" the amp control line. Since I had a bypass capability at my amp I did not install the additional toggle switch.

 

One advantage to this hack is that does not require you to have 24 VDC plus bigger heat sink to get more Pout. I guess another advantage is that it keeps the rig pure at QRP levels especially when you feel guilty about being QRO. There is also the advantage that with a bit of padding on the Bitx40 Pout, you could drive an LDMOS amp to 600 watts out. It just plain gives you a few more range of options.

 

73's

Pete N6QW

 

 

 

N6QW 1st QSO, and Hacks for the BITX 40

I have my Bitx40 on the air and all seems to be going well. Good reports just that the signal is not 40/9.  

• I suggest have two wire bundles with a red and black wires instead of the brown black. As this would prevent an accidental connecting the right wire to the wrong connector. In the board I have the PTT header is right next to the header that is the main power to the board. If you got those two connections wrong say after removing the connectors and re-installing them a lot of magic smoke.
• Provide a note that if you hook up a homebrew DDS (and not your board) to be sure to include an isolating cap, as you are feeding directly into the base of one of the transistors.

Here is a video of the 1st Contact on the Bitx40.  https://www.youtube.com/watch?v=DWe5vFpX9fY

73’s

Pete N6QW

A more stable Bitx40 (fun with ceramic resonators)

Peter VK3YE has a very neat hack for the BITX40

He explains :

If you don't wish to add a DDS to the Bitx40 (eg need low current consumption) I now have an alternative.  A cheap as chips 4.92 MHz ceramic resonator can be substituted for L4 in the VFO and be pulled by over 100 kHz.  I discuss and demonstrate the ins and outs of this and other VXO approaches in this video. 

Troubleshooting the BITX40

Here is how you can troubleshoot your BITX40 board's receiver with just a piece of wire and a Volt-Ohm-Meter. Keep a print out of the circuit handy while you troubleshooting it.