Aligning and Tuning the LM373 Transceiver


I have found that the most successful way I build transceivers is to start from sections that are easy to build and test outside of a complete radio and to also employ the "heathkit" approach where what you have tested then becomes a part of the test system testing other sections. This rig was no different!

A word here about construction techniques. I happen to have access to a CNC Milling machine (Thank You, son, Nick). With this I have been able to design stock modules using a program called G Simple that when I want a specific circuit block (audio amp, mic amp etc) I simply call up the dxf file and cut away. This made the construction go very quickly. But you can build this rig using alternate techniques and one of those is to purchase the MePads and MeSquares from Rex Harper, W1REX. Essentially my CNC programs create groupings of island squares. The W1REX pads are those same squares and you simply glue them down to a PC Board. So you can have nice looking and functional boards without having a CNC Milling Machine. Manhattan construction works too as does etching your own PC Boards.

You cannot successfuly build this rig without some proper test equipment such as a DVM, Oscilloscope and a Signal Generator. A half dim flashlight and two rusty screwdrivers form Harbor Frieght will not get you there! If you want to be a homebrewer these mentioned test items are mandatory. There are many high quality test instruments that are very reasonably priced -- $100 will buy you both a DVM and Signal Generator. A Digital O'Scope is another matter but a 50 MHz Analog scope can be found quite reasonably and will do the job. But you do need test equipmnet or you will be lost otherwise.

This is not a hurry up project or one that is built over a weekend. Figure about one to two months but that is half the fun of the hobby. By taking your time you will experience far less start up problems. Earlier you heard me refer to the heathkit approach. There was a reason why heathkkit did that -- they wanted the kit builders to be successful!

To that end there are some good common sense rules to building circuits. The first is having other good tools like a heat controlled isolated soldering iron and good quality needle nose pliers, nippers and several sizes of screwdrivers both flat head and phillips. Invest in tools and they will last a lifetime. The other factor is to build in small chunks and don't build circuits when you are tired --you'll make a mistakes --often costly.

I have a procees that before I apply any power to a circuit, I review everything I have done. Did I put in the right components in the right place? Are the soldered joints solid and not cold soldered. Are there any cross over shorts? Are there any solder splash shorts? Is the power connected with the correct polarity?

[Speaking of solder splash shorts --back in the 1970's I built a Heathkit HW32A for use as a mobile rig. I got it all built and it received fine and would TUNE with RF into the antenna --but nothing happened when you talked into the microphone. I thought maybe a bad tube --but since the HW32A had many common tubes -- I swapped the tubes around and the mic amp tube was good. Then I started looking at the mic amp tube solder connections and that is when I spotted an almost invisible hairline piece of solder splash that was shorting the input. Boom a quick heat with the soldering iron and it now worked! It happened then and can happen today.]

Back to my approach. I started first with the Audio Amplifier stage and got that working. My Easy Peasey Functional Test: once powered on and connected to a speaker put your finger on the input --loud hum and buzz --it works. Next build the Microphone Amp and you can even connect its output to the Audio Amp --same test.

The approach I used was to build the receiver portion first and add the transmitter stages after the receiver was working. Again slow cautious work will put you up higher on the success curve. Temporarily after building the main receiver board I have a stock RF amplifier test board that I connected to the main board to provide RF amplification of the signals from the antenna.

We now should turn our attention to the si5351 PLL Clock Generator. Spend some time carefully wiring this device and use the techniques suggested on the earlier page on how to build this device. After checking your wiring you now need to load the code onto the Arduino. At this stage do not install the Si5351 but only the Nano and the display and encoder. Powering the Arduino/encoder and color TFT display from the USB port on the computer will enable you to see that display is working. Move the encoder to assure that the digits will change. If that test is successful you now can supply power to the onboard 9 VDC regulator and insure the display boots up. With this test now install the Si5351 and the other switches. With your O' Scope connected to Clock 0 you should see significant output at 23 MHz. It will not be a pretty square wave. Then check Clock 2 you should see output at 9 MHz. With test we will now proceed to test the low pass filter used on the receiver.

Before making connections to the receiver low pass filter I made an initial alignment of the BPF by lightly coupling (i.e. bringing a wire lead connected to my signal generator near the input side) and then terminating the output with a 50 ohm resistor and connecting my O' Scope across the resistor and setting the generator to 14.2 MHz. Next I did the TFMS alignment (Tune For Maximum Smoke) or the max scope reading. Once that is done you can check the network by moving the generator down to 14.0 MHz and then 14.35 MHz. Note the reading at all three places --you can take even more readings and plot a curve of the response. You will find that with a bit of "twizzling" (very slight adjustment of the trimmers) it is possible to have a uniform response curve over the frequency range. Otherwise since this is a SSB transceiver TFMS for 14.2 MHz and call it a day.

Now assuming you have wired up the LM373 and the SBL-1 receiver mixer and have connected the LO and BFO plus have all of the steering diodes wired properly and finally the audio amplifier, you are not ready to test the rig. Now is the time to build the 11 VDC regulator board. The Solid State Switch will not be needed until you finish the transmit stages. This will also give you a break and a breather space so that you can recheck all the wiring and insure you have all power connections proper (and not reversed). Now proceed with powering on the rig. There are two trimpots that will require adjustment and these are identified as R1 and R2 and connected to Pins 3 and 6 (TO-5) or Pins 3 and 8 (DIP 14). See the original W5BAA article for information regarding these two pots and their adjustment.