WØBA
Last week we created the environment and populated it with tools and test equipment. After that, it makes sense to consider just what we want to build. Some kits are useful, some are complicated, others easy. They all have one thing in common; They can be built by you and will provide a showplace for your personal craftsmanship. There are kits that provide blinking LEDs and others that are complete transceivers. These are all within your ability, if you break them down into small, testable modules, or this has been engineered into the kit. The bottom line here is, it makes sense to start small and build to larger kits and projects but don’t avoid a larger kit that is designed and tested as a collection of smaller kits. Elecraft kits come to mind in this category as did (a moment of silence here) Heathkit. In fact some of the Heathkits are still in service when similarly functioning devices of the same time are often lost in history and landfills.
I like VHF/UHF/SHF so a transverter is a good project to work with. The 222MHz band has very few options in pre built transceivers and transverters and not a lot of kits. This is a band that could use more operators and there is a new transverter kit that will provide us the opportunity to expand our kit building skills as well as have an extremely useful device when the project is complete. So, there we go. The decision has been made. We will build the 222 to 144MHz transverter kit recently released by W7BAS (Google it to get yours). This kit has only 50 or so parts and has been successfully built and operated by Dave (W6OAL) and me. Generally I build a couple of kits of any given type if they are sufficiently inexpensive and useful. This kit qualifies.
I have my kit in hand so the temptation is to jump right into building it. However, it has very small Surface Mount Device components and I need a review on my SMD soldering technique so let’s do that first.
SMD Soldering Technique:
Typically the SMD components are supplied in small capped vials if the kit has been designed with the builder in mind. Others are provided on strips with individual pockets enclosing the devices and plastic covering a number of them. Either form is workable but just be careful when opening packaging that you avoid sudden movements that may launch your components all over the room.
The first order of business is to get a damp solder sponge, this is important. You must keep your soldering iron tip very clean and the damp sponge is perfect for wiping the tip of your soldering iron. Throughout the solder process clean your soldering iron, and give it a few seconds to recover from the temperature drop that the sponge will cause.
If the kit Printed Circuit Board is well made it will have pads for each contact of a SMD to be soldered to the board. Equip your soldering station with the small (1/16″) chisel tip and set it to 700 degrees. While the soldering iron comes up to temperature clean the PCB with a rubbing alcohol wetted Qtip or small lintless cloth. Locate your SMD component and place it on the board somewhere near its new home. With your iron up to temperature and using the 0.015 diameter 63Sn/37Pb P2 flux core solder “wet” the most convenient solder pad with solder. Wetting is not a gob of solder. It isn’t even much of a bump. It is just enough solder to allow the component to be placed in position and held there without much lift off the PCB. When one side of the PCB component mounting pad is solder wetted find that component you put somewhere close to the target spot and put it in place with one side resting on the wetted pad. Align it as perfectly as you can with tweezers and dulled toothpick. (What a dulled toothpick is and why; If we use a nice new and sharp toothpick to move our components around the board we will eventually launch a SMD into that place where components go when we cannot find them. Cut the point to just smaller than the component you are working with and carefully round off the end so it can be used to push components around without launching them.) Now, holding the perfectly aligned component in place with the toothpick, and doing our absolute best to keep it still, reheat the previously wetted connection pad until the SMD sinks down on the pad and the solder reflows enough to hold, or tack, the component in place. Remove the soldering iron from the work and hold the SMD in place 5 to 7 seconds to allow the solder to cool and solidify. Reconfirm the alignment and placement of the component. Now orient the board so you can solder the other end of the SMD. Again using the 0.015 diameter solder place the solder at the junction of the SMD and the PCB and apply the soldering iron to the same junction. The solder will melt and attach itself to the SMD and the PCB forming a “fillet”. A fillet is a small fill where the solder attaches itself to the SMD and the PCB and forms a concave finished solder joint between the two making them one. It is a beautiful transition between the two entities and has no solder “bump” associated with it. Now proceed to the other side which is “tacked” in place and similarly solder that side. This whole process including both ends used less than 3/8″ of solder off your roll. Here is a picture of a good SMD solder joint.
Multiple contact SMDs are similarly soldered and if you have a choice don’t tack the input of active devices. Output or ground connections are less likely to be damaged by soldering heat and a possible static discharge.
By the way, I know this is contrary to the method of soldering non SMD devices where you heat the junction and then flow solder to the joint. The reason for this is the SMD components are so small they heat to soldering temperature very quickly with a 700 degree iron and the solder can be simultaneously applied with the application of heat. Also, this is not the only way to solder SMDs. It is simply the way that works best for me. If you have a better way, use it.
Well, it’s about time we started to build our kit. Let’s check out the package. My kit came with two 8.5X11 documents and a baggie of components and PCB. Before we do anything else we inventory the kit to assure each and every part is there. This does more than just assure we are not missing components. It familiarizes us with the component sizes and shapes. We have now checked off the components on the parts list and are satisfied we have everything we need to start the build.
Here is a picture of the kit, PCB cleaned and ready for assembly. The soldering station is heating up and my solder sponge is at the ready.
I generally start at the upper right side and work top to bottom in sections. A journey of a thousand miles starts with a single step so let’s just do five components for now. . . . The first five components went in with no problems. They were a few resistors, capacitors, and a filter. The filter has a red mark on it that is supposed to be down and toward the input. That can cause a bit of uncertainty as when the red mark is down, you can’t see it. You are not absolutely sure that it is facing the input after a little bit of positioning. If you check it to assure you have the red mark toward the input you need to position again and this could be a stressful situation you don’t need. Simply use your sharpie to put a dot on the top side of the component at the input end and the uncertainty is solved. We are more than 10% complete on the PCB build. And, more important, we have momentum. The project is truly underway.
The mixer is installed with the dot on the lower left side when viewing the board such that you can read the labeling normally.
This is a good time to determine how much power you will be driving the transverter with to allow you to select the proper input attenuator. I use a half Watt of drive maximum so R4/6 were 50 Ohms and R5 is 787 Ohms. The kit is supplied with resistors for 5 to 30dBm. Select the appropriate resistors corresponding to your IF TX drive level. I chose the 1/2W configuration and in my kit R4 and R6 were stacked 100 Ohm resistors to provide a total of 50 Ohms and have the ability to handle the power they are to dissipate. R5 is also a physically larger resistor for the same reason. A bit more solder is needed to accomplish correct installation of these resistors. Try to maintain the ideal concave solder though. Diode D1 is installed with the mark to the left.
After several more components you will begin to have difficulty keeping them straight. Solders will take a bit of rework to make them look right. This is break time. Take a real break. 15 minutes to a half hour is good. Do something like stepping outside or getting a bite to eat.
We return from our break ready to roll and find our place on the board to continue soldering the components in place. Diode D2 is also installed with the mark to the left toward the 5V regulator. Soon we have all the SMD components installed and inspected. All have good solder joints and are in their proper places when compared to the parts placement diagram and the component list. Now we get to install the larger components. Place the large filter in its holes. It may take a little bending of leads to align them so be careful and slow while doing so. Now that the filter is in place find that roll of larger rosin core solder and resetting your soldering iron to 800 degrees, solder the large ground connections. Set your soldering station back to 700 degrees while you clean the rosin flux off the board. Now with the smaller diameter solder attach the smaller pins on the filter. Install and solder the two relays that go on the bottom side of the board. Finally, install the local oscillator on the bottom of the board. If the alignment mark has rubbed off examine the oscillator and find the ground pin. Install the oscillator such that the ground pin connects to ground and you will be good to go.
At this point we have a populated PCB and should start thinking about how to test the transverter. Yes, it is now a transverter, not a collection of parts with potential. Examine the board one more time to assure we have components in the right place and the solder joints are good. This is the best time to find an error and fix it. When all is as it should be we can test the transverter. After inspection take a well deserved break.
Testing;
OK, we are all rested and have a transverter that has been visually inspected and double checked. Often, in a complex project I will check voltages and currents to assure each section is working as expected. However, this is a rather uncomplicated build that will either work or not. Let’s go for an actual functionality test first. We need a power input cable assembly. I use PowerPole connectors for this function and have a fused supply so a simple +/- pair for power is installed. We will be testing receive first so an SMA (f) jumper is tack soldered to the input and a BNC (f) pigtail to the IF output. This matches my test setup. You would install appropriate connectors for your configuration. If you had a SO239 on your IF you would use that, etc.
The IF is powered up and attached to the output of the transverter. Then the signal generator is set to its lowest output level (-137dBm) and connected to the RF receive input of the transverter. DC power is next. Everything is connected and it is time to flip the switch. Will the smoke be retained in the transverter and will I hear the raggedy sound of my signal generator? Will I have to crank up the level to hear? Now we will know. The fused power supply is turned on and. . . . there it is. The characteristic sound of my signal generator at 222.1MHz. The receiver is working and working well. Less than -137dBm is a darn sensitive receiver. We are on a roll so let’s test transmit. Hmmm, need PTT and another jumper. . . back to the bench. A second SMA jumper was attached to the TX output port and connected to a power meter and dummy load. The PTT was connected to ground and the IF transceiver connected to the IF port of the transverter. Time to see if we have TX power. 0.5W at 144.1 MHz yields 0.09W at 222.1 with a frequency error of 300Hz. I’d say that is in the ball park of +20dBm output (19.45dBm) and will call it good. I’ll check current consumption and a few voltages and take a break. It is now 12 and I have been working on this kit since 8. I can rest easily knowing my project is a success at this point.
Here is a picture of the completed PCB with test jumpers soldered to the board and PTT in TX position.
