Many years ago, probably in 2001, I took on a substantial kit building project. It was in the early days of Elecraft, that they were selling the K2 transceiver kit. The venerable K2 is a fine little transceiver, and what makes it really special is the fact that it was built piece by piece, component by component, with the parts numbering in the hundreds. When completed the owner had a rig that could put out nearly 15w (although most operate the radio at QRP limits), and they are still very popular today.
My K2, serial number 1427, has sadly entered the low point of the sunspot cycle wondering how to get those QSO's done with only 5w of output power. In light of this, I purchase some "boots" for the K2, a Motorola designed AN-762 Amplifier.
The AN-762 is available from Communications Concepts for $153.85 as of November 2018. I purchase my kit in August 2017, but it sat on the shelf in its box until September 2018. The amp is capable of putting out 140w before installing the required low-pass filters. There are losses due to the filters, so my version of the AN-762 puts out about 115 - 120 watts on 20m and 130 watts on 40m. I have not added low-pass harmonic filters for other bands yet...more about that later.
Building the AN-762
The AN-762 is not exceedingly difficult to build, but you do want to keep in mind several key parts of the process. Let's start with the Motorola application note on the amp. You MUST read this note thoroughly. Don't just skim it. Same thing with the Construction Hints from Communications Concepts; read them often and again before turning on your soldering station.
The very first steps of installing parts are slightly vague, and here is part of what you need to know that is not exceedingly clear from the literature. First, the output transformer T3 (the large binocular transformer in the photo), must have non-conductive paper or mylar under it to prevent contact with the board. On top of that, the T3 transformer has a side up and a side down. You want to make sure the T3 tubes are above the board, so the side up orientation leaves space between the tubes and the board.
You will also have to solder in small surface-mount capacitors with this project. Be CAREFUL with them. Don't remove them from their bags until you are ready to solder in place. Also, do not apply heat directly to the SMD capacitors; instead, form a bubble of solder and push it gently to the ends of the caps to make contact and then take the soldering tip away. Probably the trickiest part of the project is to solder these SMD capacitors in place.
You will need to mount the board on a large enough heat sink. This will require drilling and tapping 4-40 holes to mount the board and the MRF454 final transistors. If you are not good at tapping holes, then get help from someone who is. In fact, at any time while building the amp, don't be shy about asking for help, because it is better to ask for help than to ruin an expensive kit.
Once you have completed assembly, you will need to do the tests that are written in the Construction Hints. Follow the instructions. You must set the bias supply voltage properly or you could destroy your expensive MRF454 finals.
When you are ready for testing, make sure the amp is mounted on the heat sink. Let's not cook those finals and ruin your day. Clear the area and make sure there are no loose bits of conductive aluminum or other metal shards anywhere near the board or anywhere in the testing area. Run the amp to a wattmeter and then to a dummy load. From there you can check the output power and if you were careful, then you should be able to see 140w on the meter as you increase input power.
Congratulations, you are passed part 1. Next is setting up your amp in a cabinet and adding low pass filters.
Part 1 of Final Assembly
The AN-762 mounted in the cabinet
A significant amount of work is required if you want to mount the amplifier into a proper cabinet. On top of that, if you intend to use a K2 as the exciter for your amplifier, you will need to built a relay circuit as a T/R (transmit-receive) switch. In the photo above you can see the amp board on the heat sink, and also the relay board mounted to the left of the heat sink. I will discuss the relay circuit and the cabinet in the next segment. Suffice it to say, the relay circuit needs to be properly built and installed. This includes testing the relay BEFORE installing it and using it to key the amp. Towards the front of the cabinet you can see the 20m filter.
The cabinet itself is homemade. It is 1mm sheet steel which I bent in my shop. If you are not equipped with metal working tools, then you may want to buy a cabinet. In my case, I originally made a cabinet out of wood, and then decided to use a metal box instead. Unfortunately, this was after spending a lot of time building the nice wood box. So, I have a completed and nice wooden cabinet with no purpose sitting on the floor here. Oh well.
Next time, I will describe the relay circuit and putting it all together. Here are some links for you to explore in the meantime:
http://www.communication-concepts.com/an762-140/
https://elecraft.com/products/k2-the-classic-hf-transceiver-full-kit
https://www.youtube.com/watch?v=fgfw4sqJHx0
http://www.communication-concepts.com/content/AN762/AN762_Application_Note.pdf
1 comment:
Thanks for sharing the steps and tips..
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