Thursday, December 6, 2012

QROOOOOOOOOOOOOOO! (Part 1)




Yes, you read that right.  Me, a QRP evangelist if there ever was one, is getting an amp.  Well, not so much getting as building.

LA3PNA, Thomas, over in Norway tipped me off to a small mom & pop shop called Communication Concepts, Inc. near the venerable city of Dayton, OH.  They have a number of simple solid state amplifier kits for sale based off of old Motorola application notes.  After looking around, I settled on the AN762-140 model.  The AN762 is a 140 watt amplifier for 1.6-30 MHz.
The AN762 140 watt RF Amplifier (stock photo)
This is a pretty big step for me - I've built tons of kits now but this is a bit more than just a kit.  As you may notice, there's no enclosure, there's no keying circuit, there's no bandpass filters... this is really barebones.  Luckily CCI also has band filters for a mere $15 so I purchased filters for 5 HF bands 10m through 80m but the filters can probably be pressed into service on the nearby WARC bands in a pinch.  Filters will be switched by hand of course, a simple DP5T rotary switch direct from Hong Kong is on the way for that task.  

All that is left is T/R switching.  I would have loved to use PIN diodes, but I looked at some of the designs and they're all a bit more complex than I really need and since this is all homebrew, I can always upgrade in the future!  The design for the switch is nothing spectacular and is just a simple bipolar relay driver.  Initially I had thought the keyline on the KX3 produced a 3V output on keydown so I designed the switch using a single 2n4401 NPN only to find later that its just a logic zero (ground) output during keydown.  With LA3PNA's help (my analog skills are a bit rusty) we put together this circuit which "inverts" the NPN and switches the relay when the keyline is low since I had already ordered the NPNs.
Keying circuit
The value of R1 is kind of arbitrary but you want to limit the current dumping through the KX3's keyline so chose something big-ish.  I've successfully simulated the design with resistors as big as 1M Ohm so take that for what it is.  

So save for a bit of coax, a chassis, and some connectors, that's pretty much the hole concept of the amp.  It does require a heatsink of course, and I'm still trying to dig one up on the cheap.  I'm sure it will come together when I finally have all the electrical parts in hand and can figure out how I want to lay everything out inside the case I still don't have either!  

Since I ordered from multiple vendors, I will be receiving parts in stages, with the keying circuit parts arriving first so my next post will be about constructing and testing that.  I'll be sure to include pictures.  Keep an eye out for part 2!