Hello and welcome to the DxConnection ham radio Amplifier QSK site.  My name is Bill K3HZP and I became interested in Amp QSK when I decided to put my SB220 Heathkit 1KW Amp on the air with my ICOM-735.  Also, I wanted an external QSK unit that I could move between amps when I upgrade.  
There was only one external QSK box available, but it had a lot of limitations for my use.  It used PIN diode switching which made it fast, but did nothing to accommodate the first dit or dah of a transmission.  Hence, I knew that my amp Transmit/Receive (T/R) relay would hot switch and shorten (or eliminate) the first dit or dah of a transmission if I were to use it.  A friend of mine also had used this electronic QSK box on his 30L1 linear.  It suffered a failure that took out the 30L1 bias supply.  There had to be a better way.  I found no other choices on the net.
 I decided to build an external QSK T/R switch that required 'NO MODIFICATIONS' to the amplifier.  'NO MODIFICATIONS' was my overriding design goal.  This took 3 years, 7 prototypes, and 3 PC board designs to come up with what is available today.   (See pictures of the first five prototypes at the bottom of this page!)   This design attempts to overcome most of the known problems with using external QSK boxes with no amp modifications.   I hope some of the tranceiver timings that I have included are useful to other QSK users and helps them to resolve any timing problems. 
If you are unfamiliar with QSK operation, the drawing below shows the active receive time while transmitting "Hi Bi" (ignore my capitalization!).  In the receive intervals, you, the sender, can hear a DX station if it answers a station other than yours and still maintain synchronization with the DX station.
               ...Bill  K3HZP


                                         Operating W1AW  (What a blast!)

A little about my background:     I am a retired Communications Systems Engineer with degrees from Notre Dame (BSEE), U of Ill (Urbana) (MSEE), and The Ohio State University (PhD EE).  I worked 18 years designing military anti-jam radios that used both frequency hop and direct sequence spread spectrum.   I also had the good fortune to lead the design of 'single frequency' Direct Conversion and Low-IF Direct Conversion receivers.  I am having fun now designing and building ham radio products.
QSK definition
QRP:   NPOTA  (NM15)
Lincoln Boyhood Memorial 
Figure 1   Diagram showing the additional Receive time in QSK operation
Prototype  1
Prototype  2
Prototype  3
Prototype  4
Prototype  5
Prototypes  1 through 5