John, The first TU that I used was a W2JAV design. The TTL-2 showed up in the September 1967 issue of RTTY Journal. I built that from parts and it was a really great TU. I continued to use the TTL-2 until the ST-6 that showed up in RTTY Journal in 1970. I built the ST-6 from a parts kit offered by HAL Communications. I'm using the ST-6 now. 73 Russ WA3FRP