At most Air Force bases in the continental USA, commercial telecommunication firms (known as Common Carriers) performed much of the Tech Control function. These companies provided the landlines and microwave links to these bases, and performed the troubleshooting needed to keep these lines open. The USAF bases in the States that had military Tech Control facilities (TCF) were the larger installations with their own direct military trunks to other bases. These were often run over USAF high-frequency radio systems. Andrews, McClellan, Offutt, and a few other bases were in this category. This meant that there was a nagging imbalance in the manning requirements for the 307 career field, with a great need for tech controllers overseas, and much less need stateside.

Because of the classified nature of the messages that we handled, a Top Secret clearance and the need to know was required to enter a TCF. Once inside, you saw a room filled with old-fashioned switchboard operator’s panels. A closer look would reveal that these were actually patch panels for voice and teletype (TTY) equipment. Instead of telephone operators sitting in front of the panels, several controllers with long patch cords draped over their shoulders would be walking the panels while talking to people on push-to-talk telephones, or typing madly on a keyboard. Several rows of printers would be off to one side, with one or two of them printing out the old “quick brown fox” message used as a basic check of TTY circuit quality. This technique could also be used to deter other entities from pirating a legally-assigned but idle frequency. Data circuits were in use at the time, and these often appeared on the same patch panels as the TTY circuits. These early data circuits had card punch machines and card reader machines at both ends, usually IBM 066 and 067 types. The signal bit codes traveling over these lines eventually appeared as punched chads on an 80-column wide IBM card.

A separate set of panels was populated with voice and tone group circuits. A voice circuit was nominally 3 KHz wide, and often carried the familiar base switchboard voice services. They were also commonly assigned to dedicated users such as local command or wing headquarters. The 3 KHz voice circuit was also where the teletype tone groups resided. 12 or 16 teletype channels, each running at a maximum speed of 75 baud could occupy one voice channel.

A partial view of the teletype
patch panels at Tan Son Nhut's
AGL Tech Control in 1968

If the base operator could not establish contact with the phone operator at a distant base, the “go to” shop to get a problem fixed was tech control. Mind you, tech control didn’t actually “fix” anything at the TCF, they used the patch panels to identify the location and possibly type of equipment that was defective, and issued a work order to the appropriate maintenance shop to actually get the work done. At TSN, the Army’s Integrated Wideband Communications Stations (IWCS) did most of the work on voice circuits.

The tools the controllers used on this job were racks of test equipment such as oscilloscopes, signal generators, voltage and decibel meters, as well as various types of analyzers. If the problem was with nearby equipment or lines, the controller would call the associated maintenance shop on base. If the problem seemed to be off base, the controller would pass on the problem description to the next TCF up the line. This was accomplished through the use of a high-priority voice or teletype orderwire connection used only by tech control. These orderwires normally possesed the highest priority of any channel on a given trunk, because they were needed to restore the trunk itself should it become unusable. The technique often used to restore a trunk would involve placing the orderwire circuit temporarily on an alternate routing, sometimes thousands of miles removed from its normal path.

Individual TTY circuits were combined together by multiplexing, and this process resulted in what was called a tone group, which to the uninitiated, sounded like the angry buzzing of a Hornet's nest. As stated earlier, a single such tone group would occupy one voice channel. The voice channels in turn were multiplexed into subgroups and supergroups, some of which had the potential of handling thousands of voice circuits simultaneously for in-theatre transmission. These multiplexing techniques were known as Frequency Division Mux (FDM), which has today given way largely to Time Division Mux (TDM). For longer distances, such as spanning an ocean, a tone group might travel by submarine cable, or it might be placed on a high-frequency radio system. Many tone groups could be found on these short wave frequencies well into the 1970s. The multiplexing equipment used on the high frequency trunks was often more complicated than that devoted to other transmission types because for reliability purposes it needed additional diversity circuitry to overcome the common effects of radio path fading.

If there was a problem with a given TTY trunk, such as a shortwave broadcaster illegally using a frequency assigned to a TTY trunk (not that unusual) this meant the trunk would have to be routed on alternate path until the original problem was cleared up. This "alt-routing" as we called it, was often accomplished by preempting the lower priority channels that occupied the alternative trunk if there were not sufficient spare channels available. Interoperability for these communications circuits was excellent, and a troubled Air Force trunk could be alternatively routed over systems operated by any other branch of the military or naval services. This was possible because the Tech Control facilities were standardized under the Defense Communications Agency (DCA), and inter-service cooperation was a daily event. At this time the Army, Navy, and Marines had TCFs operated by dedicated tech control personnel. Bandwidth was a guarded resource, and it was parceled out to the user with the highest authority and greatest need. At TSN most work of this nature was done by IWCS, which I believe was run by the Army’s 69th Signal Bn.


It was late 1967 and I was looking forward to putting my 18-month assignment at Incirlik AB behind me. I had filled out my “dream sheet,” knowing I would have no problem in getting my base of choice, Tan Son Nhut AB in Vietnam. My motivation for doing this is unclear at this late date. I was 22 years old, single, curious about Vietnam, and certainly tired of eastern Turkey, where there was essentially no contact with the opposite sex. Social life was very limited, and I had spent way too much time bending my elbow at the club. It was also true that during my tour in Turkey, I had picked up a lot of valuable job-related experience, having worked with many different types of radio systems, crypto gear, and teletype and audio multichannel equipment.


In the 307X0 Telecommunications Systems Control career field, the Air Force had three requirements for service in Vietnam, and I met two of them handily. First, the applicant had to be a 5-level (certified competent at the job), and second was the applicant had to have at least one overseas tour under his belt. The third requirement, a few days advanced M-16 training at Hamilton AFB, California, would be taken care of before departing the USA. The M-16 training was excellent overall, as was most of the advice we got from the instructors. One exception to the good advice was being told not to bring much in the way of uniforms, as we would be issued SEA lightweight fatigues right away when we got to Vietnam. I brought only one set of fatigues-real big mistake. I didn’t get my SEA-issue uniforms for a couple of unhygienic months. The supply chain’s flaws were obvious, but my own lack of thought was inexcusable. Oh well, live and learn.


My recollection of final approach into TSN on 2 June 1968 involved some sort of briefing by a nervous flight attendant, American fighter aircraft working over a target not far from our flight path, and all passengers suddenly emerging from the plane into the hot, bright, humidity of Saigon. We were hustled across the ramp, I assume to the passenger terminal, though I think we stopped at some sort of shelter for a while first.

In-processing at the 1876th Communications Squadron began shortly thereafter. In keeping with tradition, as a newbie I was assigned a top bunk on the top floor in one of the green two-storey barracks. I quickly mastered the free-fall from the bunk at 0300 while grabbing flak jacket and helmet. It became one quick motion, once I learned which part of the plywood floor belonged to my bunkmate, and which part belonged to me. One night a 122mm rocket hit a C-130 across the street. The aircraft exploded and though I was a few hundred feet away, the concussion and heat seemed to go right through the screen sides of the barracks. Lots of hot metal fell in the 400 area that night. One night we thought we were going to be issued our M16 rifles, which were secured in a locked CONEX container. I distinctly recall that the person who had the keys could not be found. One day I eventually I got my hands on an M16 from the CONEX for a minute or two and then it was put away in the box again. No target practice, no real familiarity with the weapon. The training at Hamilton would have to suffice. Ultimately, I never had to fire the weapon in Vietnam.


At TSN in 1968-1969 there were four different locations on the base where Air Force tech controllers worked. All of these Air Force facilities were connected to IWCS. We worked daily with the Army tech controllers, many of whom were draftees with considerably less experience than their USAF counterparts. Still, they were quality troops and the service we got from them was outstanding. I talked to them daily, but never got to meet any of these guys. So big was TSN that I only had a hazy idea of where the Army controllers worked, and I recall it was somewhere near the antennas for the PCM shot. I know, that didn’t clear it up one bit, but that’s my best shot.

The primary USAF tech control facility and administrative office for the 307 career field at TSN was through an insignificant doorway in a non-descript building. To get to this TCF you walked past a little pond populated for a while by ducks that were put there by the Comm Squadron commander. Though we had no radio gear, we retained the callsign "AGL" at the main control. The function of this control facility was primarily to support the big PAFCO TTY communications center nearby.

Inside PAFCO, a single tech controller worked in support of perhaps a dozen TTY operators who were members of the 291X0 career field. Here the Air Force used an outdated system for TTY messaging known as torn tape relay. As the name implied, this involved relaying a message from one place to another. These relay centers had been replaced by more automated systems at most other locations around the globe. USAF was certainly capable of fielding newer, smaller, faster, more reliable TTY systems operated by fewer people. The answer to why USAF TTY communications at TSN were mired in obsolescence needs to be sought from someone higher up the chain of command.

One day while at the AGL patch panel there was a “wump” outside. A Huey had crashed and burned in the nearby parking lot.

Tom Hildreth
making log entries
at AGL, 1968

There were fatalities, but brave witnesses saved a couple of passengers from the fire that consumed much of the chopper. AGL had many circuits that came through Danang, and one other day while I was working they all suddenly became “garbled” (unintelligible). What could have happened at Danang? This turned out to be a big munitions explosion. Years later, while training for the Air Force munitions career field, I would learn of the details.

Speaking of new technology, it was at PAFCO in 1969 that I saw my first electronic “memory”, a device that appended four-letter station identifiers to the TTY messages. Most of the TTY circuits ran at less than 75 bits per second, the speed at which unencrypted channels operated for a throughput of 100 words per minute (WPM). The crypto equipment used in those days required a lot of support, which accounted for perhaps 20% of the tech controller’s time on the job. About 15 years later, we would learn that the notorious Walker family had compromised much of the military communications security (COMSEC) of the Viet Nam era for small payments from the Russians. My feelings about these traitors are strong.

The third location for USAF tech controllers was at 7th Air Force Headquarters. This facility had its own TTY torn tape communications center, which if I remember correctly primarily handled MACV traffic. A single 5-level controller manned the tech control position at HQ. There was no need for the tech control NCOIC to sleep here, but he did. Nightly he could be seen behind the panels on the floor, without benefit of blankets or pillow. Apparently the security of the steel reinforcement around the bottom floor of this building is what caused him to endure the visible sores and discomfort.

Through this TCF passed the “Red Rocket” TTY messages which were “heads up” for the overflight of the SR-71 Blackbird. I recall a series of messages at 7th AF HQ that originated in South Vietnam, and may have gone as far as Washington. They were about an unfortunate Vietnamese farmer who had been killed by a phone pole that fell loose from the sling of a helicopter. Much of the message series had to do with determining the worth of this hapless soul, with a lot of deliberation being based on how many animals he owned.

If the Vietnam War was micro-managed, much of this was accomplished through HQ 7th AF via TTY messages. I recall countless frag orders that were sent to Washington for review. All the details of upcoming airstrikes were contained in these sensitive messages that went back and forth halfway around the world over what may have been a compromised communications system. How many American lives were lost because of this? Perhaps my feelings towards the Walker family can be better understood in this light.

A fourth Tech Control location opened up late in my tour in the Pulsed Integrated Automated Communications System (PIACS or PIAACS). I think this was not far from PAFCO, and as I recall, the PIAACS scheme was a very large and complex one. All sorts of equipment bays, fresh from the states, were wired up at a half dozen places in Vietnam for this system. Lots of programming was involved, and I recall volunteers being used to wire up little matrix boards. The purpose of this system may have had something to do with real-time bomb damage assessment. It was still in the construction phase when I left for “the world.”

A typical shift at the various USAF TCFs at TSN had one 7-level and three 5-level USAF tech controllers on duty working the circuits over which moved many hundreds of TTY messages per hour. I think the Air Force had perhaps 250 dedicated TTY circuits that terminated at TSN, and there may have been a similar amount for the Army. Surely the Army handled hundreds if not thousands more “through circuits”, those which did not terminate at TSN, but went on to other SEA bases. In addition to the people mentioned above, there were several other 7-levels and at least the one 9-level at the AGL administrative office during the day shift, and two PIACS 5-levels. The traffic load for TSN was immense and unrelenting. It was a huge 24/7 operation.


º Customer at TSN calls AGL on the phone reporting his TTY circuit XX59 is unreadable. He knows this because there is random “garbage” printing on his roll of paper.

º AGL controller uses a patch cord to plug in his “monitor” printer, and verifies it is unreadable, while calling out to the crew, “we lost XX59.”

º AGL controller calls the Army IWCS tech control and advises them XX59 is out of service. At the same time, he plugs in a “distortion analyzer” to see if there is a pattern to the problem, which would be visible on the instruments screen. No pattern is seen, just garbage.

º IWCS calls back a few minutes later advising a certain high-frequency radio trunk had been lost between Clark AFB and McClellan. Reason unknown at this time. XX59 had been down for six minutes at this point.

º Customer on XX59 had called his boss, and AGL tech controller is now hearing from a Major about the need to have “his” circuit back in service quickly. “Working on it, sir” say the AGL controller, with a couple of other comments on the side when the push-to-talk button is not pushed. The Major didn't know it, but XX59 only had a restoration priority (RP) of 2 Alpha, and the controller was previously attempting to restore several much higher priority circuits that if not back in service NOW, would create great concern several levels up the chain of command.

º AGL phone rings again, it’s the tech controller at PAFCO relay, and they lost contact with Hickam AFB on all four circuits. Though the problem with XX59 is fading in importance, it is now eight minutes since it went down. In two more minutes a report needs to be written and a message sent to the Defense Communications Agency reporting the loss of XX59.

º AGL phone rings again. TTY maintenance wants AGL controller to patch over a piece of gear so he can perform scheduled preventative maintenance (PM) on it. “Gimme a minute-real busy right now”, says the AGL controller. “Yeah, sure, I’ve seen you guys on the night shift before, I know how busy you are” says the TTY maintenance troop jokingly.

º PAFCO calls, advising all Hickam circuits are back in service. Though no report to DCA will be necessary, it can take PAFCO hours to recover from this interruption, because the lost messages will need to be retransmitted via the slow, overloaded TTY circuits.

º Major from XX59 calls again, “What are you guys doing to help me? I need an explanation!” XX59 had been down for 12 minutes now.

º AGL calls the IWCS controller, requests status of trunk over which XX59 is routed. “Gonna be down for along time, microwave system at San Miguel was taken out by a typhoon. But guess what? I got your XX59 on alternate routing and it’s looking good. How come your not passing traffic on it yet, Air Force?”

º AGL controller looks at the analyzer and sees a good signal on XX59. Damn, “I hate it when Army gets the jump on me” he mutters.

º AGL controller calls customer at XX59, advises him circuit is back in, go ahead and resume traffic. Airman at XX59 says, “we got no traffic for it until day shift. No big deal.”

º AGL controller concludes there will be no sweat “fudging” on this one, no lost traffic on a 12-minute outage means no report to DCA necessary. Good deal, he thinks, “ I’ll get to look at the new Playboy magazine.”

º PAFCO calls, “Hickham circuits are down again-get an alternate routing, will ‘ya?” So much for the centerfold. That’s how Tech Control went at Tan Son Nhut, 24/7, for a very long time.


Story and photos © 2007 by Tom Hildreth