a question about unreliable old technology
Cohasset / Hippisley
Tue Dec 27 07:44:50 EST 2011
On Dec 27, 2011, at 2:12 AM, Donna Halper wrote:
> My Canadian friend Art, who is writing a book with a backdrop in late 1940s or early 1950s broadcasting, asked this of me, and since technology is not my strong suit, I thought I'd ask you nice folks. I'll pass along your replies to him. He was reading some old articles about technical failures in TV equipment: "Everyone who was working in TV at the time recalls how temperamental and unreliable the electronics were, but exactly how temperamental and unreliable is not spelled out. So I am trying to find comprehensive and detailed information about the extent of it: what were the most common component failures that would lead to loss of service (not necessarily loss of the signal altogether, which was quite rare.) Essentially anything that led to the "We Are Experiencing Technical Difficulties" slide appearing fits the definition, with the exception of component failure from external causes: a burst water main that shorted out the electronics, or a lightning bolt that fried the transmitter don't count. What were the most common components to fail, how long on average would it take to fix them, and what percentage of the broadcast hours would the average station expect to lose over the course of a month?" Any guidance you can offer will be appreciated!
Without a doubt, the primary sources of "temperamental and unreliable" functioning of electronics equipment in my 50s & 60s (sorry for the slight time skew, Donna) broadcasting experience were vacuum tubes. Even in the mid-60s, I think WBZ-TV had more stockroom space allocated to storing spare, new vacuum tubes that the combined Studio B / Control Room B of then-WTBS! (I even recall a major RCA tube distributor being located just a few blocks from their Soldiers Field Road facility...I wonder if that was deliberate!)
The problems we most commonly experienced had at least four origins:
1. The gain and other electronic characteristics of tubes slowly changed as the operating hours on those tubes increased. To the extent that the (analog) circuitry of the day was a function on those characteristics, the operation of the circuitry would gradually shift and/or decline over time.
2. The use of large amounts of feedback and other compensating techniques in circuit designs to stabilize the "black box" input / output performance of electronic circuits was nowhere near as universal as it became in later years. One reason for this is that — at the bandwidths required by TV baseband video and at the over-the-air frequencies assigned to commercial FM and VHF television stations -- many circuit designs and vacuum tubes were at the limits of their (gain-bandwidth) capabilities even before applying feedback techniques.
3. The heat generated by the filaments of even very low level stages with hardly any plate dissipation was, like high blood pressure, a "silent killer" over time. Worse yet, just like "second-hand smoke", that heat affected the useful life of ALL components in the same circuit, the same equipment enclosure, and even the same room. Heat further de-stabilized circuit operation; circuit designs needed to be stable not just over the temperature range provided by the (often controlled) environment but over the additional temperature rise between cold turn-on (or room) temperature and the ultimate temperature INSIDE the equipment enclosure after an hour or more of operation.
4. Many circuits of the day were sensitive to "hum" from the normal 60-cycle alternating current filament voltages. In quality audio gear, those filaments were actually fed from DC power supplies — especially in the low-level stages needed for microphone and turntable pre-amps. I can't tell you how many hours I've spent swapping out glass triodes, trying to minimize hum in the amplified output of a high-fidelity audio amplifier! (A related problem was "microphonics" — usually heard as a modulation on the desired output signal whenever the offending tube was tapped or otherwise subjected to vibration.)
No better example of the combined effects of # 1-#4 above exists than this: Even in the mid-60s I remember the techs at Channel 4 spending more time getting the studio color cameras "tweaked" to deliver the same amount of green in flesh-tones to the control room than the actual duration of the local-origination newscast they were preparing for!
Probably the second-most likely component to fail in my experience was the electrolytic capacitor. Primarily used in DC power supplies to reduce or eliminate any AC (hum) on the output of the supply, the lifetimes of these devices were seriously compromised by high temperatures. Further, they were usually found in the highest voltage sections of equipment. Typical plate voltages in ordinary studio and control room electronics devices might be 150, 300, 600 volts, for instance. (Transmitter voltages were much higher, of course.) Electrolytics were additionally temperamental in that they were best operated regularly, and at a large fraction (say 85%) of their rated working voltage or they would prematurely fail. In fact, there are published techniques for "re-forming" electrolytics that haven't been used in a long time. I don't think I've seen anything similar for any other components except high-power glass triode transmitting tubes.
In third place I would put tape decks of all kinds. Certainly by the mid-60s the video tape decks were the highest maintenance and most temperamental of the entire group — largely because of the extremely high frequency content and bandwidth of the recording techniques required for television video, coupled with the basic limitations of magnetic tape as a recording medium. But in the 50s we certainly had our share of "fun" with audio tape decks, as well. Of course, the quality and reliability of the Ampex professional machines was one of the high points of the era, but even those units needed periodic alignment and head cleaning / replacement. And they, too, were susceptible to all the vacuum tube issues listed above — especially microphonics, since the solenoid-controlled tape handling subjected the entire tape deck chassis to shock and vibration seldom felt anywhere else in the station.
The extent to which tubes or capacitors or tape decks caused stations to put up the "Technical Difficulties" slide depended primarily on two aspects of station management philosophy:
a. The Preventive Maintenance policies and procedures in place and actually practiced.
b. The extent to which critical pieces of equipment were replicated: extra channels in the station signal paths, spare studio cameras, power supplies, tubes, etc.
Without following the Boy Scout precept ("Be Prepared") through some combination of (a) and (b) above, a simple vacuum tube failure could take hours to locate. So I'm inclined to believe you can't build a purely-electronic or electro-mechanical cause-and-effect relationship between the failure of a component and the amount of dead air experienced. Failed components were a fact of life; if you built your station and its procedures around that premise, you would experience little, if any, dead air. If you didn't, you were always going to be in a "heap of hurt". An extreme example is the transmitter: A failed electrolytic capacitor might take a shift or two to identify, another shift or more to replace. The only smart work-around is a back-up transmitter.
Also, please remember that the view from inside is not the same as the general public's perception. A large part of what the technical professionals in our industry did was work their butts off, dealing with these "notoriously unreliable" devices every hour of every working day so that the image of broadcasting presented to the lay public was one of unparalleled reliability and "up-time".
There were doubtless completely separate issues associated with the "portable" equipment used for "remotes", but I won't go there....:-)
More information about the Boston-Radio-Interest