A Brief History
of Zenith Royal Transoceanic Royal 1000
A Speech by Ray Andrejasich
Before a 1998 Quarterly Meeting of the Indiana Historical Radio Society
EARLY WORKING CONDITIONS
Remember, test equipment manufacturers usually tend to lag new technology. When I joined Zenith in 1957, the transistor labs were VERY primitive. Most of the equipment was home-brewed, that is designed by the engineer to meet his needs. For example, the screen booths used to shield out RF signals (radio station transmissions) for our design environments were made by the test equipment department at Zenith (vertical integration again). They were about 12 feet by 12 feet and constructed of 2 x 4's and 2 x 2's. The exterior and interior floors, walls and ceiling were covered by 1/4 inch hardware cloth (wire mesh). The 120 volt AC line into the booth were filtered to keep out line transmissions. A wooden floor and a lab bench at each end completed our little home.
Power supplies available at the time were in the hundreds of volts range, for vacuum tube circuit designs, so again, we had internally designed 0-12 volt DC power supplies. They consisted of a panel with volt and milliamp meters, along with a variable voltage adjustment. The voltage adjustment was a 50 watt wire-wound rheostat connected across two 6 volt automobile batteries (remember those?) connected in series to give 12 volts. There was an on-off switch on the panel which put a battery charger across the batteries when the supply was turned off in the evening.
For I.F. design, we needed a 455 KHZ sweep generator to examine the bandpass of our designs. Again, these were constructed by the test equipment department. An air-gang variable capacitor was dismantled, with the stator plates being mounted stationary, while the rotor plates were mounted on a speaker cone. This was part of a 455 oscillator circuit. By using a 6 volt RMS (AC) signal to the speaker voice coil, the plates of the air capacitor would intermesh at a 60 cps rate. The width of the sweep was controlled by the amplitude of the 60 cps voltage applied to the voice coil. For calibration markers for the 455 KHZ signal at the 6 and 60 db points, we used an army surplus Collins short-wave receiver that had a very accurate built-in crystal calibrator. I remember we used this receiver in the early 1960's to listen to signals transmitted by the first Sputnik put into space by the Russians.
For R.F. work (front end design) we used an off-the-shelf General Radio GR-1000 AM generator with a radiating loop. For a speaker load, we used an old GR load box.
Everyone in the group used a reference book published by RCA Laboratories entitled "Transistors I". It gave design procedures using a more useable impedance and "Q" approach to circuit design. This was much more desirable than the Texas Instruments design approach using admittance and susceptance parameters. I still do not understand these parameters to this day!
|In the RCA design approach, you determined the maximum available gain of the transistor device and used the I.F. coil to introduce enough insertion loss (by impedance mismatch) to make the I.F. amplifier circuit stable. The problem was, how do you measure the input and output impedances of the devices at a given frequency? Again, home-brewed fixtures were constructed in conjunction with an old Boonton "Q" meter. The Boonton measured the "Q" of a tuned circuit at resonance.||
BOONTON Q METER
If you recall, the early transistors were germanium devices, not silicon devices as we know them today. The germanium devices provided several inherent problems that gave many an engineer premature gray hair! For one thing, they were very temperature sensitive and could suffer from a condition called "thermal runaway" which as the condition implies, can cause self-destruction, if not properly biased in the circuit. Also, the transistors had a very high collector-to-base feedback capacitance which meant the circuits had to be neutralized to offset any possibilities of skewing of the I.F. bandpass, or worse-case, the possibilities of self-oscillation. This neutralization in essence was a winding on the I.F. coil that provided feedback to the input of the transistor, much the same as the feedback used in audio amplifiers. This was mandatory in early transistor design, but ran against the grain of vacuum tubes engineers--as a well known author, Termin, in his text on vacuum tube amplifiers, said, "Feedback is no excuse for poor engineering." However, neutralization proved very successful in improving the sensitivity and performance of the early transistor radios.