October 1953 QST
On the page following an article titled "TVI and the Novice," the National Company ran one of its conventional full-page informational advertisements - on the subject of TVI. This is number 234, and it is the newest one I have so there likely were many more. National's suggestion for curing TVI issues was to use a shielded chassis with chokes in series and bypass capacitors in parallel with power leads. A complete list of all my National Company advertisements is at the bottom of the page.
Number two hundred thirty-four of a series
The problem of TVI has many phases, and relatively little has been written about receiver TVI. After spending much time and toil in TVI-proofing a transmitter, many an ardent amateur is surprised and disgusted to find that his receiver is causing interference, often of a fair magnitude. This is especially true if the receiver is of an old vintage.
Tests were conducted in our lab and in the field on various models of receivers to determine possible causes. In all cases, the interference was traced to either harmonic or fundamental radiation of the high frequency oscillator in the receiver. Receivers covering the appreciable frequency range of 2.1 per band, 3.1 per band, etc. present a problem in design which has a bearing on this. The high frequency oscillator in the receiver must operate over a wide frequency range with a large change in tuned circuit impedance. Reliable oscillation must be maintained at both ends of each band under conditions of low line voltage, aging tubes, and production tolerances in manufacture. This dictates that the feedback used to obtain oscillation be high - a condition which increases the harmonic content in a manner similar to that depended upon for multiplier use in a transmitter.
The shielding of the receivers was found to be generally satisfactory. Radiation of the harmonics of the high frequency oscillator was traced to leads connected to terminals on the receiver chassis sometimes accidently resonant in one of the TV channels. Coupling from the receiver oscillator was mostly by way of the cable harness in the receiver. Additional filtering of the B plus for the oscillator generally helped but little.
The best cure found was to series connect 1.5 uh chokes in the offending leads and by-pass them with 0.005 to 0.01 ceramic condensers with short leads to the receiver chassis. This is the same technique used to filter leads in a transmitter. Naturally, any choke used in the speaker lead should have low resistance to prevent loss of audio power. Shielding the B switch leads external to the receiver also prevents harmonic radiation and also helps to keep transmitter power out of the receiver. An odd twist was that radiation from leads connected to the antenna terminals was negligible, although this depends to some extent on the vintage of the receiver and the model.
Bill Bartell, W1PIJ
Here are all the National Company advertisements I have:
Posted October 11, 2016