October 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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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.
National Company Advertisement
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:
- National's Company:
Merry Christmas and Happy New Year, January 1941
- National Company:
Noise Limiters, September 1950
- National Company: Modulators, April
1942
- National Company: Switches,
May 1939
- National Company: TMS Condenser, March 1939
- National Company: TVI, October
1953
- National Company:
Amateur Radio Activity, December 1952
- National Company: HRO-50
Receiver, May 1951
- National Company:
Noise Limiters, September 1950
- National
Company: Modulation, October 1947
- National Company:
Merry Christmas and Happy New Year, January 1942
- National Company:
Merry Christmas and Happy New Year, January 1941
- National Company: Switches,
May 1939
- National Company: AC Power Strip,
April 1939
- National Company: TMS Condenser,
March 1939
- National Company Catalog: NC80X
/ NC81X Receiver Advertisement, September 1937
- National Company:
IF Circuits, September 1935
Posted October 11, 2016
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