77 Under radio color markings: What does this mean
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
77 Under radio color markings: What does this mean
I looked under my radio and there are a lot of parts with several
color markings on them but no printing. What does this mean?
There has been a color code for marking part values since the early
1930s. The numbers are always the same:
Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Grey = 8
White = 9
There are several mnemonic sentences for remembering this series, some
lewd, some not. "Bad Boys Ruin Our Young Girls Behind Victory Garden
Walls" is one of the politer versions.
Resistor markings: early-mid 30's was "body-end-dot" where the
resistor body was the first significant digit, one end was the second
digit, and a dot in the center of the body was the multiplier. After
about 1935, resistors were marked with color bands; the first
significant digit is the band nearest one end. Silver is used to
indicate 10% tolerance; Gold, 5%. These are either on the other end of
a body-end-dot resistor or a fourth band on band-marked resistors. The
scheme is simple to decipher: a resistor marked yellow-violet-green is
47 mulplied by 10 to the 5th (100,000), or 4.7 megohms.
Mica and molded paper capacitors, in little rectangular plastic
packages, used the same color values, but there were about twenty
different schemes for locating the dots, and most of them use six dots,
with three or four giving the value. These can be a nightmare to
decipher. Generally, either the first or second dot in the top row is
the first significant figure, and either the rightmost dot in the top
row or the rightmost dot in the bottom row is the multiplier. The size
of the capacitor (bigger values are physically bigger) and the circuit
application will give a clue as to the approximate value. The left
bottom dot generally gives the voltage rating in 100s of volts (red is
200; green, 500), and the center bottom dot generally gives the
temperature characteristic. The left top dot may be a significant
figure or may be a type indicator. Some types have six dot positions,
but one or more with no marking, which may mean "not used" or "brown."
Knowing the series of standard values for resistors and capacitors can
help in deciphering color codes. These were changed during WW II.
Prewar 20% resistors (no tolerance color) were 1000, 1500, 2000, 2500
ohms, etc. Postwar were 1200, 1800, 2700, 3300, 3900, etc., replacing
the old 0/5 scheme with approximately 20% jumps in value. Mica
capacitors in old radios were generally 50, 100, 150, 200
"micromicrofarads" (picofarads---term did not come into use until the
early 1960's in the US). Molded paper capacitors are generally in the
1000 pf. (0.001 microfarad) to 10000 pf. range, with 0/5 as second
figures. Postwar production switched to 12, 18, 22, 27, 33, 39 as
significant figures, although the old scheme was still commonly used.
Wattage ratings of resistors in different package sizes have been
revised several times, always increasing the rating for a given package
size. When replacing resistors, modern 1-watt metal film resistors
generally are about the right physical dimensions for older 1/4, 1/3,
and 1/2 watt resistors. Values should be derated 50%; that is a 1 watt
resistor should calculate to a dissipation of 1/2 watt or less in a
circuit.
An overstressed resistor will overheat, and discolor its color bands,
sometimes very deceptively. In particular, the red and orange
multipliers may look brown, and it may require inspection with a
magnifier to see that the center of the resistor is charred. Any
resistor that looks as though it has been heated to the point of
charring or discoloring its markings should be replaced . Also, some
compositions used for composition resistors were unstable over time,
and a resistor that looks perfectly good and is in a circuit location
where overstress is nearly impossible may be wildly out of tolerance.
Use an ohmmeter to check, but check your ohmmeter against some
known-good new resistors of similar value.
Typical resistor failures:
240 ohm 1 watt cathode resistor for a 7C5---looks like it might have
gotten warm, colors still normal, actually is 150 ohms. Inspection
with a magnifying glass after removal found more signs of overheating
on a side that was not visible with the resistor soldered in place.
Failing "low" like this is not common, and generally comes from using a
resistor with too low a wattage rating for the application. The
coupling capacitor to the 7C5 grid was leaking, pulling the grid up
enough to over-dissipate the resistor. Oddly enough, the tube survived.
240 ohm 1/2 watt screen resistor for a 6K7. This was found on visual
inspection, connected to a replacement bypass capacitor in a
suspicious-looking repair. Ohmmeter showed about 10K ohms, and the
circuit location should have a 2K ohm resistor. Closer
inspection after removal disclosed a charred center which had turned
the red multiplier brown. This resistor was originally 2400 ohms, used
to replace a 2K.
33K 2 watt screen resistor for a 6BA6. The screen bypass capacitor was
shorted, "killing" the set. Ohmmeter showed about 250K. This resistor
showed no signs of distress. A shorted bypass capacitor often takes
out the resistor in the circuit, but a further check in this radio
showed about 2/3rds of the resistors over 20% high, some as much as
twice the value, even though they were not discolored. It got 100%
resistor replacement.
A resistor that is physically broken generally has been subjected to a
short circuit condition that overheated it until it exploded. Look for
a hard short in the circuit.
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