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RADIO RESTORATIONS
BRINGING OLD RADIOS BACK TO LIFE
Marc Ellis
marcellis@monitoringtimes.com
NC-57 Electronic Restoration Completed
We finished the last NC-57 work session
with about half of the capacitors
replaced before I ran out of time
and almost out of capacitors. There’s an unusual
amount of bypassing in the various circuits, which
is an indication of the excellent quality that was
built into this fairly low-priced radio. I think I’ve
already mentioned how impressed I was by the professional
construction of this set as compared to the
Hallicrafters S-40 (restored earlier in this column)
– which had a similar price, but which was built
more like a household broadcast receiver.
❖ Mica or Paper?
At any rate, with a fresh supply of capacitors
on the bench, I went back to work and completed
the job. This took quite a bit of time because of the
tight construction of the radio, but the result was
very satisfying. I know I’ve told you in the past
about the disappointment some folks feel when they
open up a recently-purchased receiver to find that
all of the original paper caps have been replaced
by modern equivalents. They feel that the radio
has been somehow violated and, understandably,
wonder if the circuitry has been changed, either
deliberately or accidentally.
However, I see a radio chassis with a complete
set of meticulously-installed modern capacitors as a
thing of beauty. (As long as it’s a set of capacitors
that I put in!) I know that there’s an excellent chance
that the radio can be aligned to perform like new. It
should also give good service for many years without
suffering a sudden and disastrous short circuit
that might ruin hard-to-replace components.
As you know, most of the paper capacitors in
the NC-57 were Bakelite-cased units that look like
mica caps, which hardly ever need to be replaced.
However, as discussed last time, the Bakelite cased
paper caps are no more to be trusted than the waxcovered
tubular units. They all tested leaky on my
capacitor checker. So it’s important that you find
and replace any such units present in your restora-
Fig. 1b. Six-dot coding pattern. See text for
details.
tion project.
Generally, the Bakelite-cased paper caps
look much larger than similarly-cased mica ones
– which makes sense because their capacities are
much larger. The paper caps, used as they are for
bypass and coupling purposes, typically range from
.01 to .1 uf. Mica units, often used in r.f. frequencydetermining
circuits, might run from 10 to 500 pf
(uuf).
So, if in doubt about whether a capacitor is
paper or mica, check its size and function in the
circuit. Also check the parts list. The NC-57’s list
helpfully indicates which caps are which.
❖ Reading Capacitor Color
codes
The color codes used to identify Bakelitecased
paper or mica caps can be confounding because
so many versions have been used. Eventually
the system was somewhat standardized by the RMA
(Radio Manufacturer’s Association). Following are
the more common styles.
If the capacitor is the standard garden variety
(500-volt; 20% tolerance) as found in most receivers,
including the NC-57, then a simple row of three
dots is used (Fig. 1a). Make sure you read along the
row from left to right with the manufacturer’s name
right side up or in the direction of the arrows.
The first dot (A) is the first significant figure of
the rating; the second (B) is the second significant
figure; the third (C) tells how many zeros to add
following the second figure to give the capacity
in pf (uuf). For the specific values to use for the
different colors, see the table in Fig. 2. For mica
capacitors, the capacity is usually left in uuf.
However, the capacities of paper caps are usually
so much larger that it they are converted to uf. This
is simply a matter of moving the decimal point six
places to the left.
For example, a very common Bakelite-cased
unit found in the NC-57 has a three-dot color code
of brown-black-orange. Since it has just a 3-dot
code, we know that it is a 500-volt, 20% tolerance
unit. The first (black) and second (brown) significant
figures are 1 and 0. The number of zeros after
the significant figures is three (orange). So the
capacity is 10000 (same as 10000.0, of course) pf
(uuf) or – moving the decimal point six places to
the left – .01 uf.
You might sometimes see a row of five dots
(less common and not illustrated). This format is
used when voltage (fourth, or “D,” dot) and tolerance
(fifth, or “E” dot) must be specified. See Fig.
2 for the values associated with various colors of
these dots. The fourth and fifth dot might be found,
instead, on the rear of the capacitor or above the
basic row of three.
Finally, you might come across a capacitor
coded with two rows of three dots (Fig. 1b). This
might be (but see next paragraph) a unit with a capacity
requiring three significant figures to express
(let’s say 325 uuf, just to make up an example).
For this unit, the top-row colors would be Orange-
Red-Green. The third color (labeled “B1” in the
figure) is read from column “B” in Fig. 2 (or the
identical column “A.”) The number of zeros to be
added (labeled “C” as before) is the last dot in the
bottom row. In this case it would be brown. The
first two dots in that row represent voltage (“D”)
and tolerance (“E”).
Just a few caveats! If you see a six-dot coding
Fig. 1a. Two examples of three-dot coding. If no
direction arrows are present, as at left, read the
dots from left to right with the manufacturer’s
name right side up.
Fig. 2. Chart shows values corresponding to the various dot positions and colors.
64 MONITORING TIMES January 2005