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December 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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It seems
the title, "Mac's Radio Service Shop" for John Frye's tech tales might have been dropped after Radio &
Television News magazine changed its name to
Electronics World
in May of 1959. The characters' names and roles were all the same, but the title was dropped - probably
to not bias the new theme of the magazine. This episode discusses some of the strange ways in which
a faulty bypass capacitor can manifest itself. A big part of effective troubleshooting is the experience
of 'having seem that before.' Interestingly, by 1963 vacuum tubes were still in common use, but printed
wiring boards had been introduced, along with their propensity for developing broken traces. Also mentioned
is why having a safety ground on a metal chassis is essential (a rare feature
in 1963), and what is probably the first mention of the newfangled Thomas & Betts 'Ty-Rap' nylon ties that are now used for
everything from bundling wires to securing packaged items for shipping to handcuffing perps.
Mac's Radio Service Shop: Case of the Bad Bypass
John Frye
Technical curiosity can reap some satisfying rewards even when it
just involves solving a case with freak symptoms.
"I guess this knocks the props from under those fur-on-the-wooly-worms forecasters who said we wouldn't
see any cold weather until after the first of the year," Barney remarked as he hung up his heavy coat
and started rubbing his half-frozen ears. "It took me twenty minutes to get my car started this morning."
"Here's just the thing to warm you up," Mac suggested, sliding a playing little radio down the bench
toward his assistant. "It's one of your jobs that bounced. The owner admits it now takes longer for
the noise to start up after the set is turned on than it did before; but once it starts, it's just as
bad as ever. The ticket says you replaced a noisy 12BE6 tube."
"I remember that set, and I'll swear the 12BE6 was bad!" Barney exclaimed. "It was one of those jobs
in which you could trigger the noise on or off by flipping the envelope of the tube with your finger
nail. After I put in a new tube, no amount of jarring produced any noise; so I made out the bill-"
He was interrupted by a great crackling, frying sound from the little receiver. Quickly he removed
the back and struck each tube in turn with a tube tapper. The noise was unaffected. "Guess it must be
a bad i.f. transformer," he hazarded, reluctantly starting to pull the flimsy printed-circuit chassis
from the case.
But when the noise-testing probes of the signal tracer were placed across each winding and between
the windings of the i.f. transformers, there was no indication of defective coils or of leakage from
coil to coil through the plastic in which the coil-tuning capacitors were embedded. Similar tests revealed
nothing wrong with the oscillator coil. Even when Barney tried gently flexing the printed circuit board
to see if a break in a printed circuit lead might be causing the trouble, the noise kept merrily grinding
away.
Mac, who was aligning a receiver on his end of the bench, noticed the noise from Barney's receiver
was heard almost as loudly in the set he was aligning. When Barney's set was turned off, the noise disappeared
from Mac's set.
"Whatever is causing that noise must be pretty close circuit-wise to where the line enters the set,"
Mac suggested; "otherwise it wouldn't be feeding back into the line so strongly. Try removing the capacitor
that connects directly across the input when the set is turned on."
Barney did, and instantly the noise disappeared. When the capacitor was returned to the circuit,
the noise returned. A new capacitor produced no noise.
"That's a new one on me," Barney admitted. "The defective capacitor feels pretty warm. It must have
an intermittent high-resistance leakage path through the dielectric. Probably lightning caused it. The
erratic leakage current doesn't start until the capacitor reaches a certain temperature. That's why
I didn't catch it before. I was looking for one, not two, sources of noise. Having found the noisy tube,
I looked no further."
"I can't honestly criticize you too much this time," Mac admitted. "I had that set playing a good
forty-five minutes before you came in, and it was as quiet as you could wish.
Furthermore, I can't remember seeing more than two or three cases like that in all the years I've
been servicing, It's not uncommon to find a line bypass that makes an intermittent noise when it is
tapped simply because the poor connection between foil and lead is disturbed by the vibration; but this
capacitor was soldered firmly to the printed circuit, and vibrating it had no effect whatever. The leakage
path is inside the capacitor. Ordinarily the heavy line current follows across any leakage path there
and literally blows the capacitor apart. Let the defective capacitor cool down, and then we'll run some
checks on it as we gradually warm it up with a lamp. I'd like to know what's peculiar about it."
"I used to think I got all the odd-ball cases," Barney remarked; "but the other day I was talking
to a technician over at the parts store, and he came up with a really wild tale. A man who lives at
the edge of Cantorville to the west of here was sitting on his front porch one evening last fall watching
a neighbor's dog frisking about the yard. The dog happened to brush against a downspout that came down
alongside the porch, and immediately let out a howl of anguish and went yelping for home as hard as
it could lope. The man walked over to the downspout to investigate, and when he touched it he was almost
knocked flat by an electric charge.
"He called the electric company, and the electrician found a full 120-volts a.c. between the downspout
and ground; yet neither spouting nor eavestroughs came anywhere near any wiring about the house. The
electrician methodically began pulling fuses and he soon discovered one line in the house that killed
the charge when its fuse was pulled. Next he unplugged things from that line one at a time, and when
he unplugged the TV set, the charge on the downspout was gone.
"The electrician suggested a TV technician be called to see what was wrong with the set, and that's
where my friend came in. It didn't take him long to discover lightning had shorted a line bypass capacitor
between the hot side of the line and the chassis. When this was replaced, the charge disappeared from
the downspout, and the receiver seemed to work normally. It was supper time when the job was completed;
so my friend packed up his tools and left.
"But that night he couldn't sleep. He lay awake trying to figure out how on earth the 120-volt a.c.
was getting from the TV chassis to the downspout. He reasoned it must have something to do with the
owner-installed TV antenna, for he had noticed on the diagram that the center-tap of the antenna coil
was grounded to the chassis and that there were no capacitors in the antenna leads. What's more, the
antenna mast had not been grounded. But the closest the antenna lead came to the downspout or eavestrough
was right where the lead went through the wall and connected to a lightning arrester; and that arrester
was at least a foot away from the downspout, and both were fastened securely to the painted wooden siding.
"The next morning he went back to the house and asked permission to do some more investigating on
his own time. To simulate previous conditions, he disconnected the antenna lead from the receiver and
connected it to the hot side of the a.c. line through a 10,000-ohm resistor. Sure enough, the voltmeter
revealed almost 120 volts on the downspout. Looking very closely at the lightning arrester, my friend
thought he saw a faint dark line beneath the paint going over to a metal clamp that held the downspout.
When he carefully scraped away a bit of the paint, he revealed a carbonized path burned right into the
wood leading from the arrester to the clamp. Checks with an ohmmeter revealed only thirty ohms resistance
in this path. When the arrester was replaced with a new one mounted at a slightly different spot on
the siding, the downspout was cool as a cucumber.
What's more, TV reception was improved because signal pickup by the eaves trough and down spouting
fed into one side of the lead-in had been messing up the directivity of the antenna.
"My friend reasons the same flash of lightning that knocked out the line bypass must have jumped
from the arrester to the downspout and burned the carbonized path into the wood. Possibly lead in the
paint smeared over the case of the arrester made this jump easier. Anyway, my friend claims he slept
like a baby the next night."
Mac nodded in agreement. "I know exactly how he felt, and I like this friend of yours. He has that
most important characteristic of a good technician, a good engineer, or a good scientist: technical
curiosity; and he's not afraid to spend time and effort, with no prospect of immediate monetary return,
to satisfy it. You used the phrase 'on his own time' to describe the investigation your friend made
into the puzzling case of the hot downspout. In my book, every technician worth his salt puts out a
lot of effort 'on his own time' in his daily work. When he encounters a puzzling situation in his servicing,
he's not content with merely restoring the set to operation through lucky accident. For his own peace
of mind he must try to find out why the defective component made the receiver behave the way it did,
if there was any reason for the failure of the component that could be corrected, and if there was any
best way to pinpoint the trouble should it be encountered again."
"Yeah, but aren't you going to be griping because I'm not turning out sets instead of educating myself
at your expense?"
"Have I ever criticized you for following through on a puzzling service job? It's not that I'm just
interested in seeing you satisfy your curiosity. I know that each time you do this you improve yourself
as a technician, and a really smart and alert technician is worth three times as much to me as a half-baked
one capable of doing routine servicing and nothing more. You don't get smart by doing service work mechanically.
You have to think and to wonder and to check and to double-check until you know. Out of this knowledge
comes diagnoses that are quicker, surer, and more accurate."
Mac had been replacing an FP filter capacitor with an under-the-chassis cartridge type as he talked.
After all the unanchored connections were carefully taped, he bundled the leads together and wrapped
some sort of flexible white strap around them and cut off the excess length.
"Hey, what you doing there?" Barney demanded.
"I'm using a 'Ty-Rap' manufactured by the Thomas & Betts Company of Elizabeth, New Jersey, to
hold the wires in place," Mac answered, tossing one of the objects to Barney. "As you can see, it's
sort of a long, flat needle of nylon shaped something like an old-fashioned cut nail. There's an 'eye'
running crossways in the flat head, and a piece of metal is embedded in this eye so that when the tapered
tail of the 'Ty-Rap' is threaded through the eye and pulled tight around a bunch of wires, the loop
is locked solidly in place and the excess tail can be snipped off with the diagonal cutters.
"They are used in cabling in place of lacing. They come in a wide variety of shapes and sizes. Some
of them have provision for fastening each unit to a chassis or board after it has been wrapped around
the wires. In addition to these hand-installed units that I think will be most useful in our work, there
is another group designed to be installed with a tool that pulls the 'Ty-Rap' tight around a bunch of
wires, locks it in place, and snips off the tail all in one operation. These would be fine in production,
but I believe the hand-installed units will come in quite handy for us in making neater auto-radio,
custom hi-fi, and other installations where keeping wires fastened securely together and out of sight
adds to the appearance."
"It sure beats taping wires together," Barney agreed.
Posted January 16, 2017
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of none other than John T.
Frye, creator of the Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
