It's time to gather 'round for another story about fictional
radio service shop owner Mac McGregor and his trusted sidekick
technician, Barney. In this episode, an errantly wired bypass
capacitor on a chassis from one of the old AC/DC radio sets
caused Mac to get a 300-volt wakeup call when his hand brushed
against it. After explaining the situation to Barney and apprising
him of the danger it poses to an owner who unwittingly sticks
his/her hand into the back of the cabinet, Mac lists a few other
common dangers to watch for. Radios that ran on either AC or
DC power were very common back in the early days because there
were homes and businesses that had both type systems wired in
to the premises - in part due to the famous battle between Thomas
Edison's preferred DC electrical distribution system and Nikola
Tesla's preferred AC electrical distribution system. Another
reason for DC compatibility was that prior to the
Rural Electrification Act of 1936, many homes not located
in cities and nearby suburbs had no electric service at all.
Farm homes often ran on DC power supplied by banks of lead-acid
storage batteries, with wind- or water-powered generators providing
the charging current. Radios, aka 'wireless sets,' were powered
by those batteries. In order to accommodate both AC and DC supplies,
radio designers conjured up power supply circuits that would
work on either system, but unfortunately the way it was implemented
often caused the metal chassis to have a few hundred volts of
potential on them. That meant the metal shafts to volume and
tuning controls carried a deadly potential that was isolated
only by plastic or other insulative material. Even the external
antenna connection could carry the voltage. After many electrocutions,
the industry finally standardized a design strategy that provided
adequate isolation and safety.
Mac's Radio Service Shop: Safety in Servicing
By John T. Frye
Mac was aligning a receiver in which his assistant, Barney,
had just installed new filter condensers. Suddenly he grunted
heavily and jumped back from the chassis. Then, as Barney watched
guiltily, he pulled the line plug from the socket and gingerly
turned the chassis over.
"Now there," he said as he jabbed an accusing finger at the
wiring beneath the chassis, "is something I never want to see
you do again."
"What's wrong with that?" Barney asked. "That old can-type
condenser has been cut out of the circuit. See, here's where
the lead from the insulated case has been cut loose. The positive
terminal has been left wired in merely as a convenient tie-point
for the new cartridge-type electrolytic."
"And so what happens?" Mac demanded. "The shorted condenser
element inside the can conducts the full voltage from the positive
lead right to the floating case. To make matters worse, while
looking to see what size condenser you would need for replacement,
you removed the cardboard covering that was slipped over the
can of the electrolytic and failed to replace it. That is why
I got a full 300-volt jolt a few minutes ago when my hand brushed
against the case of that old electrolytic. The same thing could
have happened to a housewife dusting out the chassis when the
receiver was turned on; or a little child might have put his
hand on the condenser can and received a fatal shock. If this
had been a TV chassis, I might have jerked when I was shocked
and hit the picture tube with a tool in my hand, causing it
to implode. In the future, I want both leads of an electrolytic
that sticks above the chassis cut loose, unless the can itself
"And while we are talking about such matters, it probably
will not hurt to touch on two or three other points. For example,
the other day I noticed that you used a new 'outboard' one-megohm
resistor to replace one that had gone bad inside the socket
of a magic-eye tube. You failed to tape the exposed leads of
the resistor, although one of those leads was a full 250 volts
d.c. above the chassis. Fortunately I noticed this and taped
them up before the set got out of the shop. A good general rule
to follow is that no on-top-of-the-chassis point should be left
exposed to touch if it carries more than ten volts potential."
"I saw a dandy along that line the other day," Barney remarked.
"Someone had replaced the output transformer mounted on the
speaker and through thoughtlessness or neglect the 'B-plus'
and output-tube-plate lead junctions were left exposed where
even a crawling child could easily reach them. That was dangerous."
"Speaking of crawling children, you can't be too careful
of line cords," Mac went on. "Quite often these cords will look
all right except for a short space just back of the plug, right
where the fingers would likely be when the plug is being inserted
or removed. The reason, of course, is that the cord at this
point takes quite a beating from sweepers, dust mops, and little
children. I always try to take a quick gander at the line cord
when the customer brings a set in or when I pick it up; and
if the line cord looks the least bit suspicious, I suggest right
then and there that it be replaced. Few people will turn the
"While we're gabbing about safety, maybe we ought not squander
all our solicitude on the customer," Barney suggested. "How's
about mentioning a few things that may help the service technician
reach a mellow age."
"At least one technician I know shows promise of becoming
mellow, very mellow, long before he becomes ripe," Mac observed,
"but your idea has merit. I think we have touched on most shop-safety
points at various times in the past, but if these ideas are
to do any good, you have to keep harping on them. Anyone who
has ever studied how accidents happen knows there is a great
difference between knowing that a danger exists and being constantly
aware that the danger is present. It is this constant awareness
that keeps you from having an accident."
"The voltages we constantly deal with are one of our greatest
dangers, aren't they?" Barney asked.
"That's right, and the dangerous part is that familiarity
breeds contempt. We are constantly working on receivers having
potentials all the way from 100 volts up to fifteen kilovolts
or so. Every now and then we get nipped a little without being
seriously hurt, and each time this happens we lose a little
more respect for the potentially lethal power that lurks in
even a lowly a.c.-d.c. set. In fact, there are some pumpkin-headed
technicians I know who boast about how they take no precautions
against being shocked because they 'can take it.' Their undertaker
will be doing the taking one of these days.
"A simple precaution that should be standard procedure in
any shop is to plug all sets, both a.c.-d.c. and transformer
type, into isolation transformers while they are being serviced.
Most technicians realize the danger present when working on
hot-chassis a.c.-d.c. receivers; but they overlook the fact
that quite often lightning will cause the condenser that goes
from one side of the line to the chassis of a transformer set
to become shorted. Such a set can be a real death-trap, because
you do not expect the chassis to be hot.
"Another point I had vividly demonstrated one time is that you
should never try to pick up a heavy chassis and move it while
the cord is plugged in. I saw a person pick up such a chassis
and start to slide it into the cabinet. Suddenly he began to
shake all over and to make a peculiar noise. I jerked the cord
from the wall socket, and he dropped the chassis to the floor
and tottered to a chair. A strong odor of burned flesh was in
the room. His fingers, curling beneath the edge of the chassis,
had come into contact with the tie-points for the 117-volt line
cord. The current froze him so that he could not let go and
made his contracting muscles force his fingers tighter and tighter
against the searing wires. Grooves were burned into those fingers
that took months to heal."
"Now look, let's not try to win Oscar with this safety talk,"
Barney said with a grin. "You're waxing pretty dramatic, you
know. I suppose we ought to mention, too, that you should use
an insulated rod instead of your fingers for probing around
the wiring of a live set."
"That's right," Mac agreed. "On the whole, though, I imagine
that a service technician would be likely to be more aware of
the danger of electrical shock than he would of other possible
sources of injury. For example, we know how important it is
to avoid breathing the fumes of carbon tetra-chloride or of
getting the stuff into your eyes or a break in the skin; yet
I still see a lot of fellows handling the solvent as carelessly
as though it were water. They ought to read some of the articles
that have appeared in radio magazines telling how familiar carbon
tet can cause serious illness or even death if mishandled.
"In the same way the average technician has been pretty well
impressed with the fact that he should handle a TV picture tube
with extreme caution to avoid an explosion, but he does not
hesitate to grab hold of a receiving tube with his bare hand
and yank with all his might in an attempt pull it from a stubborn
loctal socket. If one of these tubes breaks and cuts a quarter-inch-deep
gash in his thumb, as it did in mine, he will not need be reminded
after that to use a tube-puller, leather glove, or some other
means of protection when taking out ordinary receiving tubes."
"Even if the tube does not break, you can often get a nasty
burn from trying to handle a too-hot tube," Barney remarked.
"Right again," Mac applauded, "and a burn is always dangerous
because affords a good place for infection to start. This is
one reason we should be careful not to be burned with a hot
soldering iron or hot solder. Parts being soldered should always
be held in place with pliers instead of the fingers. Even though
the part can be gripped in such a manner that heat from the
joint cannot be conducted to the fingers there is always the
danger that the point of the iron will slip off the joint and
dab against them or that a blow of excess solder will' roll
from the joint or the iron on to the fingers and burn a blister
before you can say 'ouch'!
"What really chills me, though. Mac continued, "is the sight
of a guy pulling on a wire with all his might while he softens
the solder joint with a hot iron. When the wire finally does
break loose, it invariably throws a spray of melted solder,
usually directly toward the person pulling on it. A particle
of hot solder striking the eye ball could easily cause permanent
blindness. Never, never do that. Use a soldering tool to free
the end of the wire and work it loose while the iron keeps the
joint hot; but never pull on the wire while you are doing this
so that it becomes a slingshot for throwing molten solder into
"You keep on," Barney warned, "and you're going to convince
me that a radio or TV chassis is nothing but a booby-trap. I'll
be afraid to touch one of the things."
"You'll be safe enough unless you really are a booby," Mac
said with a grin. "I'm not saying, however, that even a careful
person can always avoid a minor scratch or burn; but such a
person will take care of the minor injury promptly and so insure
that it remains just that. Whenever you break the skin, even
with the smallest scratch, I want you to go to the first-aid
cabinet at once and apply an antiseptic. The quicker you do
this the less likelihood will there be that germs can enter
the bloodstream through the cut. Burns should be treated just
as promptly. Remember, too, that the baking soda in the cabinet
is for neutralizing battery acid that may happen to splash on
"Finally, you know how strongly I feel about every technician
being an expert at administering artificial respiration. Probably
this is the best weapon we have against unnecessary death from
electrical shock, and I think it is near-criminal for anyone
who works with electricity to be ignorant of this life-saving
process. To work with electricity and not be able to administer
artificial respiration is like a professional snake hunter plying
his trade without a vial of snake-bite serum in his pocket."
"That would definitely be un-good," Barney agreed. "Perhaps
I'd better do a little brushing up on my first aid techniques!"
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no other than John T. Frye,
creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine (which
itself started as simply Radio News), and then changed its name to Mac's Service
Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his eager,
if not somewhat naive, technician assistant. 'Lessons' are taught in story format with dialogs
between Mac and Barney.
Posted August 4, 2015