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Thanks to this 1959
installment of "Mac's Service Shop," which appeared in Radio & TV News magazine,
we now know who was responsible for the
ozone hole
discovered in the 1970s: It was the electronics service industry. Thanks to
products like General Cement's "Spray-Koat Circuit-Cooler," which was pure
canned carbon dioxide, ecocriminal technicians in workshops and living rooms
across the country - and across the world - indiscriminately loosed
life-threatening (to humans, not to plants which thrive on it) volumes of the
gaseous poison into the atmosphere whilst troubleshooting radios, TVs, stereo
systems, tape players, public address systems, and other common devices. I must
confess my own mea culpa
on the matter; I am guilty of the same miscarriage of ecojustice. One circuit
freeze spray on the market today is TechSpray's "Anti-Static Freezer." The
website states: " HFC-134a (1,1,1,2-Tetrafluoroethane, CAS #811-97-2) –
HFC-134a is the most common refrigerant because it is nonflammable, readily
available, and reasonably priced. However, there are environmental concerns with
HFC-134a because it has a Global Warming Potential (GWP) of 1400, which
means it has 1400 times the impact of CO2. On the plus side, it is not
considered a VOC (Volatile Organic Compound), so does not contribute to smog."
Well, at least is doesn't produce smog. To be fair, the company - as well as
many others - make freeze spray with a much lower GWP value.
Mac's Service Shop: Cold and Hot
 By John T. Frye
Monday was not Barney's best day. Weekend dating usually left the Number Two
Man of Mac's Service Shop pretty sleepy; so Mac was not astonished when he returned
from lunch to find his assistant precariously perched on a high stool and slumped
over the service bench with his tousled red head pillowed on his folded arms, sound
asleep.
Mac glanced from the figure at the bench down to the tall round can he carried
in his hands; then he noiselessly removed the protecting cap from the spray nozzle
on top of the can and tip-toed quietly across the room. Holding the can several
inches from the head of the sleeping youth, he depressed the valve. A white, disappearing
cloud hissed forth and played around the nape of Barney's neck.
With a yowl of surprise the boy leaped to his feet. "Wow! What a draft! Must
be getting lots colder outside," he exclaimed as he rubbed the back of his neck.
"Oh, oh!" he continued as he spied the can in Mac's hands; "what are you up to?"
"That's your cold draft," Mac said with a grin as he punched the valve again.
"It's General Cement's 'Spray-Koat Circuit-Cooler'."
"So what's it good for besides going around annoying innocent people?" Barney
asked with a huge yawn.
"It's actually freon gas under high pressure," Mac explained. "You use it on
a circuit component you suspect of being temperature-sensitive. When this gas hits
a radio part, that part gets very, very cold in a great big hurry."
"Hm-m-m-m, you're filtering through to me. That ought to be just what the doctor
ordered for those radio and TV sets that display intermittent symptoms when they
are first turned on. After these sets warm up a bit, the annoying condition disappears
until the set is turned off and allowed to cool down completely; then it's right
back. When you're trying to troubleshoot one of these little dandies, you have to
act fast and catch it cutting out when it's first turned on or you're out of luck.
They are great time wasters. In the past I've seen you put these sets outside in
the winter or in the refrigerator in the summer to make them good and cold. Now
we can put the chill on them right on the bench with that bottled north wind."
"And the good part is we can make that north wind blow exactly where we want
it. We can cool off a small section of the circuit - or even a single part, such
as a dubious capacitor - without affecting the rest of the circuit. And don't overlook
the fact that it can also be used on those sets that cut out after they get warm.
You simply spray a section of the circuit at a time until the set starts to operate
again. That tells you where the defective component is. When the set cuts out again,
you can cool off a part at a time. When the right one is chilled, it will make the
set come back on."
"Man! That's real cool!"
"There are some horse-sense precautions to observe in using the stuff. For one
thing, don't play the spray on the skin at close range. It will actually freeze
a chunk of the flesh in nothing flat. The salesman was telling me one of their boys
was demonstrating the stuff by squirting it on the palm of his hand, and he developed
a nasty 'burn' that was really a frostbite. The closer the nozzle is held to an
object, the colder that object gets. You will see a sort of rime appearing on an
object sometimes, but it disappears immediately. I'm told the gas leaves no residue
to interfere with electronic action."
"I suppose another horse-sense precaution is to see the spray doesn't fall on
a hot glass tube," Barney observed. "I'll bet you could really crack a rectifier
bulb that way."
"You certainly could," Mac said as he placed the can on the shelf with the imposing
array of chemicals used in service work. There was contact cleaner, corona dope,
cement solvent, alcohol, carbon tetrachloride marked with skull and crossbones,
acrylic spray, "Lubriplate" and silicon gel, and recorder head cleaner.
Mac picked up a high-voltage doorknob capacitor from the bench and favored it
with a sour look.
"Wish I could work out a quick and accurate way of checking this cuss," he commented.
"It really gave me a hard time. The set came in with no picture. Checking revealed
the high voltage was only about three or four kilovolts. The first thing I did was
put the ohmmeter of the v.t.v.m. that reads up to 1000 megohms across the capacitor.
It showed no leakage at all.
"A drooping high-voltage symptom is often a headache because it can be produced
by so many different circuit defects. The accompanying symptoms did not help much,
either. The boost voltage was low, but cutting in an outboard boost voltage supply
did not restore the high voltage. Neither did changing the horizontal oscillator,
horizontal output, damper tube, or high-voltage rectifier. The waveform at the grid
of the output tube was somewhat lower in amplitude than rated, but this was not
enough to cause the trouble. I checked the output transformer for shorted turns,
but nothing was wrong.
"I happened to touch this capacitor while I was making the last test, and it
was noticeably warm. I determined to cut it entirely out of the circuit, even though
this took a bit of doing. When I did so, the high voltage flipped right up. Replacing
the capacitor restored everything to normal; however, it was necessary to replace
the high-voltage rectifier that probably had been damaged by the heavy current drain.
"But then I started trying to find a check of this capacitor that I knew to be
bad that would show it so. I had absolutely no luck. I used our ohmmeter that places
450 volts across the test leads and reads up to 20 megohms, but this capacitor showed
no more leakage than a brand new unit. Next I tried our leakage tester that uses
a neon bulb to indicate leakage resistance up to 500 megohms, but this also failed
to show anything wrong.
"I've finally concluded the capacitor has no leakage until a certain critical
voltage is reached; then it abruptly develops a comparatively low resistance. Any
attempt to test the capacitor with voltages below this critical potential must fail
to show anything wrong. I was talking this over with my friend, John, who works
in an experimental laboratory and also does some TV service work. He was telling
me he had run into identically the same thing and had decided to see what happened
to the capacitor when it was subjected to an increasing voltage. The lab has a source
of d.c. voltage that can be increased from zero up to twenty thousand volts and
he put this on the defective capacitor. 'When the voltage reached about 5000 volts,
the capacitor suddenly shorted and exploded and blew bits of itself all over the
lab."
"Well, that's one way to test 'em," Barney observed.
"Yes, but it's not very practical. Until I find a better solution, I'm going
to use the old tried and true method of substitution without fooling around with
useless resistance checks."
"You spoke about the capacitor feeling warm after the set had been turned on
and that checks with what a fellow was telling me about these units the other day.
He claims that quite often you can see a little arc inside a bad capacitor if you
look closely in a very dim light. He says the glow of the arc will show up right
through the case. But enough of this talk about the hot high-voltage circuits. Come
on down to the level of this little a.c.-d.c. receiver and tell me if you hear anything
wrong with the tone quality."
Mac listened critically to the little receiver as he ran the volume up and down.
"No," he said slowly; "should I?"
"Well, the boy who brought it in said that after it was on a few minutes it became
so mushy you could hardly understand it. I've had it on for an hour and I can't
see anything wrong. Just to be on the safe side, I checked the coupling capacitors
for leakage and the speaker cone for proper centering. Nothing is wrong in either
department."
"How old would you say that boy was?"
"Around sixteen, but what's that got to do with the price of hay in China?"
"What would you say was the favorite program of the teenagers?"
"That disc jockey program that comes on at eleven p.m."
"That's probably the only time the kid uses this radio. Now what is different
about using a radio late at night and using it during the day?"
"I give up, Mr. Bones. What is different about using a radio late at night and
using it during the day?"
"Ever check the line voltage late at night?"
"Yeah-h-h! It goes away up. Let me plug this thing into the variable-voltage
transformer and raise the line voltage up to about 125 volts. There we are."
It was only a minute or so until the clear sound of the radio began to blur a
little and in no time at all it was distorting so badly that speech could scarcely
be understood. Mac did not need to tell Barney what to do next. He removed the 5005
tube and put in a new one. Now the radio continued to play clearly even at the elevated
line voltage.
"The old story of secondary emission causing the plate current to run away,"
Barney said. "The only difference is that the condition does not start until the
line voltage is increased. Man, you've really got to be on your toes in this racket.
I suppose if I hadn't told you a boy brought the radio in we never would have found
what was wrong with it."
"Oh, I wouldn't say that," Mac demurred. "Elevating the line voltage should be
a standard procedure in any radio that is said to distort after it has been on for
a few minutes if that symptom fails to show up in a normal bench check. And if that
doesn't make the set distort, try lowering the line voltage. In some instances a
weak tube will cause distortion when its filament voltage is lowered."
"You know," Barney said slowly, "I'll bet Sherlock Holmes would have made a wonderful
service technician. He believed that every detail that could be observed, no matter
how minute, was significant. It certainly is in radio and TV work."
"Righto, Dr. Watson!" Mac said with a very poor imitation of a British accent:
"and now if you will hand me my spyglass and my fore-and-aft hat, we'll start on
The Strange Adventure of the Errant Electrons!"
Posted September 18, 2024
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
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.
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