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October 1960 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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After reading the first
paragraph of this "Mac's Service Shop"
technodrama™ entitled, "Technician of
Consulting Engineer?," I expected to be told a story about the seemingly excited
customer exiting the shop as he returned from lunch. Had the man made an
unreasonable demand on Mac McGregor, the proprietor, and was rebuffed
appropriately? It never materialized. In the second paragraph underling
technician Barney mentions having seen two men exiting the shop on his way in,
not just one. At that point I'm wondering what sort of melee had just occurred.
As it turned out, my interpretation of the event was totally incorrect. While
explaining the happenings to Barney, he imparts, as he is want to do, a couple
clever bits of technical information in the process. This appeared in the
October 1960 issue of Electronics World magazine.
Check out the third problem in "What's
Your EQ?" from the June 1964 issue of Radio-Electronics magazine.
Mac's Service Shop: Technician or Consulting Engineer?
 By John T. Frye
Barney, returning to the service shop after his lunch hour, stepped aside just
in time to keep from being run over by a vigorous middle-aged man dashing out the
front door. Cautiously the young technician opened the door and stepped inside.
As he entered the service department, his eyes swept the receiving bench quickly;
then he turned to Mac, his employer, already busy at the bench.
"What did those last two jokers want?" he demanded. "I saw two guys come out
of here while I was parking the truck. Neither one was carrying anything, and apparently
neither one brought anything, either. That last one was a competitor. What was he
doing? Spying?"
"Not exactly," Mac said with a chuckle; "so you can let your hackles down. I
should warn you, though, you may have to start calling me 'mister.' I'm getting
up in the world. Those two sought my services as a consulting engineer."
"OK, Mr. McGregor, don't stop there," Barney insisted.
"The first man who came out was Mr. Harkle, an engineer with the spring factory
on the north side of town. Right after you left on that last service call before
lunch, he telephoned to say he wanted to talk to me about a problem. I agreed, and
he came right away with a bunch of blueprints under his arm.
"First he showed me a print of a large metal disc about seven feet in diameter
with holes bored through it all around the rim. At the factory springs are loaded
in these holes, and the rim of the big disc rotates slowly between two high-speed
emery wheels that are properly spaced to grind both ends of each spring simultaneously.
The rotating mechanism of the big disc is indexed so that at regular intervals it
stops while finished springs are removed from a section or while unfinished springs
are loaded into empty holes.
"The problem is that now and then a shaft breaks or a key shears on the shaft
that rotates the big disc, and then the latter starts free-wheeling under the friction
drive from the emery wheels. You can imagine the damage to the spring loading and
unloading mechanism and the real danger to persons nearby which results when that
big disc tries to turn as fast as those emery wheels: What was wanted was a device
that would shut off the emery wheels the instant the big disc accelerated beyond
its normal slow rotation. To be effective, the device had to detect acceleration
that took place over a very small portion of a complete revolution."
"That rules out anything based on centrifugal force," Barney observed.
"Right. Mr. Harkle had a hazy notion the problem might be solved somehow by electronics,
and I agreed it could. For example, a light could be arranged to shine through the
holes in the centers of the springs onto a photocell. This cell could work into
an amplifier designed to cut off at low frequencies such as those produced by the
slow lighting and darkening of the cell as the disc rotated at normal speed. When
the disc started to spin, this 'interruption' frequency would go up, pass through
the amplifier, be rectified, and operate a relay to cut power off the grinding wheels.
Another possible solution would be to mount little PM magnets around the rim of
the disc so they passed close to the soft iron core of a coil. Voltage induced in
this coil by the passage of each magnet would depend, among other things, on the
speed of the magnet past the coil. Voltage thus produced could be amplified and
used to operate a cut-off relay when it exceeded a certain value-such as that resulting
when the disc started to rotate more rapidly.
"But I kept thinking of all the things that could go wrong with such arrangements:
dirt could cut down the light received by the photocell; the exciting lamp could
burn out; amplifier tubes could go bad; the magnets could weaken; the pickup coil
could be jarred out of position; and so on. Suddenly I remembered the 'velocity
trip' arrangement used on record changers that permits the tone arm to move in toward
the spindle the diameter of one groove per revolution without tripping; but when
the needle strikes the tripping groove and the tone arm swings in rapidly, the change
mechanism is set in motion. In an instant I had what seemed to me a simple and nearly
fool-proof solution.
"I suggested a small synchronous motor and cam arrangement be used to press the
end of a rubber-tipped rod firmly against the rim of the wheel once every second.
The rod would be mounted so its tip could be carried sideways a small amount by
the normal motion of the disc during the fraction of a second the two were in contact;
but when the rod pulled back each time, it would return to its original position.
When the wheel started to spin, however, the end of the rod would be carried sideways
much farther during the contact period. This increased lateral motion could easily
be arranged to operate a micro-switch that would remove power from the grinding
wheels. I pointed out to Mr. Harker that the arrangement would be very quick acting
and positive; it used only a few parts; and it could be visually inspected at any
time for proper operation. He gathered up his blueprints and took off in a happy
frame of mind.
"Just before Mr. Harker left, Tony Klinck dropped in to report a solution I had
worked out for one of his problems was a great success. Tony is a good TV service
technician, but he got into the repair game since the advent of TV and lacks the
broad background of general electronic experience that comes from growing up with
the business since the early days of radio. He never hesitates to yell for help
when he needs it, however; and a couple of weeks back he came to me with quite an
interesting little problem.
"A drive-in eating place over on the west side has installed a whole new intercom
system with speakers on posts in the parking area to relay orders to the kitchen.
When a button on a parking lot speaker is pushed, this turns on a light above a
switch in the kitchen. The order-taker throws this switch down, steps on a foot
switch that operates a relay performing the push-to-talk function of the intercom
system, and says, 'May I take your order?' Releasing the foot switch transfers the
kitchen speaker transformer back to the output of the intercom amplifier and the
post speaker transformer to the input of the amplifier so the customer can place
his order.
"An added feature is the playing of music through post speakers not being used
for talking. A tuner or record player working into a forty-watt amplifier accomplishes
this. When all the switches in the kitchen are in the 'up' position, all post speaker
transformer primaries are connected in parallel across the output of the music amplifier.
Throwing anyone switch down removes the attendant speaker transformer from the music
bus and connects it to the intercom circuit. The rub was that music was heard through
post and kitchen speakers, even when they were switched to the intercom circuit,
interfering with orders.
"A couple of things complicated the situation: first, I could not make my own
tests. It was Tony's job; so it simply would not do for me to go nosing around.
I had to depend on him to do the testing. Second, the system had been struck by
lightning shortly after being installed, and the owner could not be sure if the
trouble had been present before then or not.
"Tony had checked over all the wiring carefully to make sure it was correct and
that there were no short-circuits feeding the music into the intercom circuits.
I suggested several tests aimed at isolating the part of the circuit where this
transfer was taking place, and he soon reported that it was happening somewhere
in the banks of switches that transferred each post speaker from the music bus to
the intercom circuit, and vice versa. Remembering the lightning, I thought possibly
an arc had carbonized a path across the insulation of one or more switches; so I
suggested the switch banks be disconnected and individually checked for leakage
with an ohmmeter. This test revealed no degradation of switch insulation, and that
left just one possibility: the music was being transferred across open switch contacts
through the capacity between them. Capacity between contacts on one switch would
be quite small; but when dozens of switches were wired in parallel, the total would
be high .
"I could think of no practicable way of reducing this capacity appreciably; so
I decided to try to 'buck out' the music in the same way a transmitting tube is
neutralized; that is, supply a bucking signal equal in amplitude but 180° out-of-phase
with the signal being transferred through the switch capacities. I worked out a
circuit to do this and tested it here on the bench. I let the audio generator represent
the music amplifier. One half the primary of a small output transformer was connected
across the output of the generator, the center-tap being grounded. The output of
the generator was fed through a 20-μμf. capacitor to the a.c. v.t.v.m.; and
the output of the generator, operating at 1000 cycles, was adjusted for a 10 millivolt
reading. A 35-μμf. variable capacitor was connected between the loose end
of the transformer primary winding and the hot terminal of the v.t.v.m. The 20-μμf.
capacitor represented the capacity between switch contacts through which the unwanted
signal - 10 millivolts in this case - was feeding. I knew any signal appearing at
the hot end of the portion of the transformer winding across the signal generator
would produce another signal equal in amplitude but 180° out-of-phase at the
other end of the winding. This phase-shifted signal would be transferred through
the variable capacitor to the v.t.v.m., representing the intercom bus in our test
setup. When the variable capacitor was adjusted to 20 μμf., the two signals
should cancel - and they did! With the capacitor adjusted for maximum attenuation,
the reading could be dropped 35 or 40 db; and since the two signal paths were identical,
this attenuation would hold all the way from 50 to 5000 cycles.
"All Tony had to do was connect one end of a center-tapped output transformer
winding to the music bus, ground the center-tap, and connect the other end of the
winding through a small variable capacitor to the intercom bus. He stopped by to
tell me that when he set the variable capacitor equal to the capacity between switch
contacts, the music dropped out of the intercom as if by magic, and stayed out."
"How about pay, Mr. Consulting Engineer?" Barney inquired.
"In Tony's case, when he asked for my bill I suggested he could pay me by doing
some antenna tower work out at the house. He is a crackerjack at this and well equipped.
As for the spring company, I'll send them a bill for consultation and advice. The
point I want to make, though, is that the radio and TV service technician has acquired
a wealth of knowledge, not necessarily restricted to repairing ailing radio and
TV sets, that others are willing and eager to buy - if he is alert to this fact.
"On the other hand, the technician must be careful not to get in over his head.
For example, yesterday I had a long distance call from the director of the hospital
over in Carlston. He wanted to install a two-way radio system at the hospital and
in ambulances so that the emergency ward and doctors could be alerted and ready
to handle accident patients when they arrived. He had received some quotations from
people manufacturing and selling this type of equipment, but he did not like the
prices. He had heard of the fabulous 'bargains' to be had in surplus electronic
equipment, and he wanted me to go with him to a war surplus depot, pick out suitable
equipment, convert it, and install it in the hospital at a big fat saving to the
hospital. I politely declined after pointing out several of the many pitfalls involved
in converting war surplus, getting the equipment licensed, maintaining it, etc."
"I don't blame, you," Barney remarked. "Dependability would be very important
in such a setup: He had better wait until he can afford commercial equipment. That's
a case where you probably gave away advice worth several hundred dollars to the
hospital director."
"Oh well, it's a good cause," Mac said with a grin.
Posted May 31, 2023
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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